Add rom

.gitea/workflows/ci.yaml

@@ -0,0 +1,30 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Zig
+        uses: https://codeberg.org/mlugg/setup-zig@v2
+        with:
+          version: master
+
+      - name: Zig version
+        run: zig version
+
+      - name: Test
+        run: zig build test --summary all
+
+      - name: Headless release build
+        run: zig build -Dheadless=true -Doptimize=ReleaseSafe
+
+      - name: Release build
+        run: zig build -Doptimize=ReleaseSafe

.radicle/native.yaml

@@ -2,9 +2,10 @@ shell: |
   set -euxo pipefail
 
   echo "=== Radicle CI ==="
-  echo "Repository:"
-  pwd
-
+  echo "Repository: $(pwd)"
   zig version
 
+  # test suite & release builds
+  zig build test --summary all
+  zig build -Dheadless=true -Doptimize=ReleaseSafe
   zig build -Doptimize=ReleaseSafe

README.md

@@ -1,4 +1,4 @@
-# 6soz
+# 6soz - Retro Game Emulator
 
 A 6502 / 6510 CPU emulator engine, disassembler, and system architecture library written in [Zig](https://ziglang.org/).
 

build.zig

@@ -85,16 +85,11 @@ pub fn build(b: *std.Build) void {
     const test_step = b.step("test", "Run 6soz tests");
 
     const test_options = b.addOptions();
-    if (b.lazyDependency("nes_test_roms", .{})) |roms_dep| {
-        test_options.addOption(?[]const u8, "nes_test_roms_dir", roms_dep.path("").getPath(b));
-    } else {
-        test_options.addOption(?[]const u8, "nes_test_roms_dir", null);
-    }
-    if (b.lazyDependency("m6502_functional_tests", .{})) |func_dep| {
-        test_options.addOption(?[]const u8, "m6502_functional_tests_dir", func_dep.path("").getPath(b));
-    } else {
-        test_options.addOption(?[]const u8, "m6502_functional_tests_dir", null);
-    }
+    const roms_dep = b.dependency("nes_test_roms", .{});
+    test_options.addOption(?[]const u8, "nes_test_roms_dir", roms_dep.path("").getPath(b));
+
+    const func_dep = b.dependency("m6502_functional_tests", .{});
+    test_options.addOption(?[]const u8, "m6502_functional_tests_dir", func_dep.path("").getPath(b));
 
     const options_mod = test_options.createModule();
 

build.zig.zon

@@ -6,12 +6,10 @@
         .nes_test_roms = .{
             .url = "https://github.com/christopherpow/nes-test-roms/archive/refs/heads/master.tar.gz",
             .hash = "N-V-__8AANPxaAENPnXYuDVuqgUcOdrux1zKqz8bZZYf0RA2",
-            .lazy = true,
         },
         .m6502_functional_tests = .{
             .url = "https://github.com/Klaus2m5/6502_65C02_functional_tests/archive/refs/heads/master.tar.gz",
             .hash = "N-V-__8AAKmLHACkwIMCMRdqnsyLtL5hbQdi8cm3nGs8n-O7",
-            .lazy = true,
         },
         .raylib = .{
             .url = "https://github.com/raysan5/raylib/archive/refs/heads/master.tar.gz",

src/main.zig

@@ -1,5 +1,12 @@
 const std = @import("std");
 const Nes = @import("system").nes;
+const platform = @import("platform");
+const frontend = @import("frontend");
+
+const PressSpec = struct {
+    frame: u64,
+    key: platform.Key,
+};
 
 pub fn main(init: std.process.Init) !void {
     const allocator = init.gpa;
@@ -44,10 +51,13 @@ pub fn main(init: std.process.Init) !void {
 
     const rom_path = args[2];
     var target_frames: u64 = 60;
+    var dump_frame_path: ?[]const u8 = null;
+    var dump_audio_path: ?[]const u8 = null;
     var save_state_path: ?[]const u8 = null;
     var load_state_path: ?[]const u8 = null;
     var save_sram_path: ?[]const u8 = null;
     var load_sram_path: ?[]const u8 = null;
+    var press_spec: ?PressSpec = null;
 
     var i: usize = 3;
     while (i < args_count) : (i += 1) {
@@ -55,6 +65,39 @@ pub fn main(init: std.process.Init) !void {
         if (std.mem.eql(u8, arg, "--frames") and i + 1 < args_count) {
             i += 1;
             target_frames = try std.fmt.parseInt(u64, args[i], 10);
+        } else if (std.mem.eql(u8, arg, "--press-at") and i + 1 < args_count) {
+            i += 1;
+            const val = args[i];
+            if (std.mem.indexOfScalar(u8, val, ':')) |colon| {
+                const frame_str = val[0..colon];
+                const btn_str = val[colon + 1 ..];
+                const frame_num = std.fmt.parseInt(u64, frame_str, 10) catch 0;
+                var key: platform.Key = .enter;
+                if (std.mem.eql(u8, btn_str, "a")) {
+                    key = .z;
+                } else if (std.mem.eql(u8, btn_str, "b")) {
+                    key = .x;
+                } else if (std.mem.eql(u8, btn_str, "select")) {
+                    key = .right_shift;
+                } else if (std.mem.eql(u8, btn_str, "start") or std.mem.eql(u8, btn_str, "enter")) {
+                    key = .enter;
+                } else if (std.mem.eql(u8, btn_str, "up")) {
+                    key = .up;
+                } else if (std.mem.eql(u8, btn_str, "down")) {
+                    key = .down;
+                } else if (std.mem.eql(u8, btn_str, "left")) {
+                    key = .left;
+                } else if (std.mem.eql(u8, btn_str, "right")) {
+                    key = .right;
+                }
+                press_spec = .{ .frame = frame_num, .key = key };
+            }
+        } else if (std.mem.eql(u8, arg, "--dump-frame") and i + 1 < args_count) {
+            i += 1;
+            dump_frame_path = args[i];
+        } else if (std.mem.eql(u8, arg, "--dump-audio") and i + 1 < args_count) {
+            i += 1;
+            dump_audio_path = args[i];
         } else if (std.mem.eql(u8, arg, "--save-state") and i + 1 < args_count) {
             i += 1;
             save_state_path = args[i];
@@ -104,19 +147,33 @@ pub fn main(init: std.process.Init) !void {
         }
     }
 
-    const frontend = @import("frontend");
-    const platform = @import("platform");
-
     const nes_bindings = frontend.Bindings{
         .keys = &.{
+            // Primary A / B buttons
             .{ .key = .z, .target = .{ .button = .{ .device = 0, .button = 0 } } },
             .{ .key = .x, .target = .{ .button = .{ .device = 0, .button = 1 } } },
+            .{ .key = .c, .target = .{ .button = .{ .device = 0, .button = 0 } } },
+            .{ .key = .v, .target = .{ .button = .{ .device = 0, .button = 1 } } },
+            .{ .key = .k, .target = .{ .button = .{ .device = 0, .button = 0 } } },
+            .{ .key = .j, .target = .{ .button = .{ .device = 0, .button = 1 } } },
+
+            // Select & Start
             .{ .key = .right_shift, .target = .{ .button = .{ .device = 0, .button = 2 } } },
+            .{ .key = .left_shift, .target = .{ .button = .{ .device = 0, .button = 2 } } },
             .{ .key = .enter, .target = .{ .button = .{ .device = 0, .button = 3 } } },
+            .{ .key = .space, .target = .{ .button = .{ .device = 0, .button = 3 } } },
+
+            // D-Pad: Arrow keys & WASD
             .{ .key = .up, .target = .{ .button = .{ .device = 0, .button = 4 } } },
             .{ .key = .down, .target = .{ .button = .{ .device = 0, .button = 5 } } },
             .{ .key = .left, .target = .{ .button = .{ .device = 0, .button = 6 } } },
             .{ .key = .right, .target = .{ .button = .{ .device = 0, .button = 7 } } },
+            .{ .key = .w, .target = .{ .button = .{ .device = 0, .button = 4 } } },
+            .{ .key = .s, .target = .{ .button = .{ .device = 0, .button = 5 } } },
+            .{ .key = .a, .target = .{ .button = .{ .device = 0, .button = 6 } } },
+            .{ .key = .d, .target = .{ .button = .{ .device = 0, .button = 7 } } },
+
+            // System actions
             .{ .key = .f5, .target = .{ .action = .quick_save } },
             .{ .key = .f8, .target = .{ .action = .quick_load } },
             .{ .key = .p, .target = .{ .action = .toggle_pause } },
@@ -143,9 +200,23 @@ pub fn main(init: std.process.Init) !void {
         std.debug.print("[6soz] Running NES emulator for {d} frames...\n", .{target_frames});
         const start_time = std.Io.Clock.Timestamp.now(init.io, .awake);
 
+        var audio_accumulator: std.ArrayListUnmanaged(i16) = .empty;
+        defer audio_accumulator.deinit(allocator);
+
         var frame: u64 = 0;
         while (frame < target_frames and app.state == .running) : (frame += 1) {
+            if (press_spec) |spec| {
+                if (frame >= spec.frame and frame < spec.frame + 20) {
+                    app.handleEvent(.{ .key_down = .{ .key = spec.key } });
+                } else if (frame == spec.frame + 20) {
+                    app.handleEvent(.{ .key_up = .{ .key = spec.key } });
+                }
+            }
             app.tickFrame(&driver);
+            if (dump_audio_path != null) {
+                const pcm = nes.bus.apu.getSamples();
+                try audio_accumulator.appendSlice(allocator, pcm);
+            }
         }
 
         const end_time = std.Io.Clock.Timestamp.now(init.io, .awake);
@@ -157,6 +228,57 @@ pub fn main(init: std.process.Init) !void {
         std.debug.print("  - Frames: {d}\n", .{target_frames});
         std.debug.print("  - Elapsed Time: {d:.3}s\n", .{elapsed_sec});
         std.debug.print("  - Performance: {d:.1} FPS\n", .{fps});
+
+        if (dump_audio_path) |path| {
+            const num_samples: u32 = @truncate(audio_accumulator.items.len);
+            const data_bytes: u32 = num_samples * 2;
+            const total_file_size: u32 = 44 + data_bytes;
+            const wav_buf = try allocator.alloc(u8, total_file_size);
+            defer allocator.free(wav_buf);
+
+            @memcpy(wav_buf[0..4], "RIFF");
+            std.mem.writeInt(u32, wav_buf[4..8], 36 + data_bytes, .little);
+            @memcpy(wav_buf[8..12], "WAVE");
+            @memcpy(wav_buf[12..16], "fmt ");
+            std.mem.writeInt(u32, wav_buf[16..20], 16, .little);
+            std.mem.writeInt(u16, wav_buf[20..22], 1, .little);
+            std.mem.writeInt(u16, wav_buf[22..24], 1, .little);
+            std.mem.writeInt(u32, wav_buf[24..28], 44100, .little);
+            std.mem.writeInt(u32, wav_buf[28..32], 44100 * 2, .little);
+            std.mem.writeInt(u16, wav_buf[32..34], 2, .little);
+            std.mem.writeInt(u16, wav_buf[34..36], 16, .little);
+            @memcpy(wav_buf[36..40], "data");
+            std.mem.writeInt(u32, wav_buf[40..44], data_bytes, .little);
+            @memcpy(wav_buf[44..total_file_size], std.mem.sliceAsBytes(audio_accumulator.items));
+
+            cwd_dir.writeFile(init.io, .{ .sub_path = path, .data = wav_buf }) catch |err| {
+                std.debug.print("Error writing dumped audio to '{s}': {s}\n", .{ path, @errorName(err) });
+            };
+            std.debug.print("[6soz] Dumped audio ({d} samples, {d:.2}s) to '{s}'\n", .{
+                num_samples,
+                @as(f64, @floatFromInt(num_samples)) / 44100.0,
+                path,
+            });
+        }
+    }
+
+    if (dump_frame_path) |path| {
+        const vf = nes.videoFrame(0);
+        const raw_pixels = std.mem.bytesAsSlice(u16, vf.data);
+        var ppm_buf: [32 + 256 * 240 * 3]u8 = undefined;
+        const header = "P6\n256 240\n255\n";
+        @memcpy(ppm_buf[0..header.len], header);
+        var offset: usize = header.len;
+        for (raw_pixels) |p| {
+            ppm_buf[offset] = @truncate(((p >> 11) & 0x1f) * 255 / 31);
+            ppm_buf[offset + 1] = @truncate(((p >> 5) & 0x3f) * 255 / 63);
+            ppm_buf[offset + 2] = @truncate((p & 0x1f) * 255 / 31);
+            offset += 3;
+        }
+        cwd_dir.writeFile(init.io, .{ .sub_path = path, .data = ppm_buf[0..offset] }) catch |err| {
+            std.debug.print("Error writing dumped frame to '{s}': {s}\n", .{ path, @errorName(err) });
+        };
+        std.debug.print("[6soz] Dumped frame to '{s}'\n", .{path});
     }
 
     if (save_state_path) |path| {
@@ -189,6 +311,9 @@ fn printUsage() void {
         \\
         \\Options:
         \\  --frames <N>         Number of frames to execute (default: 60)
+        \\  --press-at <F>:<B>   Press button (a, b, select, start, etc.) at frame F for 20 frames
+        \\  --dump-frame <file>  Dump final video frame to PPM image file
+        \\  --dump-audio <file>  Dump audio output to standard WAV file
         \\  --save-state <file>  Save state binary file output
         \\  --load-state <file>  Load state binary file input
         \\  --save-sram <file>   Save battery PRG RAM output

src/platform/raylib.zig

@@ -16,9 +16,10 @@ pub const Driver = struct {
     audio_ring_head: usize = 0,
     audio_ring_tail: usize = 0,
 
-    event_queue: [16]platform.Event = undefined,
+    event_queue: [64]platform.Event = undefined,
     event_head: usize = 0,
     event_tail: usize = 0,
+    polled_this_frame: bool = false,
 
     pub fn init(title: [*:0]const u8, width: i32, height: i32, scale: i32) Driver {
         c.InitWindow(width * scale, height * scale, title);
@@ -63,10 +64,22 @@ pub const Driver = struct {
             return ev;
         }
 
+        if (self.polled_this_frame) {
+            return null;
+        }
+        self.polled_this_frame = true;
+        self.event_head = 0;
+        self.event_tail = 0;
+
         // Poll Raylib Key Events
         const key_mappings = [_]struct { c_key: c_int, key: platform.Key }{
             .{ .c_key = c.KEY_Z, .key = .z },
             .{ .c_key = c.KEY_X, .key = .x },
+            .{ .c_key = c.KEY_C, .key = .c },
+            .{ .c_key = c.KEY_V, .key = .v },
+            .{ .c_key = c.KEY_J, .key = .j },
+            .{ .c_key = c.KEY_K, .key = .k },
+            .{ .c_key = c.KEY_SPACE, .key = .space },
             .{ .c_key = c.KEY_RIGHT_SHIFT, .key = .right_shift },
             .{ .c_key = c.KEY_LEFT_SHIFT, .key = .left_shift },
             .{ .c_key = c.KEY_ENTER, .key = .enter },
@@ -74,6 +87,10 @@ pub const Driver = struct {
             .{ .c_key = c.KEY_DOWN, .key = .down },
             .{ .c_key = c.KEY_LEFT, .key = .left },
             .{ .c_key = c.KEY_RIGHT, .key = .right },
+            .{ .c_key = c.KEY_W, .key = .w },
+            .{ .c_key = c.KEY_S, .key = .s },
+            .{ .c_key = c.KEY_A, .key = .a },
+            .{ .c_key = c.KEY_D, .key = .d },
             .{ .c_key = c.KEY_F5, .key = .f5 },
             .{ .c_key = c.KEY_F8, .key = .f8 },
             .{ .c_key = c.KEY_P, .key = .p },
@@ -81,13 +98,43 @@ pub const Driver = struct {
 
         for (key_mappings) |mapping| {
             if (c.IsKeyPressed(mapping.c_key)) {
-                return .{ .key_down = .{ .key = mapping.key } };
+                const next = (self.event_head + 1) % self.event_queue.len;
+                if (next != self.event_tail) {
+                    self.event_queue[self.event_head] = .{ .key_down = .{ .key = mapping.key } };
+                    self.event_head = next;
+                }
             }
             if (c.IsKeyReleased(mapping.c_key)) {
-                return .{ .key_up = .{ .key = mapping.key } };
+                const next = (self.event_head + 1) % self.event_queue.len;
+                if (next != self.event_tail) {
+                    self.event_queue[self.event_head] = .{ .key_up = .{ .key = mapping.key } };
+                    self.event_head = next;
+                }
             }
         }
 
+        // Support mouse left click as Start/Action button
+        if (c.IsMouseButtonPressed(c.MOUSE_BUTTON_LEFT)) {
+            const next = (self.event_head + 1) % self.event_queue.len;
+            if (next != self.event_tail) {
+                self.event_queue[self.event_head] = .{ .key_down = .{ .key = .enter } };
+                self.event_head = next;
+            }
+        }
+        if (c.IsMouseButtonReleased(c.MOUSE_BUTTON_LEFT)) {
+            const next = (self.event_head + 1) % self.event_queue.len;
+            if (next != self.event_tail) {
+                self.event_queue[self.event_head] = .{ .key_up = .{ .key = .enter } };
+                self.event_head = next;
+            }
+        }
+
+        if (self.event_head != self.event_tail) {
+            const ev = self.event_queue[self.event_tail];
+            self.event_tail = (self.event_tail + 1) % self.event_queue.len;
+            return ev;
+        }
+
         return null;
     }
 
@@ -116,6 +163,8 @@ pub const Driver = struct {
             .height = @floatFromInt(self.height * self.scale),
         };
         c.DrawTexturePro(self.texture, src, dest, c.Vector2{ .x = 0, .y = 0 }, 0.0, c.WHITE);
+
+        self.polled_this_frame = false;
     }
 
     pub fn queueAudio(self: *Driver, output: usize, buffer: contract.AudioBuffer) void {

src/system/nes/apu.zig

@@ -1,276 +0,0 @@
-const std = @import("std");
-
-const Apu = @This();
-
-pub const sample_rate: u32 = 44_100;
-
-sample_buffer: [2048]i16 = [_]i16{0} ** 2048,
-sample_count: usize = 0,
-cycle_counter: u32 = 0,
-
-// Pulse 1
-pulse1_enabled: bool = false,
-pulse1_timer_reload: u11 = 0,
-pulse1_timer: u11 = 0,
-pulse1_duty: u2 = 0,
-pulse1_duty_pos: u3 = 0,
-pulse1_volume: u4 = 0,
-
-// Pulse 2
-pulse2_enabled: bool = false,
-pulse2_timer_reload: u11 = 0,
-pulse2_timer: u11 = 0,
-pulse2_duty: u2 = 0,
-pulse2_duty_pos: u3 = 0,
-pulse2_volume: u4 = 0,
-
-// Triangle
-triangle_enabled: bool = false,
-triangle_timer_reload: u11 = 0,
-triangle_timer: u11 = 0,
-triangle_seq_pos: u5 = 0,
-
-// Noise
-noise_enabled: bool = false,
-noise_timer_reload: u12 = 0,
-noise_timer: u12 = 0,
-noise_lfsr: u16 = 1,
-noise_volume: u4 = 0,
-
-// DMC
-dmc_enabled: bool = false,
-dmc_loop: bool = false,
-dmc_rate_index: u4 = 0,
-dmc_timer: u12 = 0,
-dmc_timer_reload: u12 = 0,
-dmc_output_level: u7 = 0,
-dmc_sample_address: u16 = 0xC000,
-dmc_sample_length: u16 = 1,
-dmc_current_address: u16 = 0xC000,
-dmc_bytes_remaining: u16 = 0,
-dmc_shift_register: u8 = 0,
-dmc_bits_remaining: u4 = 0,
-dmc_silence: bool = true,
-
-const duty_tables: [4][8]u8 = .{
-    .{ 0, 1, 0, 0, 0, 0, 0, 0 }, // 12.5%
-    .{ 0, 1, 1, 0, 0, 0, 0, 0 }, // 25%
-    .{ 0, 1, 1, 1, 1, 0, 0, 0 }, // 50%
-    .{ 1, 0, 0, 1, 1, 1, 1, 1 }, // 75%
-};
-
-const triangle_table: [32]u4 = .{
-    15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,
-    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
-};
-
-const noise_periods: [16]u12 = .{
-    4, 8, 16, 32, 64, 96, 128, 160, 202, 254, 380, 508, 762, 1016, 2034, 4068,
-};
-
-const dmc_periods: [16]u12 = .{
-    428, 380, 340, 320, 286, 254, 226, 214, 190, 160, 142, 128, 106, 84, 72, 54,
-};
-
-pub fn reset(self: *Apu) void {
-    self.sample_count = 0;
-    self.cycle_counter = 0;
-    self.pulse1_enabled = false;
-    self.pulse2_enabled = false;
-    self.triangle_enabled = false;
-    self.noise_enabled = false;
-    self.noise_lfsr = 1;
-    self.dmc_enabled = false;
-    self.dmc_output_level = 0;
-}
-
-pub inline fn tick(self: *Apu) void {
-    // Pulse 1 timer
-    if (self.pulse1_timer == 0) {
-        self.pulse1_timer = self.pulse1_timer_reload;
-        self.pulse1_duty_pos +%= 1;
-    } else {
-        self.pulse1_timer -= 1;
-    }
-
-    // Pulse 2 timer
-    if (self.pulse2_timer == 0) {
-        self.pulse2_timer = self.pulse2_timer_reload;
-        self.pulse2_duty_pos +%= 1;
-    } else {
-        self.pulse2_timer -= 1;
-    }
-
-    // Triangle timer
-    if (self.triangle_timer == 0) {
-        self.triangle_timer = self.triangle_timer_reload;
-        self.triangle_seq_pos +%= 1;
-    } else {
-        self.triangle_timer -= 1;
-    }
-
-    // Noise timer
-    if (self.noise_timer == 0) {
-        self.noise_timer = self.noise_timer_reload;
-        const feedback = (self.noise_lfsr & 1) ^ ((self.noise_lfsr >> 1) & 1);
-        self.noise_lfsr = (self.noise_lfsr >> 1) | (feedback << 14);
-    } else {
-        self.noise_timer -= 1;
-    }
-
-    // DMC timer
-    if (self.dmc_timer == 0) {
-        self.dmc_timer = self.dmc_timer_reload;
-        if (!self.dmc_silence) {
-            if ((self.dmc_shift_register & 1) != 0) {
-                if (self.dmc_output_level <= 125) self.dmc_output_level += 2;
-            } else {
-                if (self.dmc_output_level >= 2) self.dmc_output_level -= 2;
-            }
-            self.dmc_shift_register >>= 1;
-        }
-
-        if (self.dmc_bits_remaining > 0) {
-            self.dmc_bits_remaining -= 1;
-        }
-        if (self.dmc_bits_remaining == 0) {
-            self.dmc_silence = true;
-        }
-    } else {
-        self.dmc_timer -= 1;
-    }
-
-    // Sample downsampling at ~44.1 kHz (every ~40 CPU cycles)
-    self.cycle_counter += 1;
-    if (self.cycle_counter >= 40) {
-        self.cycle_counter = 0;
-        if (self.sample_count < self.sample_buffer.len) {
-            const p1_val: i32 = if (self.pulse1_enabled and duty_tables[self.pulse1_duty][self.pulse1_duty_pos] != 0 and self.pulse1_timer_reload > 8)
-                @as(i32, self.pulse1_volume)
-            else
-                0;
-
-            const p2_val: i32 = if (self.pulse2_enabled and duty_tables[self.pulse2_duty][self.pulse2_duty_pos] != 0 and self.pulse2_timer_reload > 8)
-                @as(i32, self.pulse2_volume)
-            else
-                0;
-
-            const tri_val: i32 = if (self.triangle_enabled and self.triangle_timer_reload > 2)
-                @as(i32, triangle_table[self.triangle_seq_pos])
-            else
-                0;
-
-            const noise_val: i32 = if (self.noise_enabled and (self.noise_lfsr & 1) == 0)
-                @as(i32, self.noise_volume)
-            else
-                0;
-
-            const dmc_val: i32 = if (self.dmc_enabled) @as(i32, self.dmc_output_level) else 0;
-
-            const sample_i32 = (p1_val + p2_val) * 400 + tri_val * 350 + noise_val * 300 + dmc_val * 150;
-            self.sample_buffer[self.sample_count] = @intCast(std.math.clamp(sample_i32, -32000, 32000));
-            self.sample_count += 1;
-        }
-    }
-}
-
-pub fn write(self: *Apu, address: u16, value: u8) void {
-    switch (address) {
-        // Pulse 1
-        0x4000 => {
-            self.pulse1_duty = @truncate((value >> 6) & 3);
-            self.pulse1_volume = @truncate(value & 0x0f);
-        },
-        0x4002 => {
-            self.pulse1_timer_reload = (self.pulse1_timer_reload & 0x0700) | value;
-        },
-        0x4003 => {
-            self.pulse1_timer_reload = (self.pulse1_timer_reload & 0x00ff) | (@as(u11, value & 7) << 8);
-            self.pulse1_duty_pos = 0;
-        },
-
-        // Pulse 2
-        0x4004 => {
-            self.pulse2_duty = @truncate((value >> 6) & 3);
-            self.pulse2_volume = @truncate(value & 0x0f);
-        },
-        0x4006 => {
-            self.pulse2_timer_reload = (self.pulse2_timer_reload & 0x0700) | value;
-        },
-        0x4007 => {
-            self.pulse2_timer_reload = (self.pulse2_timer_reload & 0x00ff) | (@as(u11, value & 7) << 8);
-            self.pulse2_duty_pos = 0;
-        },
-
-        // Triangle
-        0x400a => {
-            self.triangle_timer_reload = (self.triangle_timer_reload & 0x0700) | value;
-        },
-        0x400b => {
-            self.triangle_timer_reload = (self.triangle_timer_reload & 0x00ff) | (@as(u11, value & 7) << 8);
-        },
-
-        // Noise
-        0x400c => {
-            self.noise_volume = @truncate(value & 0x0f);
-        },
-        0x400e => {
-            self.noise_timer_reload = noise_periods[value & 0x0f];
-        },
-
-        // DMC
-        0x4010 => {
-            self.dmc_loop = (value & 0x40) != 0;
-            self.dmc_rate_index = @truncate(value & 0x0f);
-            self.dmc_timer_reload = dmc_periods[self.dmc_rate_index];
-        },
-        0x4011 => {
-            self.dmc_output_level = @truncate(value & 0x7f);
-        },
-        0x4012 => {
-            self.dmc_sample_address = 0xC000 + (@as(u16, value) << 6);
-        },
-        0x4013 => {
-            self.dmc_sample_length = (@as(u16, value) << 4) + 1;
-        },
-        else => {},
-    }
-}
-
-pub fn readStatus(_: *Apu) u8 {
-    return 0;
-}
-
-pub fn writeStatus(self: *Apu, value: u8) void {
-    self.pulse1_enabled = (value & 0x01) != 0;
-    self.pulse2_enabled = (value & 0x02) != 0;
-    self.triangle_enabled = (value & 0x04) != 0;
-    self.noise_enabled = (value & 0x08) != 0;
-    self.dmc_enabled = (value & 0x10) != 0;
-}
-
-pub fn writeFrameCounter(_: *Apu, _: u8) void {}
-
-pub fn irqAsserted(_: *const Apu) bool {
-    return false;
-}
-
-test "NES APU - Sound Synthesis & Sample Generation" {
-    var apu = Apu{};
-    apu.reset();
-
-    // Enable Pulse 1 and Pulse 2
-    apu.writeStatus(0x03);
-
-    // Setup Pulse 1 volume and frequency
-    apu.write(0x4000, 0xbf); // 50% duty, max volume 15
-    apu.write(0x4002, 100);
-    apu.write(0x4003, 0);
-
-    // Tick APU for 100 cycles to produce PCM samples
-    for (0..100) |_| {
-        apu.tick();
-    }
-
-    try std.testing.expect(apu.sample_count > 0);
-}

src/system/nes/apu/dmc.zig

@@ -0,0 +1,877 @@
+const std = @import("std");
+
+pub const Dmc = @This();
+
+pub const Region = @import("../common.zig").Region;
+
+const ntsc_rates = [16]u16{
+    428, 380, 340, 320,
+    286, 254, 226, 214,
+    190, 160, 142, 128,
+    106, 84,  72,  54,
+};
+
+const pal_rates = [16]u16{
+    398, 354, 316, 298,
+    276, 236, 210, 198,
+    176, 148, 132, 118,
+    98,  78,  66,  50,
+};
+
+region: Region,
+
+// $4010
+// IL--.RRRR
+
+irq_enabled: bool = false,
+loop: bool = false,
+rate_index: u4 = 0,
+
+/// Set when the final sample byte is fetched and IRQs are enabled.
+irq_flag: bool = false,
+
+// $4011
+// -DDD.DDDD
+
+/// 7-bit DAC output level.
+///
+/// This is always sent to the mixer, even if DMC playback is disabled.
+output_level: u7 = 0,
+
+// $4012/$4013 configuration registers
+
+/// Starting CPU address derived from $4012.
+sample_address: u16 = 0xC000,
+
+/// Number of bytes derived from $4013.
+sample_length: u16 = 1,
+
+// Memory reader
+
+current_address: u16 = 0xC000,
+bytes_remaining: u16 = 0,
+
+/// One-byte buffer between DMA reader and output unit.
+sample_buffer: ?u8 = null,
+
+/// True once we've emitted a DMA request and are waiting for the
+/// CPU/bus to deliver the byte.
+dma_pending: bool = false,
+
+// Timer
+
+timer_counter: u16 = 0,
+
+// Output unit
+
+shift_register: u8 = 0,
+
+/// Number of bits left in the current 8-bit output cycle.
+bits_remaining: u4 = 8,
+
+/// When true, timer clocks do not modify output_level.
+///
+/// The shifter and bit counter still continue to clock.
+silence: bool = true,
+
+pub fn init(region: Region) Dmc {
+    return .{
+        .region = region,
+    };
+}
+
+// Register writes
+
+/// $4010
+///
+///     IL--.RRRR
+///
+/// I = IRQ enabled
+/// L = loop
+/// R = rate index
+pub fn writeControl(self: *Dmc, value: u8) void {
+    self.irq_enabled = (value & 0x80) != 0;
+    self.loop = (value & 0x40) != 0;
+    self.rate_index = @truncate(value & 0x0F);
+
+    //
+    // Clearing IRQ enable immediately clears the DMC IRQ flag.
+    //
+    if (!self.irq_enabled) {
+        self.irq_flag = false;
+    }
+
+    //
+    // Do NOT reset timer_counter here.
+    //
+}
+
+/// $4011
+///
+/// Directly sets the 7-bit DAC.
+pub fn writeDirectLoad(self: *Dmc, value: u8) void {
+    self.output_level = @truncate(value & 0x7F);
+}
+
+/// $4012
+///
+///     address = $C000 + value * 64
+///
+///             = %11AAAAAA.AA000000
+pub fn writeSampleAddress(self: *Dmc, value: u8) void {
+    self.sample_address =
+        0xC000 | (@as(u16, value) << 6);
+
+    //
+    // Changing this register does NOT alter a sample already
+    // being played.
+    //
+}
+
+/// $4013
+///
+///     length = value * 16 + 1
+pub fn writeSampleLength(self: *Dmc, value: u8) void {
+    self.sample_length =
+        (@as(u16, value) << 4) | 1;
+
+    //
+    // Changing this register does NOT alter the current
+    // bytes_remaining counter.
+    //
+}
+
+// $4015
+
+/// Called when $4015 is written.
+///
+/// This method also clears the DMC IRQ because every write to
+/// $4015 acknowledges the DMC interrupt.
+pub fn writeEnabled(self: *Dmc, enabled: bool) void {
+    self.irq_flag = false;
+
+    if (!enabled) {
+        //
+        // Disabling DMC stops the memory reader by setting
+        // bytes remaining to zero.
+        //
+        // It does NOT:
+        //
+        // - clear sample_buffer
+        // - clear shift_register
+        // - clear output_level
+        // - reset bits_remaining
+        //
+        self.bytes_remaining = 0;
+        return;
+    }
+
+    //
+    // Enabling restarts the sample ONLY if there isn't already
+    // a sample in progress.
+    //
+    if (self.bytes_remaining == 0) {
+        self.restartSample();
+    }
+}
+
+/// Bit 4 returned by $4015.
+///
+/// This reflects bytes remaining, NOT whether the DAC is producing
+/// a nonzero value.
+pub fn active(self: *const Dmc) bool {
+    return self.bytes_remaining != 0;
+}
+
+pub fn irqAsserted(self: *const Dmc) bool {
+    return self.irq_flag;
+}
+
+// Sample restart
+
+fn restartSample(self: *Dmc) void {
+    self.current_address = self.sample_address;
+    self.bytes_remaining = self.sample_length;
+}
+
+// DMC DMA interface
+
+/// The DMC needs a DMA whenever:
+///
+/// - sample buffer is empty
+/// - bytes remain in the sample
+/// - there isn't already a DMA in progress
+pub fn needsDma(self: *const Dmc) bool {
+    return self.sample_buffer == null and
+        self.bytes_remaining != 0 and
+        !self.dma_pending;
+}
+
+/// Start a DMA request.
+///
+/// Returns the CPU address that must be read.
+///
+/// The CPU/Bus layer should:
+///
+///     1. stall the CPU appropriately
+///     2. read this address
+///     3. call completeDma(value)
+///
+pub fn beginDma(self: *Dmc) ?u16 {
+    if (!self.needsDma())
+        return null;
+
+    self.dma_pending = true;
+
+    return self.current_address;
+}
+
+/// Called when the CPU DMA machinery has obtained the requested byte.
+pub fn completeDma(self: *Dmc, value: u8) void {
+    std.debug.assert(self.dma_pending);
+
+    self.dma_pending = false;
+
+    //
+    // Reader fills the one-byte sample buffer.
+    //
+    self.sample_buffer = value;
+
+    //
+    // Advance sample address.
+    //
+    // DMC has a special wrap:
+    //
+    //     $FFFF -> $8000
+    //
+    // NOT:
+    //
+    //     $FFFF -> $0000
+    //
+    if (self.current_address == 0xFFFF) {
+        self.current_address = 0x8000;
+    } else {
+        self.current_address += 1;
+    }
+
+    std.debug.assert(self.bytes_remaining != 0);
+    self.bytes_remaining -= 1;
+
+    if (self.bytes_remaining == 0) {
+        if (self.loop) {
+            //
+            // Looping immediately reloads address/length.
+            //
+            // sample_buffer is already full, so another DMA will
+            // not happen until the output unit consumes it.
+            //
+            self.restartSample();
+        } else if (self.irq_enabled) {
+            //
+            // Important:
+            //
+            // The IRQ happens when the FINAL BYTE IS FETCHED,
+            // not after its final 8 bits have been played.
+            //
+            self.irq_flag = true;
+        }
+    }
+}
+
+// Timer
+
+pub fn timerPeriod(self: *const Dmc) u16 {
+    const index: usize = @intCast(self.rate_index);
+
+    return switch (self.region) {
+        .ntsc => ntsc_rates[index],
+        .pal => pal_rates[index],
+    };
+}
+
+/// Clock once per CPU cycle.
+pub fn clockTimer(self: *Dmc) void {
+    if (self.timer_counter == 0) {
+        //
+        // NESdev's table gives the exact number of CPU cycles
+        // between output clocks.
+        //
+        // Therefore reload with period - 1.
+        //
+        self.timer_counter = self.timerPeriod() - 1;
+
+        self.clockOutputUnit();
+    } else {
+        self.timer_counter -= 1;
+    }
+}
+
+// Output unit
+
+fn clockOutputUnit(self: *Dmc) void {
+    //
+    // Step 1:
+    //
+    // Modify DAC using bit 0 of the shift register, unless this
+    // output cycle is silent.
+    //
+    if (!self.silence) {
+        if ((self.shift_register & 0x01) != 0) {
+            //
+            // Delta bit 1 => +2
+            //
+            // Don't wrap beyond 127.
+            //
+            if (self.output_level <= 125) {
+                self.output_level += 2;
+            }
+        } else {
+            //
+            // Delta bit 0 => -2
+            //
+            // Don't wrap below zero.
+            //
+            if (self.output_level >= 2) {
+                self.output_level -= 2;
+            }
+        }
+    }
+
+    //
+    // Step 2:
+    //
+    // Shift regardless of silence.
+    //
+    self.shift_register >>= 1;
+
+    //
+    // Step 3:
+    //
+    // Advance bit counter.
+    //
+    self.bits_remaining -= 1;
+
+    //
+    // An output cycle consists of exactly 8 timer clocks.
+    //
+    if (self.bits_remaining == 0) {
+        self.startOutputCycle();
+    }
+}
+
+fn startOutputCycle(self: *Dmc) void {
+    self.bits_remaining = 8;
+
+    if (self.sample_buffer) |sample| {
+        //
+        // A buffered sample can only enter the shifter at an
+        // 8-bit output-cycle boundary.
+        //
+        self.shift_register = sample;
+        self.sample_buffer = null;
+        self.silence = false;
+
+        //
+        // sample_buffer has just become empty.
+        //
+        // needsDma() will now become true if there are more bytes.
+        //
+    } else {
+        //
+        // No sample available.
+        //
+        // The output unit continues ticking, but it stops modifying
+        // the DAC.
+        //
+        self.silence = true;
+    }
+}
+
+// Mixer
+
+/// Current DMC mixer input.
+///
+/// This must NOT be gated by:
+///
+/// - bytes_remaining
+/// - $4015 enable
+/// - silence
+///
+/// The 7-bit DAC retains its value.
+pub fn output(self: *const Dmc) u7 {
+    return self.output_level;
+}
+
+// Tests
+
+test "$4010 decodes IRQ loop and rate" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.writeControl(0b1100_1010);
+
+    try std.testing.expect(dmc.irq_enabled);
+    try std.testing.expect(dmc.loop);
+    try std.testing.expectEqual(
+        @as(u4, 10),
+        dmc.rate_index,
+    );
+}
+
+test "clearing IRQ enable clears IRQ flag" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.irq_flag = true;
+
+    dmc.writeControl(0x00);
+
+    try std.testing.expect(!dmc.irq_flag);
+}
+
+test "$4011 directly sets output level" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.writeDirectLoad(0x55);
+
+    try std.testing.expectEqual(
+        @as(u7, 0x55),
+        dmc.output(),
+    );
+}
+
+test "$4011 ignores bit seven" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.writeDirectLoad(0xFF);
+
+    try std.testing.expectEqual(
+        @as(u7, 127),
+        dmc.output(),
+    );
+}
+
+test "$4012 sample address calculation" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.writeSampleAddress(0x00);
+
+    try std.testing.expectEqual(
+        @as(u16, 0xC000),
+        dmc.sample_address,
+    );
+
+    dmc.writeSampleAddress(0x01);
+
+    try std.testing.expectEqual(
+        @as(u16, 0xC040),
+        dmc.sample_address,
+    );
+
+    dmc.writeSampleAddress(0xFF);
+
+    try std.testing.expectEqual(
+        @as(u16, 0xFFC0),
+        dmc.sample_address,
+    );
+}
+
+test "$4013 sample length calculation" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.writeSampleLength(0x00);
+
+    try std.testing.expectEqual(
+        @as(u16, 1),
+        dmc.sample_length,
+    );
+
+    dmc.writeSampleLength(0x01);
+
+    try std.testing.expectEqual(
+        @as(u16, 17),
+        dmc.sample_length,
+    );
+
+    dmc.writeSampleLength(0xFF);
+
+    try std.testing.expectEqual(
+        @as(u16, 4081),
+        dmc.sample_length,
+    );
+}
+
+test "enabling starts sample if no bytes remain" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.writeSampleAddress(0x20);
+    dmc.writeSampleLength(0x02);
+
+    dmc.writeEnabled(true);
+
+    try std.testing.expectEqual(
+        dmc.sample_address,
+        dmc.current_address,
+    );
+
+    try std.testing.expectEqual(
+        dmc.sample_length,
+        dmc.bytes_remaining,
+    );
+}
+
+test "enabling does not restart active sample" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.current_address = 0xD123;
+    dmc.bytes_remaining = 5;
+
+    dmc.writeEnabled(true);
+
+    try std.testing.expectEqual(
+        @as(u16, 0xD123),
+        dmc.current_address,
+    );
+
+    try std.testing.expectEqual(
+        @as(u16, 5),
+        dmc.bytes_remaining,
+    );
+}
+
+test "disabling clears bytes remaining" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.bytes_remaining = 123;
+
+    dmc.writeEnabled(false);
+
+    try std.testing.expectEqual(
+        @as(u16, 0),
+        dmc.bytes_remaining,
+    );
+}
+
+test "disabling does not clear DAC output" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.output_level = 70;
+    dmc.bytes_remaining = 10;
+
+    dmc.writeEnabled(false);
+
+    try std.testing.expectEqual(
+        @as(u7, 70),
+        dmc.output(),
+    );
+}
+
+test "$4015 write clears DMC IRQ" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.irq_flag = true;
+
+    dmc.writeEnabled(true);
+
+    try std.testing.expect(!dmc.irq_flag);
+}
+
+test "active is based only on bytes remaining" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.bytes_remaining = 0;
+    dmc.sample_buffer = 0xAA;
+
+    try std.testing.expect(!dmc.active());
+
+    dmc.bytes_remaining = 1;
+
+    try std.testing.expect(dmc.active());
+}
+
+test "DMC requests DMA when buffer is empty" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.current_address = 0xC123;
+    dmc.bytes_remaining = 4;
+
+    try std.testing.expect(dmc.needsDma());
+
+    const address = dmc.beginDma();
+
+    try std.testing.expectEqual(
+        @as(?u16, 0xC123),
+        address,
+    );
+
+    try std.testing.expect(dmc.dma_pending);
+    try std.testing.expect(!dmc.needsDma());
+}
+
+test "DMA fills buffer and advances reader" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.current_address = 0xC123;
+    dmc.bytes_remaining = 4;
+
+    _ = dmc.beginDma();
+
+    dmc.completeDma(0xAB);
+
+    try std.testing.expectEqual(
+        @as(?u8, 0xAB),
+        dmc.sample_buffer,
+    );
+
+    try std.testing.expectEqual(
+        @as(u16, 0xC124),
+        dmc.current_address,
+    );
+
+    try std.testing.expectEqual(
+        @as(u16, 3),
+        dmc.bytes_remaining,
+    );
+
+    try std.testing.expect(!dmc.dma_pending);
+}
+
+test "DMA address wraps $FFFF to $8000" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.current_address = 0xFFFF;
+    dmc.bytes_remaining = 2;
+
+    _ = dmc.beginDma();
+    dmc.completeDma(0x00);
+
+    try std.testing.expectEqual(
+        @as(u16, 0x8000),
+        dmc.current_address,
+    );
+}
+
+test "final fetch raises IRQ" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.irq_enabled = true;
+    dmc.loop = false;
+
+    dmc.current_address = 0xC000;
+    dmc.bytes_remaining = 1;
+
+    _ = dmc.beginDma();
+    dmc.completeDma(0xAA);
+
+    try std.testing.expectEqual(
+        @as(u16, 0),
+        dmc.bytes_remaining,
+    );
+
+    try std.testing.expect(dmc.irq_flag);
+}
+
+test "loop restarts sample instead of raising IRQ" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.sample_address = 0xD000;
+    dmc.sample_length = 17;
+
+    dmc.current_address = 0xD010;
+    dmc.bytes_remaining = 1;
+
+    dmc.loop = true;
+    dmc.irq_enabled = true;
+
+    _ = dmc.beginDma();
+    dmc.completeDma(0xAA);
+
+    try std.testing.expectEqual(
+        @as(u16, 0xD000),
+        dmc.current_address,
+    );
+
+    try std.testing.expectEqual(
+        @as(u16, 17),
+        dmc.bytes_remaining,
+    );
+
+    try std.testing.expect(!dmc.irq_flag);
+}
+
+test "sample buffer prevents another DMA" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.bytes_remaining = 10;
+    dmc.sample_buffer = 0xAA;
+
+    try std.testing.expect(!dmc.needsDma());
+}
+
+test "NTSC rate table" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.rate_index = 0;
+    try std.testing.expectEqual(
+        @as(u16, 428),
+        dmc.timerPeriod(),
+    );
+
+    dmc.rate_index = 15;
+    try std.testing.expectEqual(
+        @as(u16, 54),
+        dmc.timerPeriod(),
+    );
+}
+
+test "PAL rate table" {
+    var dmc = Dmc.init(.pal);
+
+    dmc.rate_index = 0;
+    try std.testing.expectEqual(
+        @as(u16, 398),
+        dmc.timerPeriod(),
+    );
+
+    dmc.rate_index = 15;
+    try std.testing.expectEqual(
+        @as(u16, 50),
+        dmc.timerPeriod(),
+    );
+}
+
+test "delta one increases output by two" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.output_level = 50;
+    dmc.shift_register = 0b0000_0001;
+    dmc.bits_remaining = 8;
+    dmc.silence = false;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u7, 52),
+        dmc.output_level,
+    );
+}
+
+test "delta zero decreases output by two" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.output_level = 50;
+    dmc.shift_register = 0;
+    dmc.bits_remaining = 8;
+    dmc.silence = false;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u7, 48),
+        dmc.output_level,
+    );
+}
+
+test "delta increment does not overflow" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.output_level = 126;
+    dmc.shift_register = 1;
+    dmc.bits_remaining = 8;
+    dmc.silence = false;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u7, 126),
+        dmc.output_level,
+    );
+}
+
+test "delta decrement does not underflow" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.output_level = 1;
+    dmc.shift_register = 0;
+    dmc.bits_remaining = 8;
+    dmc.silence = false;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u7, 1),
+        dmc.output_level,
+    );
+}
+
+test "silent cycle does not change DAC" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.output_level = 50;
+    dmc.shift_register = 1;
+    dmc.bits_remaining = 8;
+    dmc.silence = true;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u7, 50),
+        dmc.output_level,
+    );
+}
+
+test "output shifter shifts right" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.shift_register = 0b1010_1011;
+    dmc.bits_remaining = 8;
+    dmc.silence = false;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u8, 0b0101_0101),
+        dmc.shift_register,
+    );
+}
+
+test "new output cycle consumes sample buffer" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.bits_remaining = 1;
+    dmc.sample_buffer = 0b1010_1010;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u4, 8),
+        dmc.bits_remaining,
+    );
+
+    try std.testing.expectEqual(
+        @as(u8, 0b1010_1010),
+        dmc.shift_register,
+    );
+
+    try std.testing.expectEqual(
+        @as(?u8, null),
+        dmc.sample_buffer,
+    );
+
+    try std.testing.expect(!dmc.silence);
+}
+
+test "new output cycle becomes silent when buffer empty" {
+    var dmc = Dmc.init(.ntsc);
+
+    dmc.bits_remaining = 1;
+    dmc.sample_buffer = null;
+    dmc.silence = false;
+
+    dmc.clockOutputUnit();
+
+    try std.testing.expectEqual(
+        @as(u4, 8),
+        dmc.bits_remaining,
+    );
+
+    try std.testing.expect(dmc.silence);
+}

src/system/nes/apu/envelope.zig

@@ -0,0 +1,132 @@
+const std = @import("std");
+
+const Envelope = @This();
+
+/// Bit 5: envelope loop / length-counter halt.
+loop: bool = false,
+
+/// Bit 4: select constant volume instead of decay level.
+constant_volume: bool = false,
+
+/// Bits 3-0.
+///
+/// When constant_volume is true this is the output volume.
+/// Otherwise this is the divider reload value.
+volume: u4 = 0,
+
+/// Set by writes to:
+///
+/// Pulse 1: $4003
+/// Pulse 2: $4007
+/// Noise:   $400F
+start: bool = false,
+
+/// Envelope divider counter.
+///
+/// Reloaded with `volume`.
+divider: u4 = 0,
+
+/// Current envelope volume, 0...15.
+decay_level: u4 = 0,
+
+pub fn init() Envelope {
+    return .{};
+}
+
+/// Write the envelope portion of:
+///
+/// $4000 - Pulse 1
+/// $4004 - Pulse 2
+/// $400C - Noise
+///
+/// Register:
+///
+///     --LC.VVVV
+///
+///     L = loop envelope / halt length counter
+///     C = constant volume
+///     V = volume / envelope divider period
+pub fn write(self: *Envelope, value: u8) void {
+    self.loop = (value & 0x20) != 0;
+    self.constant_volume = (value & 0x10) != 0;
+    self.volume = @truncate(value);
+}
+
+/// Writing $4003/$4007/$400F does not immediately restart the
+/// envelope.
+///
+/// Instead it sets this flag. The restart occurs at the next
+/// quarter-frame clock.
+pub fn restart(self: *Envelope) void {
+    self.start = true;
+}
+
+/// Clocked by the APU frame counter on every quarter-frame.
+pub fn clockQuarterFrame(self: *Envelope) void {
+    //
+    // If the start flag is set:
+    //
+    //   1. clear start
+    //   2. set decay to 15
+    //   3. reload divider
+    //
+    if (self.start) {
+        self.start = false;
+        self.decay_level = 15;
+        self.divider = self.volume;
+        return;
+    }
+
+    //
+    // Clock the divider.
+    //
+    if (self.divider != 0) {
+        self.divider -= 1;
+        return;
+    }
+
+    //
+    // Divider reached zero:
+    //
+    //   1. reload divider
+    //   2. clock decay counter
+    //
+    self.divider = self.volume;
+
+    if (self.decay_level != 0) {
+        self.decay_level -= 1;
+    } else if (self.loop) {
+        self.decay_level = 15;
+    }
+}
+
+/// Current 4-bit volume sent to the channel's output gate.
+pub fn output(self: *const Envelope) u4 {
+    return if (self.constant_volume)
+        self.volume
+    else
+        self.decay_level;
+}
+
+// consolidated envelope generator unit test suite
+test "envelope register write, quarter-frame clocking, decay and loop" {
+    var envelope = Envelope.init();
+    envelope.write(0b0010_1010); // loop=1, const_vol=0, vol=10
+    try std.testing.expect(envelope.loop and !envelope.constant_volume);
+
+    // Restart & quarter-frame clocking
+    envelope.write(0b0000_0010);
+    envelope.restart();
+    try std.testing.expect(envelope.start);
+    envelope.clockQuarterFrame();
+    try std.testing.expect(!envelope.start and envelope.decay_level == 15 and envelope.divider == 2);
+
+    // Decay countdown & loop
+    envelope.write(0b0010_0000); // loop enabled, vol=0
+    envelope.restart();
+    envelope.clockQuarterFrame();
+    for (0..15) |_| envelope.clockQuarterFrame();
+    try std.testing.expectEqual(@as(u4, 0), envelope.decay_level);
+    envelope.clockQuarterFrame();
+    try std.testing.expectEqual(@as(u4, 15), envelope.decay_level); // wrapped back
+}

src/system/nes/apu/frame_counter.zig

@@ -0,0 +1,237 @@
+const std = @import("std");
+const Region = @import("../common.zig").Region;
+
+pub const FrameCounter = @This();
+
+pub const Mode = enum {
+    four_step,
+    five_step,
+};
+
+pub const Events = struct {
+    quarter: bool = false,
+    half: bool = false,
+};
+
+region: Region,
+
+mode: Mode = .four_step,
+pending_mode: Mode = .four_step,
+
+irq_inhibit: bool = false,
+irq_flag: bool = false,
+irq_just_set: bool = false,
+
+/// CPU cycles since the current frame sequence started.
+cycle: u32 = 0,
+
+/// $4017 reset delay.
+///
+/// Hardware applies the reset after either 3 or 4 CPU cycles.
+reset_delay: u3 = 0,
+
+pub fn init(region: Region) FrameCounter {
+    return .{
+        .region = region,
+    };
+}
+
+/// Write $4017:
+///
+///     MI--.----
+///
+/// M = 5-step mode
+/// I = frame IRQ inhibit
+pub fn write(
+    self: *FrameCounter,
+    value: u8,
+    cpu_cycle: u64,
+) void {
+    self.pending_mode = if ((value & 0x80) != 0)
+        .five_step
+    else
+        .four_step;
+
+    self.irq_inhibit = (value & 0x40) != 0;
+
+    //
+    // Setting interrupt inhibit immediately clears frame IRQ.
+    //
+    if (self.irq_inhibit) {
+        self.irq_flag = false;
+    }
+
+    //
+    // $4017 reset occurs after 3 or 4 CPU cycles depending on
+    // APU clock phase.
+    //
+    // This assumes even cpu_cycle values are our APU-cycle phase.
+    //
+    // If your CPU core defines cycle parity oppositely, flip this.
+    //
+    self.reset_delay = if ((cpu_cycle & 1) == 0)
+        3
+    else
+        4;
+}
+
+pub fn clockCpu(self: *FrameCounter) Events {
+    self.irq_just_set = false;
+    var events = self.clockSequence();
+
+    //
+    // The existing sequence keeps running during the delayed
+    // $4017 reset.
+    //
+    if (self.reset_delay != 0) {
+        self.reset_delay -= 1;
+
+        if (self.reset_delay == 0) {
+            self.mode = self.pending_mode;
+            self.cycle = 0;
+
+            //
+            // Entering 5-step mode immediately generates both
+            // quarter-frame and half-frame clocks.
+            //
+            if (self.mode == .five_step) {
+                events.quarter = true;
+                events.half = true;
+            }
+        }
+    }
+
+    return events;
+}
+
+fn clockSequence(self: *FrameCounter) Events {
+    self.cycle += 1;
+
+    // NESdev timing constants (expressed in CPU cycles)
+    const Timing = struct {
+        s1: u32,
+        s2: u32,
+        s3: u32,
+        s4: u32,
+        s5: u32,
+        m0_irq1: u32,
+        m0_irq2: u32,
+        m0_reset: u32,
+        m1_reset: u32,
+    };
+
+    const t: Timing = switch (self.region) {
+        .ntsc => .{
+            .s1 = 7457,
+            .s2 = 14913,
+            .s3 = 22371,
+            .s4 = 29829,
+            .s5 = 37281,
+            .m0_irq1 = 29828,
+            .m0_irq2 = 29829,
+            .m0_reset = 29830,
+            .m1_reset = 37282,
+        },
+        .pal => .{
+            .s1 = 8313,
+            .s2 = 16627,
+            .s3 = 24939,
+            .s4 = 33253,
+            .s5 = 41565,
+            .m0_irq1 = 33252,
+            .m0_irq2 = 33253,
+            .m0_reset = 33254,
+            .m1_reset = 41566,
+        },
+    };
+
+    var events = Events{};
+
+    switch (self.mode) {
+        .four_step => {
+            if (self.cycle == t.s1) {
+                events.quarter = true;
+            } else if (self.cycle == t.s2) {
+                events.quarter = true;
+                events.half = true;
+            } else if (self.cycle == t.s3) {
+                events.quarter = true;
+            } else if (self.cycle == t.m0_irq1) {
+                if (!self.irq_inhibit) {
+                    self.irq_flag = true;
+                    self.irq_just_set = true;
+                }
+            } else if (self.cycle == t.m0_irq2) {
+                events.quarter = true;
+                events.half = true;
+                if (!self.irq_inhibit) {
+                    self.irq_flag = true;
+                    self.irq_just_set = true;
+                }
+            } else if (self.cycle >= t.m0_reset) {
+                if (!self.irq_inhibit) {
+                    self.irq_flag = true;
+                    self.irq_just_set = true;
+                }
+                self.cycle = 0;
+            }
+        },
+
+        .five_step => {
+            if (self.cycle == t.s1) {
+                events.quarter = true;
+            } else if (self.cycle == t.s2) {
+                events.quarter = true;
+                events.half = true;
+            } else if (self.cycle == t.s3) {
+                events.quarter = true;
+            } else if (self.cycle == t.s5) {
+                events.quarter = true;
+                events.half = true;
+            } else if (self.cycle >= t.m1_reset) {
+                self.cycle = 0;
+            }
+        },
+    }
+
+    return events;
+}
+
+pub fn reset(self: *FrameCounter) void {
+    self.mode = .four_step;
+    self.pending_mode = .four_step;
+    self.irq_inhibit = false;
+    self.irq_flag = false;
+    self.irq_just_set = false;
+    self.cycle = 0;
+    self.reset_delay = 0;
+}
+
+pub fn irqAsserted(self: *const FrameCounter) bool {
+    return self.irq_flag;
+}
+
+pub fn clearIrq(self: *FrameCounter) void {
+    // simultaneous read and IRQ set does not clear flag
+    if (!self.irq_just_set) {
+        self.irq_flag = false;
+    }
+}
+
+// consolidated frame counter unit test suite
+test "frame counter 4-step and 5-step mode timings, irq window and write delays" {
+    var fc = FrameCounter.init(.ntsc);
+
+    // 4-step mode timings and IRQ window
+    for (0..29828) |_| _ = fc.clockCpu();
+    try std.testing.expect(fc.irqAsserted());
+    fc.clearIrq(); // simultaneous set/read check
+    try std.testing.expect(fc.irqAsserted()); // remains set until next cycle
+
+    // 5-step mode
+    fc = FrameCounter.init(.ntsc);
+    fc.write(0x80, 0); // 5-step write
+    for (0..4) |_| _ = fc.clockCpu();
+    try std.testing.expect(!fc.irqAsserted());
+    try std.testing.expectEqual(Mode.five_step, fc.mode);
+}

src/system/nes/apu/length_counter.zig

@@ -0,0 +1,78 @@
+const std = @import("std");
+
+const LengthCounter = @This();
+
+pub const table = [32]u8{
+    10,  254, 20, 2,
+    40,  4,   80, 6,
+    160, 8,   60, 10,
+    14,  12,  26, 14,
+
+    12,  16,  24, 18,
+    48,  20,  96, 22,
+    192, 24,  72, 26,
+    16,  28,  32, 30,
+};
+
+enabled: bool = false,
+value: u8 = 0,
+
+pub fn init() LengthCounter {
+    return .{};
+}
+
+/// Corresponds to this channel's bit in $4015.
+///
+/// Clearing the enable bit immediately clears the length counter.
+/// Setting it does not reload the counter.
+pub fn setEnabled(self: *LengthCounter, enabled: bool) void {
+    self.enabled = enabled;
+
+    if (!enabled) {
+        self.value = 0;
+    }
+}
+
+/// Load from bits 7-3 of $4003/$4007.
+///
+/// The counter may only be loaded while the channel is enabled
+/// through $4015.
+pub fn load(self: *LengthCounter, index: u5) void {
+    if (!self.enabled)
+        return;
+
+    self.value = table[@intCast(index)];
+}
+
+/// Clocked on a half-frame.
+pub fn clock(self: *LengthCounter, halt: bool) void {
+    if (self.value != 0 and !halt) {
+        self.value -= 1;
+    }
+}
+
+pub fn active(self: *const LengthCounter) bool {
+    return self.value != 0;
+}
+
+// consolidated length counter unit test suite
+test "length counter enable, loading, immediate clearing, clocking and halt" {
+    try std.testing.expectEqual(@as(u8, 10), table[0]);
+
+    var counter = LengthCounter.init();
+    counter.load(0);
+    try std.testing.expectEqual(@as(u8, 0), counter.value);
+
+    counter.setEnabled(true);
+    counter.load(0);
+    try std.testing.expectEqual(@as(u8, 10), counter.value);
+
+    counter.clock(false); // decrement
+    try std.testing.expectEqual(@as(u8, 9), counter.value);
+
+    counter.clock(true); // halt prevents decrement
+    try std.testing.expectEqual(@as(u8, 9), counter.value);
+
+    counter.setEnabled(false); // clears immediately
+    try std.testing.expectEqual(@as(u8, 0), counter.value);
+}

src/system/nes/apu/noise.zig

@@ -0,0 +1,175 @@
+const std = @import("std");
+
+const Envelope = @import("envelope.zig");
+const LengthCounter = @import("length_counter.zig");
+const Region = @import("../common.zig").Region;
+
+pub const Noise = @This();
+
+const ntsc_periods = [16]u16{
+    4,   8,    16,   32,
+    64,  96,   128,  160,
+    202, 254,  380,  508,
+    762, 1016, 2034, 4068,
+};
+
+const pal_periods = [16]u16{
+    4,   8,   14,   30,
+    60,  88,  118,  148,
+    188, 236, 354,  472,
+    708, 944, 1890, 3778,
+};
+
+region: Region,
+
+envelope: Envelope = .{},
+length_counter: LengthCounter = .{},
+
+/// $400E bit 7.
+mode: bool = false,
+
+/// $400E bits 3..0.
+period_index: u4 = 0,
+
+/// CPU-cycle timer.
+timer_counter: u16 = 0,
+
+/// 15-bit LFSR.
+///
+/// Must not be initialized to zero or it will remain there forever.
+shift_register: u15 = 1,
+
+pub fn init(region: Region) Noise {
+    return .{
+        .region = region,
+    };
+}
+
+// $400C
+// --LC.VVVV
+
+pub fn writeControl(self: *Noise, value: u8) void {
+    self.envelope.write(value);
+}
+
+// $400E
+// M---.PPPP
+
+pub fn writePeriod(self: *Noise, value: u8) void {
+    self.mode = (value & 0x80) != 0;
+    self.period_index = @truncate(value & 0x0f);
+}
+
+// $400F
+// LLLL.L---
+
+pub fn writeLength(self: *Noise, value: u8) void {
+    const index: u5 = @truncate(value >> 3);
+
+    self.length_counter.load(index);
+    self.envelope.restart();
+}
+
+// $4015
+
+pub fn setEnabled(self: *Noise, enabled: bool) void {
+    self.length_counter.setEnabled(enabled);
+}
+
+pub fn active(self: *const Noise) bool {
+    return self.length_counter.active();
+}
+
+// Frame sequencer
+
+pub fn clockQuarterFrame(self: *Noise) void {
+    self.envelope.clockQuarterFrame();
+}
+
+pub fn clockHalfFrame(self: *Noise) void {
+    self.length_counter.clock(self.envelope.loop);
+}
+
+// Timer
+
+pub fn timerPeriod(self: *const Noise) u16 {
+    const index: usize = @intCast(self.period_index);
+
+    return switch (self.region) {
+        .ntsc => ntsc_periods[index],
+        .pal => pal_periods[index],
+    };
+}
+
+/// Called every CPU cycle.
+pub fn clockCpu(self: *Noise) void {
+    if (self.timer_counter == 0) {
+        self.timer_counter = self.timerPeriod() - 1;
+        self.clockShiftRegister();
+    } else {
+        self.timer_counter -= 1;
+    }
+}
+
+fn clockShiftRegister(self: *Noise) void {
+    const tap: u4 = if (self.mode) 6 else 1;
+
+    const feedback: u1 = @truncate(
+        (self.shift_register ^
+            (self.shift_register >> tap)) & 1,
+    );
+
+    self.shift_register >>= 1;
+    self.shift_register |= @as(u15, feedback) << 14;
+}
+
+// Output
+
+pub fn output(self: *const Noise) u4 {
+    if (!self.length_counter.active())
+        return 0;
+
+    //
+    // Noise channel is muted whenever LFSR bit 0 is 1.
+    //
+    if ((self.shift_register & 1) != 0)
+        return 0;
+
+    return self.envelope.output();
+}
+
+pub fn reset(self: *Noise) void {
+    self.shift_register = 1;
+    self.timer_counter = 0;
+    self.period_index = 0;
+    self.mode = false;
+    self.length_counter.setEnabled(false);
+}
+
+// consolidated noise unit test suite
+test "noise channel control, period, length and LFSR shift register" {
+    var noise = Noise.init(.ntsc);
+    noise.writeControl(0b0011_1010);
+    try std.testing.expect(noise.envelope.constant_volume);
+    try std.testing.expectEqual(@as(u4, 10), noise.envelope.volume);
+
+    noise.writePeriod(0b1000_0101); // Mode 1, period index 5
+    try std.testing.expect(noise.mode);
+    try std.testing.expectEqual(@as(u16, 96), noise.timerPeriod());
+
+    noise.setEnabled(true);
+    noise.writeLength(0b0000_1000);
+    try std.testing.expect(noise.length_counter.active());
+
+    // Output gating by bit 0
+    noise.shift_register = 1;
+    try std.testing.expectEqual(@as(u4, 0), noise.output());
+    noise.shift_register = 2;
+    try std.testing.expectEqual(@as(u4, 10), noise.output());
+
+    // Shift register mode 0 vs mode 1
+    noise.shift_register = 1;
+    noise.mode = false;
+    noise.clockShiftRegister();
+    try std.testing.expectEqual(@as(u15, 0x4000), noise.shift_register);
+}

src/system/nes/apu/pulse.zig

@@ -0,0 +1,301 @@
+const std = @import("std");
+
+const Envelope = @import("envelope.zig");
+const Sweep = @import("sweep.zig");
+const LengthCounter = @import("length_counter.zig");
+
+pub const Pulse = @This();
+pub const Channel = Sweep.Channel;
+
+/// Internal lookup table.
+///
+/// The hardware sequencer starts at index 0 and counts DOWN:
+///
+///     0, 7, 6, 5, 4, 3, 2, 1
+///
+/// Producing:
+///
+/// duty 0: 0 1 0 0 0 0 0 0
+/// duty 1: 0 1 1 0 0 0 0 0
+/// duty 2: 0 1 1 1 1 0 0 0
+/// duty 3: 1 0 0 1 1 1 1 1
+const duty_table = [4][8]u1{
+    .{ 0, 0, 0, 0, 0, 0, 0, 1 },
+    .{ 0, 0, 0, 0, 0, 0, 1, 1 },
+    .{ 0, 0, 0, 0, 1, 1, 1, 1 },
+    .{ 1, 1, 1, 1, 1, 1, 0, 0 },
+};
+
+channel: Channel,
+
+envelope: Envelope = .{},
+sweep: Sweep,
+length_counter: LengthCounter = .{},
+
+/// DD from $4000/$4004.
+duty: u2 = 0,
+
+/// Internal 8-step sequencer position.
+///
+/// Counts downward.
+sequence_position: u3 = 0,
+
+/// 11-bit period from:
+///
+///     HHHLLLLLLLL
+///
+/// We intentionally keep this as u16 because the sweep target
+/// calculation needs to detect values > $7FF.
+timer_period: u16 = 0,
+
+/// Current timer divider value.
+///
+/// This is separate from timer_period because writing $4003/$4007
+/// does NOT reset this divider.
+timer_counter: u16 = 0,
+
+pub fn init(channel: Channel) Pulse {
+    return .{
+        .channel = channel,
+        .sweep = Sweep.init(channel),
+    };
+}
+
+// $4015
+
+/// Set this pulse channel's enable bit from $4015.
+pub fn setEnabled(self: *Pulse, enabled: bool) void {
+    self.length_counter.setEnabled(enabled);
+}
+
+/// Used when constructing $4015 reads.
+pub fn active(self: *const Pulse) bool {
+    return self.length_counter.active();
+}
+
+// $4000 / $4004
+// DDLC.VVVV
+
+pub fn writeControl(self: *Pulse, value: u8) void {
+    self.duty = @truncate(value >> 6);
+
+    self.envelope.write(value);
+
+    // Important:
+    //
+    // Changing duty does NOT reset sequence_position.
+}
+
+// $4001 / $4005
+// EPPP.NSSS
+
+pub fn writeSweep(self: *Pulse, value: u8) void {
+    self.sweep.write(value);
+}
+
+// $4002 / $4006
+// LLLL.LLLL
+
+pub fn writeTimerLow(self: *Pulse, value: u8) void {
+    self.timer_period =
+        (self.timer_period & 0x0700) |
+        @as(u16, value);
+}
+
+// $4003 / $4007
+// LLLL.LHHH
+
+pub fn writeTimerHigh(self: *Pulse, value: u8) void {
+    //
+    // Replace high 3 timer bits.
+    //
+    self.timer_period =
+        (self.timer_period & 0x00ff) |
+        (@as(u16, value & 0x07) << 8);
+
+    //
+    // Load length counter from bits 7-3.
+    //
+    // LengthCounter itself checks whether $4015 has enabled
+    // this channel.
+    //
+    const length_index: u5 = @truncate(value >> 3);
+    self.length_counter.load(length_index);
+
+    //
+    // Restart waveform sequencer immediately.
+    //
+    self.sequence_position = 0;
+
+    //
+    // Restart envelope on next quarter-frame clock.
+    //
+    self.envelope.restart();
+
+    //
+    // IMPORTANT:
+    //
+    // Do NOT modify timer_counter here.
+    //
+    // $4003/$4007 reset waveform phase but don't reset the
+    // period divider.
+    //
+}
+
+// Timer
+
+/// Clock once per APU cycle.
+///
+/// For the NES pulse channels:
+///
+///     1 APU cycle = 2 CPU cycles
+///
+/// This means the caller should invoke this every second CPU cycle.
+pub fn clockTimer(self: *Pulse) void {
+    if (self.timer_counter == 0) {
+        //
+        // Timer runs:
+        //
+        //     t, t-1, ... 1, 0
+        //
+        // and then reloads with t.
+        //
+        self.timer_counter = self.timer_period;
+
+        //
+        // Hardware sequencer counts downward.
+        //
+        self.sequence_position -%= 1;
+    } else {
+        self.timer_counter -= 1;
+    }
+}
+
+// Frame sequencer
+//
+/// Quarter-frame clock:
+///
+/// Pulse channels clock their envelopes.
+pub fn clockQuarterFrame(self: *Pulse) void {
+    self.envelope.clockQuarterFrame();
+}
+
+/// Half-frame clock:
+///
+/// Pulse channels clock:
+///
+/// - sweep
+/// - length counter
+pub fn clockHalfFrame(self: *Pulse) void {
+    self.sweep.clockHalfFrame(&self.timer_period);
+
+    //
+    // $4000/$4004 bit 5 serves BOTH purposes:
+    //
+    //     envelope loop
+    //     length-counter halt
+    //
+    self.length_counter.clock(self.envelope.loop);
+}
+
+// Output
+fn sequencerOutput(self: *const Pulse) u1 {
+    const duty: usize = @intCast(self.duty);
+    const position: usize = @intCast(self.sequence_position);
+
+    return duty_table[duty][position];
+}
+
+/// Current raw pulse-channel output: 0...15.
+///
+/// This is the value that should be passed to the APU pulse mixer.
+pub fn output(self: *const Pulse) u4 {
+    //
+    // Length-counter gate.
+    //
+    if (!self.length_counter.active())
+        return 0;
+
+    //
+    // Sweep gate.
+    //
+    // Covers:
+    //
+    //     timer_period < 8
+    //     target_period > $7FF
+    //
+    if (self.sweep.isMuted(self.timer_period))
+        return 0;
+
+    //
+    // Duty sequencer gate.
+    //
+    if (self.sequencerOutput() == 0)
+        return 0;
+
+    //
+    // Everything is allowing output, so the current envelope
+    // volume reaches the mixer.
+    //
+    return self.envelope.output();
+}
+
+// Debug / inspection
+pub fn timerPeriod(self: *const Pulse) u16 {
+    return self.timer_period;
+}
+
+pub fn length(self: *const Pulse) u8 {
+    return self.length_counter.value;
+}
+
+pub fn sequencePosition(self: *const Pulse) u3 {
+    return self.sequence_position;
+}
+
+// consolidated pulse unit test suite
+test "pulse channel register writes, timing, duty sequences and output gating" {
+    var pulse = Pulse.init(.pulse1);
+
+    // Period low and high writes
+    pulse.timer_period = 0x500;
+    pulse.writeTimerLow(0xab);
+    try std.testing.expectEqual(@as(u16, 0x5ab), pulse.timer_period);
+    pulse.writeTimerHigh(0x05);
+    try std.testing.expectEqual(@as(u16, 0x5ab), pulse.timer_period);
+    try std.testing.expectEqual(@as(u3, 0), pulse.sequence_position);
+    try std.testing.expect(pulse.envelope.start);
+
+    // Length loading and clearing
+    pulse.setEnabled(true);
+    pulse.writeTimerHigh(0x00);
+    try std.testing.expectEqual(@as(u8, 10), pulse.length_counter.value);
+    pulse.setEnabled(false);
+    try std.testing.expectEqual(@as(u8, 0), pulse.length_counter.value);
+
+    // Duty sequences (0..3)
+    const expected_sequences = [4][8]u1{
+        .{ 0, 1, 0, 0, 0, 0, 0, 0 },
+        .{ 0, 1, 1, 0, 0, 0, 0, 0 },
+        .{ 0, 1, 1, 1, 1, 0, 0, 0 },
+        .{ 1, 0, 0, 1, 1, 1, 1, 1 },
+    };
+    const positions = [8]u3{ 0, 7, 6, 5, 4, 3, 2, 1 };
+    for (expected_sequences, 0..) |expected, duty_idx| {
+        pulse.duty = @truncate(duty_idx);
+        for (positions, expected) |pos, exp| {
+            pulse.sequence_position = pos;
+            try std.testing.expectEqual(exp, pulse.sequencerOutput());
+        }
+    }
+
+    // Output gating checks (zero length, timer < 8, envelope volume)
+    pulse.writeControl(0b0011_1010);
+    pulse.length_counter.value = 10;
+    pulse.timer_period = 100;
+    pulse.sequence_position = 7;
+    try std.testing.expectEqual(@as(u4, 10), pulse.output());
+
+    pulse.timer_period = 7;
+    try std.testing.expectEqual(@as(u4, 0), pulse.output());
+}

src/system/nes/apu/root.zig

@@ -0,0 +1,493 @@
+const std = @import("std");
+
+pub const common = @import("../common.zig");
+pub const Region = common.Region;
+
+const Pulse = @import("pulse.zig");
+const Triangle = @import("triangle.zig");
+const Noise = @import("noise.zig");
+const Dmc = @import("dmc.zig");
+const FrameCounter = @import("frame_counter.zig");
+
+pub const Apu = @This();
+
+pub const sample_rate: u32 = 44_100;
+
+region: Region,
+
+pulse1: Pulse,
+pulse2: Pulse,
+
+triangle: Triangle,
+noise: Noise,
+dmc: Dmc,
+
+frame_counter: FrameCounter,
+
+/// Absolute CPU cycle as seen by the APU.
+cpu_cycle: u64 = 0,
+
+// Downsampling buffer for audio output
+sample_buffer: [2048]i16 = [_]i16{0} ** 2048,
+sample_count: usize = 0,
+sample_accumulator: f32 = 0.0,
+sample_cycles: u32 = 0,
+prev_input: f32 = 0.0,
+prev_output: f32 = 0.0,
+
+pub fn init(region: Region) Apu {
+    return .{
+        .region = region,
+
+        .pulse1 = Pulse.init(.pulse1),
+        .pulse2 = Pulse.init(.pulse2),
+
+        .triangle = Triangle.init(),
+
+        .noise = Noise.init(
+            switch (region) {
+                .ntsc => .ntsc,
+                .pal => .pal,
+            },
+        ),
+
+        .dmc = Dmc.init(
+            switch (region) {
+                .ntsc => .ntsc,
+                .pal => .pal,
+            },
+        ),
+
+        .frame_counter = FrameCounter.init(
+            switch (region) {
+                .ntsc => .ntsc,
+                .pal => .pal,
+            },
+        ),
+    };
+}
+
+pub fn reset(self: *Apu) void {
+    // reset silences all channels and clears APU state
+    self.writeStatus(0);
+    self.frame_counter.reset();
+    self.noise.reset();
+    self.dmc.writeDirectLoad(0);
+    self.cpu_cycle = 0;
+    self.sample_count = 0;
+    self.sample_accumulator = 0.0;
+    self.sample_cycles = 0;
+    self.prev_input = 0.0;
+    self.prev_output = 0.0;
+}
+
+// CPU register interface
+
+pub fn write(
+    self: *Apu,
+    address: u16,
+    value: u8,
+) void {
+    switch (address) {
+        // Pulse 1
+
+        0x4000 => self.pulse1.writeControl(value),
+        0x4001 => self.pulse1.writeSweep(value),
+        0x4002 => self.pulse1.writeTimerLow(value),
+        0x4003 => self.pulse1.writeTimerHigh(value),
+
+        // Pulse 2
+
+        0x4004 => self.pulse2.writeControl(value),
+        0x4005 => self.pulse2.writeSweep(value),
+        0x4006 => self.pulse2.writeTimerLow(value),
+        0x4007 => self.pulse2.writeTimerHigh(value),
+
+        // Triangle
+
+        0x4008 => self.triangle.writeLinear(value),
+
+        // $4009 unused
+        0x4009 => {},
+
+        0x400A => self.triangle.writeTimerLow(value),
+        0x400B => self.triangle.writeTimerHigh(value),
+
+        // Noise
+
+        0x400C => self.noise.writeControl(value),
+
+        // $400D unused
+        0x400D => {},
+
+        0x400E => self.noise.writePeriod(value),
+        0x400F => self.noise.writeLength(value),
+
+        // DMC
+
+        0x4010 => self.dmc.writeControl(value),
+        0x4011 => self.dmc.writeDirectLoad(value),
+        0x4012 => self.dmc.writeSampleAddress(value),
+        0x4013 => self.dmc.writeSampleLength(value),
+
+        // $4014 is OAM DMA.
+        //
+        // That belongs to the CPU/PPU DMA controller, NOT the APU.
+
+        0x4014 => {},
+
+        // Status
+
+        0x4015 => self.writeStatus(value),
+
+        // $4016 joypad
+
+        0x4016 => {},
+
+        // Frame counter
+
+        0x4017 => self.frame_counter.write(
+            value,
+            self.cpu_cycle,
+        ),
+
+        else => {},
+    }
+}
+
+/// Only $4015 is actually an APU-readable register.
+///
+/// null means:
+///
+///     "APU does not drive the CPU data bus here."
+///
+/// This lets your Bus preserve open-bus behavior instead of the
+/// APU incorrectly returning 0.
+pub fn read(self: *Apu, address: u16) ?u8 {
+    return self.readWithOpenBus(address, 0);
+}
+
+pub fn readWithOpenBus(self: *Apu, address: u16, open_bus: u8) ?u8 {
+    return switch (address) {
+        0x4015 => self.readStatus(open_bus),
+        else => null,
+    };
+}
+
+// $4015
+
+fn writeStatus(self: *Apu, value: u8) void {
+    self.pulse1.setEnabled((value & 0x01) != 0);
+    self.pulse2.setEnabled((value & 0x02) != 0);
+
+    self.triangle.setEnabled((value & 0x04) != 0);
+    self.noise.setEnabled((value & 0x08) != 0);
+
+    self.dmc.writeEnabled((value & 0x10) != 0);
+}
+
+fn readStatus(self: *Apu, open_bus: u8) u8 {
+    // Bit 5 is unmapped on the APU and returns open bus from the data bus.
+    var result: u8 = open_bus & 0x20;
+
+    // Pulse 1 length counter.
+    if (self.pulse1.active())
+        result |= 0x01;
+
+    // Pulse 2 length counter.
+    if (self.pulse2.active())
+        result |= 0x02;
+
+    // Triangle length counter.
+    if (self.triangle.active())
+        result |= 0x04;
+
+    // Noise length counter.
+    if (self.noise.active())
+        result |= 0x08;
+
+    // DMC bytes remaining.
+    if (self.dmc.active())
+        result |= 0x10;
+
+    // Frame IRQ.
+    if (self.frame_counter.irqAsserted())
+        result |= 0x40;
+
+    // DMC IRQ.
+    if (self.dmc.irqAsserted())
+        result |= 0x80;
+
+    //
+    // Reading $4015 acknowledges ONLY frame IRQ.
+    //
+    // DMC IRQ remains asserted.
+    //
+    self.frame_counter.clearIrq();
+
+    return result;
+}
+
+// IRQ
+
+/// APU's contribution to the CPU IRQ line.
+///
+/// Your NES/CPU layer should OR this with mapper IRQs.
+pub fn irqAsserted(self: *const Apu) bool {
+    return self.frame_counter.irqAsserted() or
+        self.dmc.irqAsserted();
+}
+
+// Main CPU clock
+
+/// Clock once for EVERY CPU cycle.
+///
+/// This should be your only ordinary clock entrypoint into the APU.
+pub fn clockCpu(self: *Apu) void {
+    // Triangle
+    //
+    // Timer runs every CPU cycle.
+
+    self.triangle.clockTimer();
+
+    // Noise
+    //
+    // Our Noise implementation uses the NESdev table expressed
+    // directly in CPU cycles.
+
+    self.noise.clockCpu();
+
+    // DMC
+    //
+    // Our DMC rate table is also expressed directly in CPU cycles.
+
+    self.dmc.clockTimer();
+
+    // Pulse timers
+    //
+    // Pulse timer period is expressed in APU cycles.
+    //
+    // One APU cycle = two CPU cycles.
+
+    if ((self.cpu_cycle & 1) == 0) {
+        self.pulse1.clockTimer();
+        self.pulse2.clockTimer();
+    }
+
+    // Frame sequencer
+
+    const events = self.frame_counter.clockCpu();
+
+    if (events.quarter) {
+        self.clockQuarterFrame();
+    }
+
+    if (events.half) {
+        self.clockHalfFrame();
+    }
+
+    // audio downsampler (~40.58 CPU cycles per 44.1kHz sample) with 1st-order DC blocker
+    self.sample_accumulator += self.output();
+    self.sample_cycles += 1;
+    if (self.sample_cycles >= 40) {
+        if (self.sample_count < self.sample_buffer.len) {
+            const avg = self.sample_accumulator / @as(f32, @floatFromInt(self.sample_cycles));
+            // 1st-order IIR DC blocker filter: y[n] = x[n] - x[n-1] + 0.995 * y[n-1]
+            const filtered = avg - self.prev_input + (0.995 * self.prev_output);
+            self.prev_input = avg;
+            self.prev_output = filtered;
+
+            const sample_f = filtered * 32000.0 * 2.5;
+            self.sample_buffer[self.sample_count] = @intFromFloat(std.math.clamp(sample_f, -32767.0, 32767.0));
+            self.sample_count += 1;
+        }
+        self.sample_accumulator = 0.0;
+        self.sample_cycles = 0;
+    }
+
+    self.cpu_cycle +%= 1;
+}
+
+pub fn getSamples(self: *const Apu) []const i16 {
+    return self.sample_buffer[0..self.sample_count];
+}
+
+pub fn clearSamples(self: *Apu) void {
+    self.sample_count = 0;
+}
+
+// Quarter-frame
+
+fn clockQuarterFrame(self: *Apu) void {
+    //
+    // Pulse envelopes
+    //
+    self.pulse1.clockQuarterFrame();
+    self.pulse2.clockQuarterFrame();
+
+    //
+    // Triangle linear counter
+    //
+    self.triangle.clockQuarterFrame();
+
+    //
+    // Noise envelope
+    //
+    self.noise.clockQuarterFrame();
+
+    //
+    // DMC has no frame-sequencer-controlled unit.
+    //
+}
+
+// Half-frame
+
+fn clockHalfFrame(self: *Apu) void {
+    //
+    // Pulse:
+    //
+    // - sweep
+    // - length
+    //
+    self.pulse1.clockHalfFrame();
+    self.pulse2.clockHalfFrame();
+
+    //
+    // Triangle length
+    //
+    self.triangle.clockHalfFrame();
+
+    //
+    // Noise length
+    //
+    self.noise.clockHalfFrame();
+
+    //
+    // DMC again has no half-frame unit.
+    //
+}
+
+// DMC DMA
+
+/// Does the DMC memory reader currently need another byte?
+pub fn needsDmcDma(self: *const Apu) bool {
+    return self.dmc.needsDma();
+}
+
+/// Begin a DMC DMA operation.
+///
+/// Returns the CPU address that should eventually be read.
+pub fn beginDmcDma(self: *Apu) ?u16 {
+    return self.dmc.beginDma();
+}
+
+/// Deliver the result of the CPU DMA read.
+pub fn completeDmcDma(
+    self: *Apu,
+    value: u8,
+) void {
+    self.dmc.completeDma(value);
+}
+
+// Individual mixer inputs
+
+pub fn pulse1Output(self: *const Apu) u4 {
+    return self.pulse1.output();
+}
+
+pub fn pulse2Output(self: *const Apu) u4 {
+    return self.pulse2.output();
+}
+
+pub fn triangleOutput(self: *const Apu) u4 {
+    return self.triangle.output();
+}
+
+pub fn noiseOutput(self: *const Apu) u4 {
+    return self.noise.output();
+}
+
+pub fn dmcOutput(self: *const Apu) u7 {
+    return self.dmc.output();
+}
+
+// Nonlinear NES mixer
+
+/// Raw nonlinear APU output.
+///
+/// Nominal range is roughly 0...1.
+///
+/// This does NOT implement the analog high-pass / low-pass filter
+/// chain following the NES DAC.
+pub fn output(self: *const Apu) f32 {
+    const p1: f32 =
+        @floatFromInt(self.pulse1.output());
+
+    const p2: f32 =
+        @floatFromInt(self.pulse2.output());
+
+    const triangle: f32 =
+        @floatFromInt(self.triangle.output());
+
+    const noise: f32 =
+        @floatFromInt(self.noise.output());
+
+    const dmc: f32 =
+        @floatFromInt(self.dmc.output());
+
+    // Pulse mixer
+
+    const pulse_sum = p1 + p2;
+
+    const pulse_out: f32 =
+        if (pulse_sum == 0.0)
+            0.0
+        else
+            95.88 /
+                ((8128.0 / pulse_sum) + 100.0);
+
+    // Triangle / Noise / DMC mixer
+
+    const tnd_input =
+        (triangle / 8227.0) +
+        (noise / 12241.0) +
+        (dmc / 22638.0);
+
+    const tnd_out: f32 =
+        if (tnd_input == 0.0)
+            0.0
+        else
+            159.79 /
+                ((1.0 / tnd_input) + 100.0);
+
+    return pulse_out + tnd_out;
+}
+
+// consolidated APU root test suite
+test "apu initialization, status reads, IRQ clearing, sample generation and DMC DMA" {
+    var apu = Apu.init(.ntsc);
+
+    // Init, reset & channel status
+    apu.write(0x4015, 0x0d);
+    apu.write(0x4003, 0x08);
+    apu.write(0x400b, 0x08);
+    apu.write(0x400f, 0x08);
+    const status = apu.readWithOpenBus(0x4015, 0x20);
+    try std.testing.expectEqual(@as(u8, 0b0010_1101), status.?);
+
+    apu.reset();
+    try std.testing.expect(!apu.pulse1.active() and apu.cpu_cycle == 0);
+
+    // Frame/DMC IRQ clearing
+    apu.frame_counter.irq_flag = true;
+    apu.dmc.irq_flag = true;
+    _ = apu.read(0x4015);
+    try std.testing.expect(!apu.frame_counter.irqAsserted() and apu.dmc.irqAsserted());
+
+    // Audio sampling & DMC DMA
+    apu.write(0x4015, 0x11);
+    for (0..200) |_| apu.clockCpu();
+    try std.testing.expect(apu.getSamples().len > 0);
+    try std.testing.expect(apu.needsDmcDma());
+    try std.testing.expectEqual(@as(?u16, 0xc000), apu.beginDmcDma());
+}

src/system/nes/apu/sweep.zig

@@ -0,0 +1,171 @@
+const std = @import("std");
+
+const Sweep = @This();
+
+pub const Channel = enum {
+    pulse1,
+    pulse2,
+};
+
+channel: Channel,
+
+enabled: bool = false,
+
+/// Raw P value from EPPP.NSSS.
+///
+/// Hardware clocks the sweep every P + 1 half-frame clocks because
+/// the divider counts:
+///
+///     P, P-1, ..., 0
+///
+period: u3 = 0,
+
+negate: bool = false,
+shift: u3 = 0,
+
+/// Current divider counter.
+divider: u3 = 0,
+
+/// Set whenever $4001/$4005 is written.
+reload: bool = false,
+
+pub fn init(channel: Channel) Sweep {
+    return .{
+        .channel = channel,
+    };
+}
+
+/// Write $4001 or $4005:
+///
+///     EPPP.NSSS
+///     ││││ │└┴┴─ shift
+///     ││││ └──── negate
+///     │└┴┴────── divider period
+///     └───────── enabled
+pub fn write(self: *Sweep, value: u8) void {
+    self.enabled = (value & 0x80) != 0;
+    self.period = @truncate((value >> 4) & 0x07);
+    self.negate = (value & 0x08) != 0;
+    self.shift = @truncate(value & 0x07);
+
+    self.reload = true;
+}
+
+/// Calculate the continuously-evaluated sweep target period.
+///
+/// timer_period is the pulse channel's current 11-bit timer period.
+///
+/// This returns u16 rather than an 11-bit type intentionally:
+/// values > 0x7FF must remain visible so the sweep muting logic
+/// can detect overflow.
+pub fn targetPeriod(self: *const Sweep, timer_period: u16) u16 {
+    std.debug.assert(timer_period <= 0x07ff);
+
+    const change = timer_period >> @intCast(self.shift);
+
+    if (!self.negate) {
+        return timer_period + change;
+    }
+
+    return switch (self.channel) {
+        // Pulse 1 uses one's-complement negation:
+        //
+        //     period + ~change
+        //
+        // Equivalent mathematically to:
+        //
+        //     period - change - 1
+        //
+        // Clamp a negative target to zero.
+        .pulse1 => if (timer_period <= change)
+            0
+        else
+            timer_period - change - 1,
+
+        // Pulse 2 uses ordinary two's-complement subtraction:
+        //
+        //     period - change
+        .pulse2 => timer_period - change,
+    };
+}
+
+/// The sweep can mute a pulse channel even when sweep updating itself
+/// is disabled.
+///
+/// A pulse is muted when:
+///
+///   current period < 8
+///
+/// OR
+///
+///   target period > 0x7FF
+pub fn isMuted(self: *const Sweep, timer_period: u16) bool {
+    const target = self.targetPeriod(timer_period);
+
+    return timer_period < 8 or target > 0x07ff;
+}
+
+/// Clocked by the frame sequencer on a half-frame clock.
+///
+/// This potentially updates the pulse channel's timer period and
+/// advances/reloads the sweep divider.
+pub fn clockHalfFrame(
+    self: *Sweep,
+    timer_period: *u16,
+) void {
+    std.debug.assert(timer_period.* <= 0x07ff);
+
+    // The target must be evaluated BEFORE potentially updating
+    // timer_period.
+    const target = self.targetPeriod(timer_period.*);
+    const muted = timer_period.* < 8 or target > 0x07ff;
+
+    const divider_zero = self.divider == 0;
+
+    // If the divider emits a clock, apply the sweep.
+    //
+    // shift == 0 disables period updates, even if enabled == true.
+    if (divider_zero and
+        self.enabled and
+        self.shift != 0 and
+        !muted)
+    {
+        timer_period.* = target;
+    }
+
+    // Divider operation happens regardless of whether the timer
+    // was actually changed.
+    if (divider_zero or self.reload) {
+        self.divider = self.period;
+        self.reload = false;
+    } else {
+        self.divider -= 1;
+    }
+}
+
+// consolidated sweep unit test suite
+test "sweep unit target calculation, gating and clocking" {
+    var sweep1 = Sweep.init(.pulse1);
+    sweep1.write(0b1010_0001); // P=2, shift=1
+    try std.testing.expectEqual(@as(u16, 1500), sweep1.targetPeriod(1000));
+
+    // Pulse 1 ones complement vs Pulse 2 twos complement negation
+    sweep1.write(0b1000_1010); // negate, shift=2
+    try std.testing.expectEqual(@as(u16, 749), sweep1.targetPeriod(1000));
+
+    var sweep2 = Sweep.init(.pulse2);
+    sweep2.write(0b1000_1010);
+    try std.testing.expectEqual(@as(u16, 750), sweep2.targetPeriod(1000));
+
+    // Muting checks
+    try std.testing.expect(sweep1.isMuted(7));
+    try std.testing.expect(!sweep1.isMuted(8));
+    sweep1.write(0b0000_0000);
+    try std.testing.expect(sweep1.isMuted(0x400)); // overflow mutes
+
+    // Clocking & reload
+    var timer_period: u16 = 1000;
+    sweep2.write(0b1010_0001);
+    sweep2.clockHalfFrame(&timer_period);
+    try std.testing.expectEqual(@as(u16, 1500), timer_period);
+}

src/system/nes/apu/triangle.zig

@@ -0,0 +1,672 @@
+const std = @import("std");
+
+const LengthCounter = @import("length_counter.zig");
+
+pub const Triangle = @This();
+/// 32-step triangle DAC sequence.
+///
+/// Unlike pulse, the triangle directly sends one of these
+/// 4-bit values to the mixer.
+const sequence = [32]u4{
+    15, 14, 13, 12, 11, 10, 9,  8,
+    7,  6,  5,  4,  3,  2,  1,  0,
+
+    0,  1,  2,  3,  4,  5,  6,  7,
+    8,  9,  10, 11, 12, 13, 14, 15,
+};
+
+length_counter: LengthCounter = .{},
+
+// $4008
+// CRRR.RRRR
+
+/// Bit 7 of $4008.
+///
+/// This has two functions:
+///
+/// - halts the length counter
+/// - controls whether linear_counter_reload remains set
+control: bool = false,
+
+/// Bits 6..0 of $4008.
+linear_reload_value: u7 = 0,
+
+/// Current linear counter.
+linear_counter: u7 = 0,
+
+/// Set by every write to $400B.
+linear_reload: bool = false,
+
+// Timer
+
+/// 11-bit timer period:
+///
+///     HHHLLLLLLLL
+timer_period: u16 = 0,
+
+/// Internal timer divider.
+///
+/// Triangle timer runs at the CPU clock, unlike pulse timers
+/// which run at CPU / 2.
+timer_counter: u16 = 0,
+
+// Sequencer
+
+/// Current position in the 32-step waveform.
+sequence_position: u5 = 0,
+
+pub fn init() Triangle {
+    return .{};
+}
+
+// $4015
+
+/// Triangle is bit 2 of $4015.
+///
+/// Clearing it immediately clears the length counter.
+///
+/// Importantly, this does NOT reset the sequencer or linear counter.
+pub fn setEnabled(self: *Triangle, enabled: bool) void {
+    self.length_counter.setEnabled(enabled);
+}
+
+/// Status returned as bit 2 when reading $4015.
+///
+/// $4015 status reports whether the length counter is non-zero.
+pub fn active(self: *const Triangle) bool {
+    return self.length_counter.active();
+}
+
+// $4008
+// CRRR.RRRR
+//
+// C = control / length-counter halt
+// R = linear-counter reload value
+
+pub fn writeLinear(self: *Triangle, value: u8) void {
+    self.control = (value & 0x80) != 0;
+    self.linear_reload_value = @truncate(value & 0x7f);
+
+    //
+    // IMPORTANT:
+    //
+    // Writing $4008 itself does NOT set linear_reload.
+    //
+    // $400B does that.
+    //
+}
+
+// $400A
+// LLLL.LLLL
+
+pub fn writeTimerLow(self: *Triangle, value: u8) void {
+    self.timer_period =
+        (self.timer_period & 0x0700) |
+        @as(u16, value);
+}
+
+// $400B
+// LLLL.LHHH
+//
+// bits 7..3 = length counter index
+// bits 2..0 = high timer bits
+//
+// Side effect:
+//     set linear-counter reload flag
+
+pub fn writeTimerHigh(self: *Triangle, value: u8) void {
+    self.timer_period =
+        (self.timer_period & 0x00ff) |
+        (@as(u16, value & 0x07) << 8);
+
+    const length_index: u5 = @truncate(value >> 3);
+    self.length_counter.load(length_index);
+
+    //
+    // Every $400B write sets the reload flag.
+    //
+    self.linear_reload = true;
+
+    //
+    // Unlike pulse $4003/$4007:
+    //
+    // DO NOT reset sequence_position.
+    //
+    // DO NOT reset timer_counter.
+    //
+}
+
+// $4009
+
+/// $4009 is unused.
+pub fn writeUnused(_: *Triangle, _: u8) void {}
+
+// Linear counter
+
+/// Clocked on every quarter-frame.
+///
+/// NES behavior, in this exact order:
+///
+/// 1. If reload flag set:
+///        counter = reload value
+///
+///    Otherwise, if counter > 0:
+///        counter--
+///
+/// 2. If control flag clear:
+///        reload flag = false
+///
+pub fn clockQuarterFrame(self: *Triangle) void {
+    if (self.linear_reload) {
+        self.linear_counter = self.linear_reload_value;
+    } else if (self.linear_counter != 0) {
+        self.linear_counter -= 1;
+    }
+
+    if (!self.control) {
+        self.linear_reload = false;
+    }
+}
+
+// Length counter
+
+/// Clocked on every half-frame.
+pub fn clockHalfFrame(self: *Triangle) void {
+    //
+    // $4008 bit 7 is also the length-counter halt flag.
+    //
+    self.length_counter.clock(self.control);
+}
+
+// Timer / waveform sequencer
+
+/// Clock EVERY CPU cycle.
+///
+/// This differs from Pulse:
+///
+///     Pulse timer    -> CPU / 2
+///     Triangle timer -> CPU
+///
+pub fn clockTimer(self: *Triangle) void {
+    if (self.timer_counter == 0) {
+        self.timer_counter = self.timer_period;
+
+        //
+        // The waveform advances only when BOTH counters are non-zero.
+        //
+        if (self.linear_counter != 0 and
+            self.length_counter.active())
+        {
+            self.sequence_position +%= 1;
+        }
+    } else {
+        self.timer_counter -= 1;
+    }
+}
+
+// Output
+
+/// Current 4-bit DAC output.
+///
+/// IMPORTANT:
+///
+/// Do NOT do this:
+///
+///     if (linear_counter == 0) return 0;
+///
+/// and do NOT do:
+///
+///     if (length_counter == 0) return 0;
+///
+/// Those counters stop the waveform sequencer. They do not force
+/// the triangle DAC to zero.
+///
+/// When halted, the channel retains its current sequence value.
+pub fn output(self: *const Triangle) u4 {
+    return sequence[@intCast(self.sequence_position)];
+}
+
+pub fn sequencerRunning(self: *const Triangle) bool {
+    return self.linear_counter != 0 and
+        self.length_counter.active();
+}
+
+// Inspection helpers
+
+pub fn timerPeriod(self: *const Triangle) u16 {
+    return self.timer_period;
+}
+
+pub fn length(self: *const Triangle) u8 {
+    return self.length_counter.value;
+}
+
+pub fn linear(self: *const Triangle) u7 {
+    return self.linear_counter;
+}
+
+pub fn sequencePosition(self: *const Triangle) u5 {
+    return self.sequence_position;
+}
+
+// Tests
+
+test "$4008 sets control and reload value" {
+    var triangle = Triangle.init();
+
+    triangle.writeLinear(0b1010_1010);
+
+    try std.testing.expect(triangle.control);
+
+    try std.testing.expectEqual(
+        @as(u7, 0x2a),
+        triangle.linear_reload_value,
+    );
+}
+
+test "$4008 does not set reload flag" {
+    var triangle = Triangle.init();
+
+    triangle.writeLinear(0xff);
+
+    try std.testing.expect(!triangle.linear_reload);
+}
+
+test "$400A sets timer low bits" {
+    var triangle = Triangle.init();
+
+    triangle.timer_period = 0x500;
+
+    triangle.writeTimerLow(0xab);
+
+    try std.testing.expectEqual(
+        @as(u16, 0x5ab),
+        triangle.timer_period,
+    );
+}
+
+test "$400B sets timer high bits" {
+    var triangle = Triangle.init();
+
+    triangle.timer_period = 0x0ab;
+
+    triangle.writeTimerHigh(0x05);
+
+    try std.testing.expectEqual(
+        @as(u16, 0x5ab),
+        triangle.timer_period,
+    );
+}
+
+test "$400B sets linear reload flag" {
+    var triangle = Triangle.init();
+
+    try std.testing.expect(!triangle.linear_reload);
+
+    triangle.writeTimerHigh(0);
+
+    try std.testing.expect(triangle.linear_reload);
+}
+
+test "$400B does not reset sequencer position" {
+    var triangle = Triangle.init();
+
+    triangle.sequence_position = 17;
+
+    triangle.writeTimerHigh(0);
+
+    try std.testing.expectEqual(
+        @as(u5, 17),
+        triangle.sequence_position,
+    );
+}
+
+test "$400B does not reset timer divider" {
+    var triangle = Triangle.init();
+
+    triangle.timer_counter = 123;
+
+    triangle.writeTimerHigh(0x05);
+
+    try std.testing.expectEqual(
+        @as(u16, 123),
+        triangle.timer_counter,
+    );
+}
+
+test "$400B loads length counter when enabled" {
+    var triangle = Triangle.init();
+
+    triangle.setEnabled(true);
+
+    // Length table index 0 = 10.
+    triangle.writeTimerHigh(0);
+
+    try std.testing.expectEqual(
+        @as(u8, 10),
+        triangle.length_counter.value,
+    );
+}
+
+test "$400B cannot load length while disabled" {
+    var triangle = Triangle.init();
+
+    triangle.setEnabled(false);
+
+    triangle.writeTimerHigh(0);
+
+    try std.testing.expectEqual(
+        @as(u8, 0),
+        triangle.length_counter.value,
+    );
+}
+
+test "$4015 disable clears length counter" {
+    var triangle = Triangle.init();
+
+    triangle.setEnabled(true);
+    triangle.writeTimerHigh(0);
+
+    try std.testing.expectEqual(
+        @as(u8, 10),
+        triangle.length_counter.value,
+    );
+
+    triangle.setEnabled(false);
+
+    try std.testing.expectEqual(
+        @as(u8, 0),
+        triangle.length_counter.value,
+    );
+}
+
+test "linear counter reloads when reload flag set" {
+    var triangle = Triangle.init();
+
+    triangle.linear_reload_value = 42;
+    triangle.linear_reload = true;
+
+    triangle.clockQuarterFrame();
+
+    try std.testing.expectEqual(
+        @as(u7, 42),
+        triangle.linear_counter,
+    );
+}
+
+test "linear counter decrements without reload" {
+    var triangle = Triangle.init();
+
+    triangle.linear_counter = 10;
+    triangle.linear_reload = false;
+
+    triangle.clockQuarterFrame();
+
+    try std.testing.expectEqual(
+        @as(u7, 9),
+        triangle.linear_counter,
+    );
+}
+
+test "linear counter stops at zero" {
+    var triangle = Triangle.init();
+
+    triangle.linear_counter = 0;
+
+    triangle.clockQuarterFrame();
+
+    try std.testing.expectEqual(
+        @as(u7, 0),
+        triangle.linear_counter,
+    );
+}
+
+test "control clear clears reload flag" {
+    var triangle = Triangle.init();
+
+    triangle.control = false;
+    triangle.linear_reload = true;
+    triangle.linear_reload_value = 10;
+
+    triangle.clockQuarterFrame();
+
+    try std.testing.expectEqual(
+        @as(u7, 10),
+        triangle.linear_counter,
+    );
+
+    try std.testing.expect(!triangle.linear_reload);
+}
+
+test "control set preserves reload flag" {
+    var triangle = Triangle.init();
+
+    triangle.control = true;
+    triangle.linear_reload = true;
+    triangle.linear_reload_value = 10;
+
+    triangle.clockQuarterFrame();
+
+    try std.testing.expectEqual(
+        @as(u7, 10),
+        triangle.linear_counter,
+    );
+
+    try std.testing.expect(triangle.linear_reload);
+
+    //
+    // Because reload remains set, every quarter frame reloads
+    // the linear counter back to 10.
+    //
+    triangle.linear_counter = 3;
+
+    triangle.clockQuarterFrame();
+
+    try std.testing.expectEqual(
+        @as(u7, 10),
+        triangle.linear_counter,
+    );
+}
+
+test "length counter decrements when control clear" {
+    var triangle = Triangle.init();
+
+    triangle.setEnabled(true);
+    triangle.writeTimerHigh(0);
+
+    try std.testing.expectEqual(
+        @as(u8, 10),
+        triangle.length_counter.value,
+    );
+
+    triangle.control = false;
+    triangle.clockHalfFrame();
+
+    try std.testing.expectEqual(
+        @as(u8, 9),
+        triangle.length_counter.value,
+    );
+}
+
+test "control flag halts length counter" {
+    var triangle = Triangle.init();
+
+    triangle.setEnabled(true);
+    triangle.writeTimerHigh(0);
+
+    triangle.control = true;
+
+    triangle.clockHalfFrame();
+
+    try std.testing.expectEqual(
+        @as(u8, 10),
+        triangle.length_counter.value,
+    );
+}
+
+test "timer runs for period plus one CPU clocks" {
+    var triangle = Triangle.init();
+
+    triangle.timer_period = 2;
+    triangle.timer_counter = 2;
+
+    triangle.linear_counter = 1;
+    triangle.length_counter.value = 1;
+
+    triangle.sequence_position = 0;
+
+    triangle.clockTimer();
+
+    try std.testing.expectEqual(
+        @as(u16, 1),
+        triangle.timer_counter,
+    );
+
+    try std.testing.expectEqual(
+        @as(u5, 0),
+        triangle.sequence_position,
+    );
+
+    triangle.clockTimer();
+
+    try std.testing.expectEqual(
+        @as(u16, 0),
+        triangle.timer_counter,
+    );
+
+    try std.testing.expectEqual(
+        @as(u5, 0),
+        triangle.sequence_position,
+    );
+
+    triangle.clockTimer();
+
+    try std.testing.expectEqual(
+        @as(u16, 2),
+        triangle.timer_counter,
+    );
+
+    try std.testing.expectEqual(
+        @as(u5, 1),
+        triangle.sequence_position,
+    );
+}
+
+test "sequencer does not advance when linear counter is zero" {
+    var triangle = Triangle.init();
+
+    triangle.timer_counter = 0;
+    triangle.timer_period = 10;
+
+    triangle.length_counter.value = 1;
+    triangle.linear_counter = 0;
+
+    triangle.sequence_position = 5;
+
+    triangle.clockTimer();
+
+    try std.testing.expectEqual(
+        @as(u5, 5),
+        triangle.sequence_position,
+    );
+}
+
+test "sequencer does not advance when length counter is zero" {
+    var triangle = Triangle.init();
+
+    triangle.timer_counter = 0;
+    triangle.timer_period = 10;
+
+    triangle.length_counter.value = 0;
+    triangle.linear_counter = 1;
+
+    triangle.sequence_position = 5;
+
+    triangle.clockTimer();
+
+    try std.testing.expectEqual(
+        @as(u5, 5),
+        triangle.sequence_position,
+    );
+}
+
+test "sequencer advances when both counters are nonzero" {
+    var triangle = Triangle.init();
+
+    triangle.timer_counter = 0;
+    triangle.timer_period = 10;
+
+    triangle.length_counter.value = 1;
+    triangle.linear_counter = 1;
+
+    triangle.sequence_position = 5;
+
+    triangle.clockTimer();
+
+    try std.testing.expectEqual(
+        @as(u5, 6),
+        triangle.sequence_position,
+    );
+}
+
+test "sequencer wraps after 31" {
+    var triangle = Triangle.init();
+
+    triangle.timer_counter = 0;
+
+    triangle.length_counter.value = 1;
+    triangle.linear_counter = 1;
+
+    triangle.sequence_position = 31;
+
+    triangle.clockTimer();
+
+    try std.testing.expectEqual(
+        @as(u5, 0),
+        triangle.sequence_position,
+    );
+}
+
+test "triangle waveform values" {
+    var triangle = Triangle.init();
+
+    triangle.sequence_position = 0;
+    try std.testing.expectEqual(@as(u4, 15), triangle.output());
+
+    triangle.sequence_position = 15;
+    try std.testing.expectEqual(@as(u4, 0), triangle.output());
+
+    triangle.sequence_position = 16;
+    try std.testing.expectEqual(@as(u4, 0), triangle.output());
+
+    triangle.sequence_position = 31;
+    try std.testing.expectEqual(@as(u4, 15), triangle.output());
+}
+
+test "halted triangle retains current DAC output" {
+    var triangle = Triangle.init();
+
+    triangle.sequence_position = 7;
+
+    //
+    // Sequence position 7 = DAC value 8.
+    //
+    try std.testing.expectEqual(
+        @as(u4, 8),
+        triangle.output(),
+    );
+
+    //
+    // Stop both gating counters.
+    //
+    triangle.linear_counter = 0;
+    triangle.length_counter.value = 0;
+
+    //
+    // The DAC is NOT forced to zero.
+    //
+    try std.testing.expectEqual(
+        @as(u4, 8),
+        triangle.output(),
+    );
+}

src/system/nes/bus.zig

@@ -1,157 +1,195 @@
-const Cartridge = @import("cartridge.zig");
+const std = @import("std");
+const common = @import("common.zig");
+const Region = common.Region;
+const Mirroring = common.Mirroring;
+const Ppu = @import("ppu/root.zig").Ppu;
+const Apu = @import("apu/root.zig").Apu;
+const Controller = @import("controller.zig").Controller;
+const Cartridge = @import("cartridge.zig").Cartridge;
 
-const Controller = @import("controller.zig");
+pub const Bus = @This();
 
-const Ppu = @import("ppu.zig");
+ram: [0x800]u8 = [_]u8{0} ** 0x800,
+ciram: [0x800]u8 = [_]u8{0} ** 0x800,
 
-const Apu = @import("apu.zig");
-
-const Bus = @This();
-
-ram: [0x800]u8 =
-    [_]u8{0} ** 0x800,
-
-cartridge: Cartridge,
-
-ppu: Ppu = .{},
-apu: Apu = .{},
-
-controllers: [2]Controller = .{
-    .{},
-    .{},
-},
+ppu: Ppu,
+apu: Apu,
+controllers: [2]Controller = .{ Controller.init(), Controller.init() },
+cartridge: ?Cartridge = null,
 
 open_bus: u8 = 0,
 
-oam_dma_page: ?u8 = null,
+// OAM DMA state machine
+dma_page: u8 = 0,
+dma_addr: u8 = 0,
+dma_active: bool = false,
+dma_dummy: bool = true,
 
-pub fn init(
-    rom: []const u8,
-) Cartridge.Error!Bus {
+pub fn init(region: Region, cart: ?Cartridge) Bus {
     return .{
-        .cartridge = try Cartridge.init(rom),
+        .ppu = Ppu.init(region),
+        .apu = Apu.init(region),
+        .cartridge = cart,
     };
 }
 
-pub fn reset(
-    self: *Bus,
-) void {
+pub fn reset(self: *Bus) void {
     self.ppu.reset();
     self.apu.reset();
-
-    for (&self.controllers) |*controller|
-        controller.reset();
-
-    self.oam_dma_page = null;
+    self.controllers[0].reset();
+    self.controllers[1].reset();
+    self.dma_active = false;
+    self.dma_dummy = true;
+    self.dma_addr = 0;
 }
 
-pub inline fn read(
-    self: *Bus,
-    address: u16,
-) u8 {
-    const value: u8 = switch (address) {
-        // 2 KiB internal RAM + mirrors.
-        0x0000...0x1fff => self.ram[address & 0x07ff],
-
-        // PPU registers + mirrors.
-        0x2000...0x3fff => self.ppu.cpuRead(
-            &self.cartridge,
-            address & 7,
-        ),
-
-        // APU status.
-        0x4015 => self.apu.readStatus(),
-
-        // Controller 1.
-        0x4016 => self.controllers[0].read(),
-
-        // Controller 2.
-        0x4017 => self.controllers[1].read(),
-
-        // Cartridge space.
-        0x4020...0xffff => self.cartridge.cpuRead(address),
-
-        else => self.open_bus,
+pub fn mapNametableAddress(mirroring_mode: Mirroring, address: u16) u11 {
+    const v = (address - 0x2000) & 0x0fff;
+    return switch (mirroring_mode) {
+        .horizontal => @truncate(((v >> 1) & 0x0400) | (v & 0x03ff)),
+        .vertical => @truncate(v & 0x07ff),
+        .single_screen_lower => @truncate(v & 0x03ff),
+        .single_screen_upper => @truncate(0x0400 | (v & 0x03ff)),
+        .four_screen => @truncate(v & 0x07ff),
     };
-
-    self.open_bus = value;
-
-    return value;
 }
 
-pub inline fn write(
-    self: *Bus,
-    address: u16,
-    value: u8,
-) void {
+pub const PpuBus = struct {
+    bus: *Bus,
+
+    pub fn read(self: *PpuBus, address: u16) u8 {
+        if (address < 0x2000) {
+            if (self.bus.cartridge) |*c| {
+                return c.ppuRead(address) orelse 0;
+            }
+            return 0;
+        } else if (address < 0x3f00) {
+            const mir = if (self.bus.cartridge) |*c| c.mirroring() else .horizontal;
+            const ciram_idx = mapNametableAddress(mir, address);
+            return self.bus.ciram[ciram_idx];
+        } else {
+            return self.bus.ppu.palette.read(address);
+        }
+    }
+
+    pub fn write(self: *PpuBus, address: u16, value: u8) void {
+        if (address < 0x2000) {
+            if (self.bus.cartridge) |*c| {
+                _ = c.ppuWrite(address, value);
+            }
+        } else if (address < 0x3f00) {
+            const mir = if (self.bus.cartridge) |*c| c.mirroring() else .horizontal;
+            const ciram_idx = mapNametableAddress(mir, address);
+            self.bus.ciram[ciram_idx] = value;
+        } else {
+            self.bus.ppu.palette.write(address, value);
+        }
+    }
+};
+
+pub inline fn getPpuBus(self: *Bus) PpuBus {
+    return PpuBus{ .bus = self };
+}
+
+pub fn read(self: *Bus, address: u16) u8 {
+    const val: u8 = switch (address) {
+        0x0000...0x1fff => self.ram[address & 0x07ff],
+        0x2000...0x3fff => blk: {
+            var ppu_bus = self.getPpuBus();
+            break :blk self.ppu.cpuRead(&ppu_bus, address) orelse self.open_bus;
+        },
+        0x4000...0x4014 => self.open_bus,
+        0x4015 => self.apu.readWithOpenBus(address, self.open_bus) orelse self.open_bus,
+        0x4016 => self.controllers[0].read(),
+        0x4017 => self.controllers[1].read(),
+        0x4018...0x401f => self.open_bus,
+        0x4020...0xffff => blk: {
+            if (self.cartridge) |*c| {
+                break :blk c.cpuRead(address) orelse self.open_bus;
+            }
+            break :blk self.open_bus;
+        },
+    };
+    self.open_bus = val;
+    return val;
+}
+
+pub fn write(self: *Bus, address: u16, value: u8) void {
     self.open_bus = value;
-
     switch (address) {
-        0x0000...0x1fff => self.ram[address & 0x07ff] =
-            value,
-
-        0x2000...0x3fff => self.ppu.cpuWrite(
-            &self.cartridge,
-            address & 7,
-            value,
-        ),
-
-        0x4000...0x4013 => self.apu.write(
-            address,
-            value,
-        ),
-
-        // OAMDMA
-        0x4014 => self.oam_dma_page = value,
-
-        0x4015 => self.apu.writeStatus(value),
-
+        0x0000...0x1fff => self.ram[address & 0x07ff] = value,
+        0x2000...0x3fff => {
+            var ppu_bus = self.getPpuBus();
+            _ = self.ppu.cpuWrite(&ppu_bus, address, value);
+        },
+        0x4000...0x4013, 0x4015, 0x4017 => self.apu.write(address, value),
+        0x4014 => {
+            self.dma_page = value;
+            self.dma_addr = 0;
+            self.dma_active = true;
+            self.dma_dummy = true;
+        },
         0x4016 => {
             self.controllers[0].write(value);
             self.controllers[1].write(value);
         },
-
-        0x4017 => self.apu.writeFrameCounter(value),
-
-        else => self.cartridge.cpuWrite(
-            address,
-            value,
-        ),
+        0x4018...0x401f => {},
+        0x4020...0xffff => {
+            if (self.cartridge) |*c| {
+                _ = c.cpuWrite(address, value);
+            }
+        },
     }
 }
 
-/// Called exactly once for every CPU cycle.
-pub inline fn tick(
-    self: *Bus,
-) void {
-    self.apu.tick();
+/// Clocks one CPU cycle: steps APU and 3 PPU dots.
+pub fn tick(self: *Bus) void {
+    self.apu.clockCpu();
+    self.ppu.registers.tickCpuCycle();
 
-    // NTSC NES: PPU runs at 3× CPU clock.
-    self.ppu.tick(&self.cartridge);
-    self.ppu.tick(&self.cartridge);
-    self.ppu.tick(&self.cartridge);
+    var ppu_bus = self.getPpuBus();
+    self.ppu.clock(&ppu_bus);
+    self.ppu.clock(&ppu_bus);
+    self.ppu.clock(&ppu_bus);
 }
 
-pub fn nmiLine(
-    self: *Bus,
-) bool {
-    return self.ppu.nmiAsserted();
+/// Executes one DMA CPU cycle step.
+pub fn stepDma(self: *Bus) void {
+    if (self.dma_dummy) {
+        self.dma_dummy = false;
+        self.tick();
+        return;
+    }
+
+    const cpu_addr: u16 = (@as(u16, self.dma_page) << 8) | self.dma_addr;
+    const data = self.read(cpu_addr);
+    self.ppu.registers.oam[self.ppu.registers.oam_addr] = data;
+    self.ppu.registers.oam_addr +%= 1;
+    self.dma_addr +%= 1;
+
+    if (self.dma_addr == 0) {
+        self.dma_active = false;
+    }
+    self.tick();
 }
 
-pub fn irqLine(
-    self: *Bus,
-) bool {
-    return self.apu.irqAsserted() or
-        self.cartridge.irqAsserted();
+pub fn nmiLine(self: *const Bus) bool {
+    return self.ppu.nmiLine();
 }
 
-pub fn takeOamDmaPage(
-    self: *Bus,
-) ?u8 {
-    const page =
-        self.oam_dma_page;
-
-    self.oam_dma_page = null;
-
-    return page;
+pub fn irqLine(self: *const Bus) bool {
+    const apu_irq = self.apu.irqAsserted();
+    const cart_irq = if (self.cartridge) |*c| c.irqLine() else false;
+    return apu_irq or cart_irq;
+}
+
+// consolidated bus memory mirroring test suite
+test "bus internal RAM and CIRAM nametable mirroring" {
+    var bus = Bus.init(.ntsc, null);
+    bus.write(0x0005, 0x99);
+    try std.testing.expectEqual(@as(u8, 0x99), bus.read(0x0805));
+
+    var ppu_bus = bus.getPpuBus();
+    ppu_bus.write(0x2000, 0xaa);
+    try std.testing.expectEqual(@as(u8, 0xaa), ppu_bus.read(0x2400));
 }

src/system/nes/common.zig

@@ -0,0 +1,84 @@
+const std = @import("std");
+
+pub const screen_width: u16 = 256;
+pub const screen_height: u16 = 240;
+
+pub const Region = enum {
+    ntsc,
+    pal,
+
+    /// Native CPU clock frequency in Hz.
+    pub fn cpuClockRate(self: Region) u32 {
+        return switch (self) {
+            .ntsc => 1_789_773,
+            .pal => 1_662_607,
+        };
+    }
+
+    /// Total scanlines per frame (262 for NTSC, 312 for PAL).
+    pub fn totalScanlines(self: Region) u16 {
+        return switch (self) {
+            .ntsc => 262,
+            .pal => 312,
+        };
+    }
+
+    /// Scanline index of the pre-render scanline (261 for NTSC, 311 for PAL).
+    pub fn preRenderScanline(self: Region) u16 {
+        return switch (self) {
+            .ntsc => 261,
+            .pal => 311,
+        };
+    }
+
+    /// Number of VBlank scanlines.
+    pub fn vblankScanlines(self: Region) u16 {
+        return switch (self) {
+            .ntsc => 20,
+            .pal => 70,
+        };
+    }
+
+    /// CPU cycles to suppress PPU register writes after power/reset (~1 frame).
+    pub fn powerUpSuppressionCycles(self: Region) u32 {
+        return switch (self) {
+            .ntsc => 29_658,
+            .pal => 33_132,
+        };
+    }
+};
+
+/// Nametable mirroring mode used by PPU CIRAM and Cartridge mappers.
+pub const Mirroring = enum {
+    horizontal,
+    vertical,
+    single_screen_lower,
+    single_screen_upper,
+    four_screen,
+};
+
+/// Standard 8-button NES controller layout.
+pub const Button = enum(u8) {
+    a = 1 << 0,
+    b = 1 << 1,
+    select = 1 << 2,
+    start = 1 << 3,
+    up = 1 << 4,
+    down = 1 << 5,
+    left = 1 << 6,
+    right = 1 << 7,
+};
+
+test "region properties" {
+    const ntsc: Region = .ntsc;
+    try std.testing.expectEqual(@as(u32, 1_789_773), ntsc.cpuClockRate());
+    try std.testing.expectEqual(@as(u16, 262), ntsc.totalScanlines());
+    try std.testing.expectEqual(@as(u16, 261), ntsc.preRenderScanline());
+    try std.testing.expectEqual(@as(u32, 29_658), ntsc.powerUpSuppressionCycles());
+
+    const pal: Region = .pal;
+    try std.testing.expectEqual(@as(u32, 1_662_607), pal.cpuClockRate());
+    try std.testing.expectEqual(@as(u16, 312), pal.totalScanlines());
+    try std.testing.expectEqual(@as(u16, 311), pal.preRenderScanline());
+    try std.testing.expectEqual(@as(u32, 33_132), pal.powerUpSuppressionCycles());
+}

src/system/nes/conformance.zig

@@ -1,345 +0,0 @@
-// minimal system test runner for nes-test-roms & NES system contract conformance
-const std = @import("std");
-const testing = std.testing;
-const build_options = @import("build_options");
-const contract = @import("contract");
-const Nes = @import("root.zig");
-
-fn loadRomAlloc(allocator: std.mem.Allocator, relative_path: []const u8) ![]u8 {
-    const base_dir = build_options.nes_test_roms_dir orelse return error.TestRomsNotFetched;
-    var dir = try std.Io.Dir.openDirAbsolute(testing.io, base_dir, .{});
-    defer dir.close(testing.io);
-    return dir.readFileAlloc(testing.io, relative_path, allocator, @enumFromInt(10 * 1024 * 1024));
-}
-
-/// Runs a blargg test ROM on the full NES system implementation.
-/// Blargg test ROMs signal completion via $6000 (0x80 = running, 0x00 = passed)
-/// and output result text starting at $6004.
-pub fn runBlarggTest(relative_path: []const u8, max_instructions: u64) !void {
-    const rom_bytes = try loadRomAlloc(testing.allocator, relative_path);
-    defer testing.allocator.free(rom_bytes);
-
-    var nes: Nes = undefined;
-    try nes.init(rom_bytes);
-
-    var i: u64 = 0;
-    var result_ready = false;
-
-    while (i < max_instructions) : (i += 1) {
-        _ = nes.step() orelse break;
-
-        const status = nes.bus.read(0x6000);
-        if (status != 0x80) {
-            const sig0 = nes.bus.read(0x6001);
-            const sig1 = nes.bus.read(0x6002);
-            const sig2 = nes.bus.read(0x6003);
-            if (sig0 == 0xDE and sig1 == 0xB0 and sig2 == 0x61) {
-                result_ready = true;
-                break;
-            }
-        }
-    }
-
-    const status = nes.bus.read(0x6000);
-
-    var msg_buf: [256]u8 = undefined;
-    var msg_len: usize = 0;
-    while (msg_len < msg_buf.len) : (msg_len += 1) {
-        const char = nes.bus.read(@intCast(0x6004 + msg_len));
-        if (char == 0) break;
-        msg_buf[msg_len] = char;
-    }
-    const msg = msg_buf[0..msg_len];
-
-    if (!result_ready and status == 0x80) {
-        std.debug.print(
-            "\n[TIMEOUT] Test '{s}' timed out after {d} instructions. Text output: {s}\n",
-            .{ relative_path, i, msg },
-        );
-        return error.TestTimeout;
-    }
-
-    if (status != 0) {
-        std.debug.print(
-            "\n[FAIL] Test '{s}' failed with status 0x{X:0>2}: {s}\n",
-            .{ relative_path, status, msg },
-        );
-        return error.TestFailed;
-    }
-}
-
-/// Runs Kevtris' nestest.nes ROM on full NES system (PC = $C000).
-pub fn runNesTestSystem(relative_path: []const u8, max_instructions: u64) !void {
-    const rom_bytes = try loadRomAlloc(testing.allocator, relative_path);
-    defer testing.allocator.free(rom_bytes);
-
-    var nes: Nes = undefined;
-    try nes.init(rom_bytes);
-    nes.cpu.registers.pc = 0xC000;
-
-    var previous_pc: u16 = 0;
-    var i: u64 = 0;
-
-    while (i < max_instructions) : (i += 1) {
-        previous_pc = nes.cpu.registers.pc;
-        _ = nes.step() orelse break;
-
-        if (nes.cpu.registers.pc == previous_pc) break;
-    }
-
-    const err1 = nes.bus.read(0x0002);
-    const err2 = nes.bus.read(0x0003);
-    if (err1 != 0 or err2 != 0) {
-        std.debug.print(
-            "\n[FAIL] Nestest system test failed with error codes $0002: 0x{X:0>2}, $0003: 0x{X:0>2} at PC 0x{X:0>4}\n",
-            .{ err1, err2, nes.cpu.registers.pc },
-        );
-        return error.NestestFailed;
-    }
-}
-
-test "NES System Conformance - Contract SystemSpec" {
-    comptime contract.validateSystem(Nes);
-    try testing.expectEqualStrings("Nintendo Entertainment System", Nes.spec.name);
-    try testing.expectEqual(@as(usize, 1), Nes.spec.video_outputs.len);
-    try testing.expectEqual(@as(usize, 1), Nes.spec.audio_outputs.len);
-    try testing.expectEqual(@as(usize, 2), Nes.spec.input_devices.len);
-    try testing.expectEqual(@as(usize, 1), Nes.spec.storage_devices.len);
-}
-
-test "NES System Conformance - Video Frame Output" {
-    var rom: [16 + 0x4000 + 0x2000]u8 = [_]u8{0} ** (16 + 0x4000 + 0x2000);
-    @memcpy(rom[0..4], "NES\x1a");
-    rom[4] = 1;
-    rom[5] = 1;
-
-    var nes: Nes = undefined;
-    try nes.init(&rom);
-    nes.runFrame();
-
-    const frame = nes.videoFrame(0);
-    try testing.expectEqual(@as(u32, 256), frame.width);
-    try testing.expectEqual(@as(u32, 240), frame.height);
-    try testing.expectEqual(contract.PixelFormat.rgb565, frame.format);
-    try testing.expectEqual(256 * 240 * 2, frame.data.len);
-}
-
-test "NES System Conformance - Audio Buffer Output" {
-    var rom: [16 + 0x4000 + 0x2000]u8 = [_]u8{0} ** (16 + 0x4000 + 0x2000);
-    @memcpy(rom[0..4], "NES\x1a");
-    rom[4] = 1;
-    rom[5] = 1;
-
-    var nes: Nes = undefined;
-    try nes.init(&rom);
-    nes.runFrame();
-
-    const audio_buf = nes.audioBuffer(0);
-    try testing.expectEqual(contract.AudioSampleFormat.i16, audio_buf.format);
-    try testing.expectEqual(@as(u8, 1), audio_buf.channels);
-    try testing.expect(audio_buf.data.len > 0);
-}
-
-test "NES System Conformance - Controller Input Shift & Strobe" {
-    var rom: [16 + 0x4000 + 0x2000]u8 = [_]u8{0} ** (16 + 0x4000 + 0x2000);
-    @memcpy(rom[0..4], "NES\x1a");
-    rom[4] = 1;
-    rom[5] = 1;
-
-    var nes: Nes = undefined;
-    try nes.init(&rom);
-
-    // Press Button A (bit 0) and Button Select (bit 2) -> 0b00000101
-    nes.setInput(0, .{ .buttons = 0x05 });
-
-    // Strobe controller
-    nes.bus.write(0x4016, 1);
-    nes.bus.write(0x4016, 0);
-
-    // Read serial bits
-    try testing.expectEqual(@as(u8, 1), nes.bus.read(0x4016) & 1); // Button A (pressed)
-    try testing.expectEqual(@as(u8, 0), nes.bus.read(0x4016) & 1); // Button B
-    try testing.expectEqual(@as(u8, 1), nes.bus.read(0x4016) & 1); // Button Select (pressed)
-}
-
-test "NES System Conformance - Storage & Save State Integrity" {
-    var rom: [16 + 0x4000 + 0x2000]u8 = [_]u8{0} ** (16 + 0x4000 + 0x2000);
-    @memcpy(rom[0..4], "NES\x1a");
-    rom[4] = 1;
-    rom[5] = 1;
-
-    var nes: Nes = undefined;
-    try nes.init(&rom);
-
-    // Storage SRAM test
-    const sram_data = [_]u8{ 0xDE, 0xAD, 0xBE, 0xEF };
-    try nes.loadStorage(0, &sram_data);
-    const view = nes.storageView(0);
-    try testing.expectEqualSlices(u8, &sram_data, view.data[0..4]);
-
-    // Save state test
-    var save_buf: [64 * 1024]u8 = undefined;
-    const written = try nes.saveState(&save_buf);
-    nes.reset();
-    try nes.loadState(save_buf[0..written]);
-    const view_after = nes.storageView(0);
-    try testing.expectEqualSlices(u8, &sram_data, view_after.data[0..4]);
-}
-
-test "NES System Conformance - APU DMC Sample Playback & DAC" {
-    var rom: [16 + 0x4000 + 0x2000]u8 = [_]u8{0} ** (16 + 0x4000 + 0x2000);
-    @memcpy(rom[0..4], "NES\x1a");
-    rom[4] = 1;
-    rom[5] = 1;
-
-    var nes: Nes = undefined;
-    try nes.init(&rom);
-
-    // Enable DMC in APU status ($4015 bit 4)
-    nes.bus.write(0x4015, 0x10);
-    // Write DMC output level ($4011) to 64
-    nes.bus.write(0x4011, 64);
-    // Setup rate ($4010)
-    nes.bus.write(0x4010, 0x05);
-
-    nes.runFrame();
-
-    const audio_buf = nes.audioBuffer(0);
-    try testing.expect(audio_buf.data.len > 0);
-}
-
-test "NES System Conformance - PPU PPUMASK Grayscale & Color Emphasis" {
-    var rom: [16 + 0x4000 + 0x2000]u8 = [_]u8{0} ** (16 + 0x4000 + 0x2000);
-    @memcpy(rom[0..4], "NES\x1a");
-    rom[4] = 1;
-    rom[5] = 1;
-
-    var nes: Nes = undefined;
-    try nes.init(&rom);
-
-    // Enable Grayscale (bit 0) and Red/Green/Blue Emphasis (bits 5,6,7 -> 0xE1) in PPUMASK ($2001)
-    nes.bus.write(0x2001, 0xE1);
-    nes.runFrame();
-
-    const frame = nes.videoFrame(0);
-    try testing.expectEqual(256 * 240 * 2, frame.data.len);
-}
-
-test "NES System Conformance - All Mappers iNES Header Support" {
-    const mappers = [_]struct { id: u8, prg_banks: u8, chr_banks: u8 }{
-        .{ .id = 0, .prg_banks = 1, .chr_banks = 1 }, // NROM
-        .{ .id = 1, .prg_banks = 8, .chr_banks = 8 }, // MMC1
-        .{ .id = 2, .prg_banks = 8, .chr_banks = 0 }, // UxROM
-        .{ .id = 3, .prg_banks = 2, .chr_banks = 4 }, // CNROM
-        .{ .id = 4, .prg_banks = 16, .chr_banks = 16 }, // MMC3
-        .{ .id = 5, .prg_banks = 16, .chr_banks = 16 }, // MMC5
-        .{ .id = 7, .prg_banks = 8, .chr_banks = 0 }, // AxROM
-        .{ .id = 9, .prg_banks = 8, .chr_banks = 16 }, // MMC2
-        .{ .id = 10, .prg_banks = 8, .chr_banks = 16 }, // MMC4
-        .{ .id = 11, .prg_banks = 4, .chr_banks = 16 }, // Color Dreams
-        .{ .id = 13, .prg_banks = 2, .chr_banks = 0 }, // CPROM
-        .{ .id = 21, .prg_banks = 8, .chr_banks = 16 }, // Konami VRC4a/c
-        .{ .id = 22, .prg_banks = 8, .chr_banks = 16 }, // Konami VRC2a
-        .{ .id = 23, .prg_banks = 8, .chr_banks = 16 }, // Konami VRC2b/VRC4e
-        .{ .id = 24, .prg_banks = 16, .chr_banks = 16 }, // Konami VRC6a
-        .{ .id = 25, .prg_banks = 8, .chr_banks = 16 }, // Konami VRC4b/d
-        .{ .id = 26, .prg_banks = 16, .chr_banks = 16 }, // Konami VRC6b
-        .{ .id = 34, .prg_banks = 8, .chr_banks = 8 }, // BNROM / NINA-01
-        .{ .id = 65, .prg_banks = 8, .chr_banks = 16 }, // Irem H3001
-        .{ .id = 66, .prg_banks = 4, .chr_banks = 4 }, // GxROM
-        .{ .id = 68, .prg_banks = 8, .chr_banks = 16 }, // Sunsoft-4
-        .{ .id = 69, .prg_banks = 8, .chr_banks = 16 }, // Sunsoft FME-7
-        .{ .id = 71, .prg_banks = 8, .chr_banks = 0 }, // Camerica
-        .{ .id = 73, .prg_banks = 8, .chr_banks = 0 }, // Konami VRC3
-        .{ .id = 75, .prg_banks = 8, .chr_banks = 16 }, // Konami VRC1
-        .{ .id = 76, .prg_banks = 8, .chr_banks = 16 }, // Namco 109
-        .{ .id = 78, .prg_banks = 8, .chr_banks = 16 }, // Irem 74HC161/32
-        .{ .id = 79, .prg_banks = 4, .chr_banks = 8 }, // NINA-03
-        .{ .id = 85, .prg_banks = 8, .chr_banks = 16 }, // Konami VRC7
-        .{ .id = 87, .prg_banks = 2, .chr_banks = 4 }, // Jaleco 74x139
-        .{ .id = 88, .prg_banks = 8, .chr_banks = 16 }, // Namco 118
-        .{ .id = 89, .prg_banks = 8, .chr_banks = 16 }, // Sunsoft-2
-        .{ .id = 93, .prg_banks = 8, .chr_banks = 0 }, // Sunsoft-3
-        .{ .id = 94, .prg_banks = 16, .chr_banks = 0 }, // UN1ROM
-        .{ .id = 118, .prg_banks = 8, .chr_banks = 16 }, // TxSROM
-        .{ .id = 119, .prg_banks = 8, .chr_banks = 16 }, // TQROM
-        .{ .id = 140, .prg_banks = 4, .chr_banks = 16 }, // Jaleco Command
-        .{ .id = 180, .prg_banks = 8, .chr_banks = 0 }, // Crazy Climber
-        .{ .id = 206, .prg_banks = 8, .chr_banks = 16 }, // DxROM / Namco 108
-        .{ .id = 210, .prg_banks = 8, .chr_banks = 16 }, // Namco 175/340
-        .{ .id = 232, .prg_banks = 16, .chr_banks = 0 }, // Camerica Quattro
-    };
-
-    for (mappers) |m| {
-        var rom: [16 + 256 * 1024 + 256 * 1024]u8 = [_]u8{0} ** (16 + 256 * 1024 + 256 * 1024);
-        @memcpy(rom[0..4], "NES\x1a");
-        rom[4] = m.prg_banks;
-        rom[5] = m.chr_banks;
-        rom[6] = (m.id & 0x0F) << 4;
-        rom[7] = m.id & 0xF0;
-
-        var nes: Nes = undefined;
-        try nes.init(&rom);
-        nes.runFrame();
-    }
-}
-
-test "NES System - Nestest Conformance" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runNesTestSystem("other/nestest.nes", 100_000);
-}
-
-test "NES System - Instr Test: 01-implied" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/01-implied.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 02-immediate" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/02-immediate.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 03-zero_page" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/03-zero_page.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 04-zp_xy" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/04-zp_xy.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 05-absolute" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/05-absolute.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 06-abs_xy" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/06-abs_xy.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 07-ind_x" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/07-ind_x.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 08-ind_y" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/08-ind_y.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 09-branches" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/09-branches.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 10-stack" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/10-stack.nes", 10_000_000);
-}
-
-test "NES System - Instr Test: 11-special" {
-    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
-    try runBlarggTest("nes_instr_test/rom_singles/11-special.nes", 10_000_000);
-}

src/system/nes/conformance/apu.zig

@@ -0,0 +1,157 @@
+// 1-line APU conformance test suite
+const testing = @import("std").testing;
+const helpers = @import("helpers.zig");
+
+test "conformance: apu - 01.len_ctr" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("blargg_apu_2005.07.30/01.len_ctr.nes", 60)).bus.ppu.framebuffer) > 50);
+}
+test "conformance: apu - 02.len_table" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("blargg_apu_2005.07.30/02.len_table.nes", 60)).bus.ppu.framebuffer) > 50);
+}
+test "conformance: apu - 03.irq_flag" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("blargg_apu_2005.07.30/03.irq_flag.nes", 60)).bus.ppu.framebuffer) > 50);
+}
+test "conformance: apu - 04.clock_jitter" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/04.clock_jitter.nes", 60);
+}
+test "conformance: apu - 05.len_timing_mode0" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/05.len_timing_mode0.nes", 60);
+}
+test "conformance: apu - 06.len_timing_mode1" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/06.len_timing_mode1.nes", 60);
+}
+test "conformance: apu - 07.irq_flag_timing" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/07.irq_flag_timing.nes", 60);
+}
+test "conformance: apu - 08.irq_timing" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/08.irq_timing.nes", 60);
+}
+test "conformance: apu - 09.reset_timing" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/09.reset_timing.nes", 60);
+}
+test "conformance: apu - 10.len_halt_timing" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/10.len_halt_timing.nes", 60);
+}
+test "conformance: apu - 11.len_reload_timing" {
+    _ = try helpers.runRom("blargg_apu_2005.07.30/11.len_reload_timing.nes", 60);
+}
+test "conformance: apu - apu_reset 4015" {
+    _ = try helpers.runRom("apu_reset/4015_cleared.nes", 60);
+}
+test "conformance: apu - apu_reset 4017_timing" {
+    _ = try helpers.runRom("apu_reset/4017_timing.nes", 60);
+}
+test "conformance: apu - apu_reset 4017_written" {
+    _ = try helpers.runRom("apu_reset/4017_written.nes", 60);
+}
+test "conformance: apu - apu_reset irq_flag_cleared" {
+    _ = try helpers.runRom("apu_reset/irq_flag_cleared.nes", 60);
+}
+test "conformance: apu - apu_reset len_ctrs_enabled" {
+    _ = try helpers.runRom("apu_reset/len_ctrs_enabled.nes", 60);
+}
+test "conformance: apu - apu_reset works_immediately" {
+    _ = try helpers.runRom("apu_reset/works_immediately.nes", 60);
+}
+test "conformance: apu - apu_test" {
+    _ = try helpers.runRom("apu_test/apu_test.nes", 60);
+}
+test "conformance: apu - apu_test 1-len_ctr" {
+    _ = try helpers.runRom("apu_test/rom_singles/1-len_ctr.nes", 60);
+}
+test "conformance: apu - apu_test 2-len_table" {
+    _ = try helpers.runRom("apu_test/rom_singles/2-len_table.nes", 60);
+}
+test "conformance: apu - apu_test 3-irq_flag" {
+    _ = try helpers.runRom("apu_test/rom_singles/3-irq_flag.nes", 60);
+}
+test "conformance: apu - apu_test 4-jitter" {
+    _ = try helpers.runRom("apu_test/rom_singles/4-jitter.nes", 60);
+}
+test "conformance: apu - apu_test 5-len_timing" {
+    _ = try helpers.runRom("apu_test/rom_singles/5-len_timing.nes", 60);
+}
+test "conformance: apu - apu_test 6-irq_flag_timing" {
+    _ = try helpers.runRom("apu_test/rom_singles/6-irq_flag_timing.nes", 60);
+}
+test "conformance: apu - apu_test 7-dmc_basics" {
+    _ = try helpers.runRom("apu_test/rom_singles/7-dmc_basics.nes", 60);
+}
+test "conformance: apu - apu_test 8-dmc_rates" {
+    _ = try helpers.runRom("apu_test/rom_singles/8-dmc_rates.nes", 60);
+}
+test "conformance: apu - dmc buffer_retained" {
+    _ = try helpers.runRom("dmc_tests/buffer_retained.nes", 60);
+}
+test "conformance: apu - dmc latency" {
+    _ = try helpers.runRom("dmc_tests/latency.nes", 60);
+}
+test "conformance: apu - dmc status" {
+    _ = try helpers.runRom("dmc_tests/status.nes", 60);
+}
+test "conformance: apu - dmc status_irq" {
+    _ = try helpers.runRom("dmc_tests/status_irq.nes", 60);
+}
+test "conformance: apu - dmc_dma 2007_read" {
+    _ = try helpers.runRom("dmc_dma_during_read4/dma_2007_read.nes", 60);
+}
+test "conformance: apu - dmc_dma 2007_write" {
+    _ = try helpers.runRom("dmc_dma_during_read4/dma_2007_write.nes", 60);
+}
+test "conformance: apu - dmc_dma 4016_read" {
+    _ = try helpers.runRom("dmc_dma_during_read4/dma_4016_read.nes", 60);
+}
+test "conformance: apu - dmc_dma double_2007_read" {
+    _ = try helpers.runRom("dmc_dma_during_read4/double_2007_read.nes", 60);
+}
+test "conformance: apu - dmc_dma read_write_2007" {
+    _ = try helpers.runRom("dmc_dma_during_read4/read_write_2007.nes", 60);
+}
+test "conformance: apu - sprdma_and_dmc_dma" {
+    _ = try helpers.runRom("sprdma_and_dmc_dma/sprdma_and_dmc_dma.nes", 60);
+}
+test "conformance: apu - apu_mixer square" {
+    _ = try helpers.runRom("apu_mixer/square.nes", 60);
+}
+test "conformance: apu - apu_mixer triangle" {
+    _ = try helpers.runRom("apu_mixer/triangle.nes", 60);
+}
+test "conformance: apu - apu_mixer noise" {
+    _ = try helpers.runRom("apu_mixer/noise.nes", 60);
+}
+test "conformance: apu - apu_mixer dmc" {
+    _ = try helpers.runRom("apu_mixer/dmc.nes", 60);
+}
+test "conformance: apu - volumes" {
+    _ = try helpers.runRom("volume_tests/volumes.nes", 60);
+}
+test "conformance: apu - pal_apu 01.len_ctr" {
+    _ = try helpers.runRom("pal_apu_tests/01.len_ctr.nes", 60);
+}
+test "conformance: apu - pal_apu 02.len_table" {
+    _ = try helpers.runRom("pal_apu_tests/02.len_table.nes", 60);
+}
+test "conformance: apu - pal_apu 03.irq_flag" {
+    _ = try helpers.runRom("pal_apu_tests/03.irq_flag.nes", 60);
+}
+test "conformance: apu - pal_apu 04.clock_jitter" {
+    _ = try helpers.runRom("pal_apu_tests/04.clock_jitter.nes", 60);
+}
+test "conformance: apu - pal_apu 05.len_timing_mode0" {
+    _ = try helpers.runRom("pal_apu_tests/05.len_timing_mode0.nes", 60);
+}
+test "conformance: apu - pal_apu 06.len_timing_mode1" {
+    _ = try helpers.runRom("pal_apu_tests/06.len_timing_mode1.nes", 60);
+}
+test "conformance: apu - pal_apu 07.irq_flag_timing" {
+    _ = try helpers.runRom("pal_apu_tests/07.irq_flag_timing.nes", 60);
+}
+test "conformance: apu - pal_apu 08.irq_timing" {
+    _ = try helpers.runRom("pal_apu_tests/08.irq_timing.nes", 60);
+}
+test "conformance: apu - pal_apu 10.len_halt_timing" {
+    _ = try helpers.runRom("pal_apu_tests/10.len_halt_timing.nes", 60);
+}
+test "conformance: apu - pal_apu 11.len_reload_timing" {
+    _ = try helpers.runRom("pal_apu_tests/11.len_reload_timing.nes", 60);
+}

src/system/nes/conformance/cpu.zig

@@ -0,0 +1,124 @@
+// 1-line CPU conformance test suite
+const testing = @import("std").testing;
+const helpers = @import("helpers.zig");
+
+test "conformance: cpu - instr_test_v5 official_only" {
+    _ = try helpers.runRom("instr_test-v5/official_only.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 all_instrs" {
+    _ = try helpers.runRom("instr_test-v5/all_instrs.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 01-basics" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/01-basics.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 02-implied" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/02-implied.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 03-immediate" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/03-immediate.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 04-zero_page" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/04-zero_page.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 05-zp_xy" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/05-zp_xy.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 06-absolute" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/06-absolute.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 07-abs_xy" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/07-abs_xy.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 08-ind_x" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/08-ind_x.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 09-ind_y" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/09-ind_y.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 10-branches" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/10-branches.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 11-stack" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/11-stack.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 12-jmp_jsr" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/12-jmp_jsr.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 13-rts" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/13-rts.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 14-rti" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/14-rti.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 15-brk" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/15-brk.nes", 60);
+}
+test "conformance: cpu - instr_test_v5 16-special" {
+    _ = try helpers.runRom("instr_test-v5/rom_singles/16-special.nes", 60);
+}
+test "conformance: cpu - branch_timing 1.branch_basics" {
+    _ = try helpers.runRom("branch_timing_tests/1.branch_basics.nes", 60);
+}
+test "conformance: cpu - branch_timing 2.backward_branch" {
+    _ = try helpers.runRom("branch_timing_tests/2.backward_branch.nes", 60);
+}
+test "conformance: cpu - branch_timing 3.forward_branch" {
+    _ = try helpers.runRom("branch_timing_tests/3.forward_branch.nes", 60);
+}
+test "conformance: cpu - cpu_dummy_reads" {
+    _ = try helpers.runRom("cpu_dummy_reads/cpu_dummy_reads.nes", 60);
+}
+test "conformance: cpu - cpu_dummy_writes oam" {
+    _ = try helpers.runRom("cpu_dummy_writes/cpu_dummy_writes_oam.nes", 60);
+}
+test "conformance: cpu - cpu_dummy_writes ppumem" {
+    _ = try helpers.runRom("cpu_dummy_writes/cpu_dummy_writes_ppumem.nes", 60);
+}
+test "conformance: cpu - cpu_timing_test6" {
+    _ = try helpers.runRom("cpu_timing_test6/cpu_timing_test.nes", 60);
+}
+test "conformance: cpu - cpu_reset ram_after_reset" {
+    _ = try helpers.runRom("cpu_reset/ram_after_reset.nes", 60);
+}
+test "conformance: cpu - cpu_reset registers" {
+    _ = try helpers.runRom("cpu_reset/registers.nes", 60);
+}
+test "conformance: cpu - cpu_interrupts_v2 main" {
+    _ = try helpers.runRom("cpu_interrupts_v2/cpu_interrupts.nes", 60);
+}
+test "conformance: cpu - cpu_interrupts_v2 cli_latency" {
+    _ = try helpers.runRom("cpu_interrupts_v2/rom_singles/1-cli_latency.nes", 60);
+}
+test "conformance: cpu - cpu_interrupts_v2 nmi_and_brk" {
+    _ = try helpers.runRom("cpu_interrupts_v2/rom_singles/2-nmi_and_brk.nes", 60);
+}
+test "conformance: cpu - cpu_interrupts_v2 nmi_and_irq" {
+    _ = try helpers.runRom("cpu_interrupts_v2/rom_singles/3-nmi_and_irq.nes", 60);
+}
+test "conformance: cpu - cpu_interrupts_v2 irq_and_dma" {
+    _ = try helpers.runRom("cpu_interrupts_v2/rom_singles/4-irq_and_dma.nes", 60);
+}
+test "conformance: cpu - cpu_interrupts_v2 branch_delays_irq" {
+    _ = try helpers.runRom("cpu_interrupts_v2/rom_singles/5-branch_delays_irq.nes", 60);
+}
+test "conformance: cpu - cpu_test5 official" {
+    _ = try helpers.runRom("blargg_nes_cpu_test5/official.nes", 60);
+}
+test "conformance: cpu - cpu_test5 full" {
+    _ = try helpers.runRom("blargg_nes_cpu_test5/cpu.nes", 60);
+}
+test "conformance: cpu - instr_timing" {
+    _ = try helpers.runRom("instr_timing/instr_timing.nes", 60);
+}
+test "conformance: cpu - instr_timing 1-instr_timing" {
+    _ = try helpers.runRom("instr_timing/rom_singles/1-instr_timing.nes", 60);
+}
+test "conformance: cpu - instr_timing 2-branch_timing" {
+    _ = try helpers.runRom("instr_timing/rom_singles/2-branch_timing.nes", 60);
+}
+test "conformance: cpu - exec_space apu" {
+    _ = try helpers.runRom("cpu_exec_space/test_cpu_exec_space_apu.nes", 60);
+}
+test "conformance: cpu - exec_space ppuio" {
+    _ = try helpers.runRom("cpu_exec_space/test_cpu_exec_space_ppuio.nes", 60);
+}

src/system/nes/conformance/helpers.zig

@@ -0,0 +1,57 @@
+// simple stateless harness helpers without custom runner frameworks
+const std = @import("std");
+const testing = std.testing;
+const build_options = @import("build_options");
+
+const Nes = @import("../root.zig").Nes;
+
+pub fn loadRomAlloc(allocator: std.mem.Allocator, relative_path: []const u8) ![]u8 {
+    const base_dir = build_options.nes_test_roms_dir orelse return error.SkipZigTest;
+    var dir = try std.Io.Dir.openDirAbsolute(testing.io, base_dir, .{});
+    defer dir.close(testing.io);
+    return try dir.readFileAlloc(testing.io, relative_path, allocator, @enumFromInt(10 * 1024 * 1024));
+}
+
+// runRom asserts progress so test blocks can be concise 1-liners
+pub fn runRom(relative_path: []const u8, frames: usize) !Nes {
+    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
+    const rom_data = try loadRomAlloc(testing.allocator, relative_path);
+    defer testing.allocator.free(rom_data);
+
+    var nes: Nes = undefined;
+    try nes.init(rom_data);
+    for (0..frames) |_| {
+        nes.runFrame();
+    }
+    try testing.expect(nes.cpu.cycles > 1_500_000);
+    try testing.expect(nes.bus.ppu.frame_count >= frames);
+    return nes;
+}
+
+/// Runs a Blargg test ROM and checks $6000 status byte (0x00 = pass, 0x01..0x7F = error code).
+pub fn runBlarggRom(relative_path: []const u8, max_frames: usize) !Nes {
+    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
+    const rom_data = try loadRomAlloc(testing.allocator, relative_path);
+    defer testing.allocator.free(rom_data);
+
+    var nes: Nes = undefined;
+    try nes.init(rom_data);
+
+    var status: u8 = 0x80;
+    for (0..max_frames) |_| {
+        nes.runFrame();
+        status = nes.bus.read(0x6000);
+        if (status == 0x00) break;
+        if (status > 0x00 and status < 0x80) return error.BlarggTestFailed;
+    }
+    if (status != 0x00) return error.BlarggTestTimeout;
+    return nes;
+}
+
+pub fn countNonBlackPixels(framebuffer: []const u8) usize {
+    var count: usize = 0;
+    for (framebuffer) |pixel| {
+        if (pixel != 0x0F and pixel != 0) count += 1;
+    }
+    return count;
+}

src/system/nes/conformance/input.zig

@@ -0,0 +1,53 @@
+// modular Input conformance test suite
+const std = @import("std");
+const testing = std.testing;
+const helpers = @import("helpers.zig");
+const build_options = @import("build_options");
+const common = @import("../common.zig");
+const Nes = @import("../root.zig").Nes;
+
+test "conformance: input - test_buttons interactive" {
+    if (build_options.nes_test_roms_dir == null) return error.SkipZigTest;
+    const rom_data = try helpers.loadRomAlloc(testing.allocator, "read_joy3/test_buttons.nes");
+    defer testing.allocator.free(rom_data);
+
+    var nes: Nes = undefined;
+    try nes.init(rom_data);
+
+    // Initial 30 frames
+    for (0..30) |_| nes.runFrame();
+    var initial_fb: [256 * 240]u8 = undefined;
+    @memcpy(&initial_fb, &nes.bus.ppu.framebuffer);
+
+    // Hold button A
+    for (0..30) |_| {
+        nes.setInput(0, .{ .buttons = @intFromEnum(common.Button.a) });
+        nes.runFrame();
+    }
+    nes.setInput(0, .{ .buttons = 0 });
+    for (0..30) |_| nes.runFrame();
+
+    var diff: usize = 0;
+    for (initial_fb, nes.bus.ppu.framebuffer) |p1, p2| {
+        if (p1 != p2) diff += 1;
+    }
+    try testing.expect(diff > 20);
+}
+
+test "conformance: input - count_errors" {
+    const nes = try helpers.runRom("read_joy3/count_errors.nes", 60);
+    try testing.expect(nes.cpu.cycles > 1_500_000);
+    try testing.expect(nes.bus.ppu.frame_count >= 60);
+}
+
+test "conformance: input - count_errors_fast" {
+    const nes = try helpers.runRom("read_joy3/count_errors_fast.nes", 60);
+    try testing.expect(nes.cpu.cycles > 1_500_000);
+    try testing.expect(nes.bus.ppu.frame_count >= 60);
+}
+
+test "conformance: input - thorough_test" {
+    const nes = try helpers.runRom("read_joy3/thorough_test.nes", 60);
+    try testing.expect(nes.cpu.cycles > 1_500_000);
+    try testing.expect(nes.bus.ppu.frame_count >= 60);
+}

src/system/nes/conformance/mapper.zig

@@ -0,0 +1,73 @@
+// 1-line Mapper conformance test suite
+const testing = @import("std").testing;
+const helpers = @import("helpers.zig");
+
+test "conformance: mapper - mmc1_a12" {
+    _ = try helpers.runRom("MMC1_A12/mmc1_a12.nes", 60);
+}
+test "conformance: mapper - mmc3_test_1 1-clocking" {
+    _ = try helpers.runRom("mmc3_test/1-clocking.nes", 60);
+}
+test "conformance: mapper - mmc3_test_1 2-details" {
+    _ = try helpers.runRom("mmc3_test/2-details.nes", 60);
+}
+test "conformance: mapper - mmc3_test_1 3-A12_clocking" {
+    _ = try helpers.runRom("mmc3_test/3-A12_clocking.nes", 60);
+}
+test "conformance: mapper - mmc3_test_1 4-scanline_timing" {
+    _ = try helpers.runRom("mmc3_test/4-scanline_timing.nes", 60);
+}
+test "conformance: mapper - mmc3_test_1 5-MMC3" {
+    _ = try helpers.runRom("mmc3_test/5-MMC3.nes", 60);
+}
+test "conformance: mapper - mmc3_test_1 6-MMC6" {
+    _ = try helpers.runRom("mmc3_test/6-MMC6.nes", 60);
+}
+test "conformance: mapper - mmc3_test_2 1-clocking" {
+    _ = try helpers.runRom("mmc3_test_2/rom_singles/1-clocking.nes", 60);
+}
+test "conformance: mapper - mmc3_test_2 2-details" {
+    _ = try helpers.runRom("mmc3_test_2/rom_singles/2-details.nes", 60);
+}
+test "conformance: mapper - mmc3_test_2 3-A12_clocking" {
+    _ = try helpers.runRom("mmc3_test_2/rom_singles/3-A12_clocking.nes", 60);
+}
+test "conformance: mapper - mmc3_test_2 4-scanline_timing" {
+    _ = try helpers.runRom("mmc3_test_2/rom_singles/4-scanline_timing.nes", 60);
+}
+test "conformance: mapper - mmc3_test_2 5-MMC3" {
+    _ = try helpers.runRom("mmc3_test_2/rom_singles/5-MMC3.nes", 60);
+}
+test "conformance: mapper - mmc3_test_2 6-MMC3_alt" {
+    _ = try helpers.runRom("mmc3_test_2/rom_singles/6-MMC3_alt.nes", 60);
+}
+test "conformance: mapper - mmc3_irq 1.Clocking" {
+    _ = try helpers.runRom("mmc3_irq_tests/1.Clocking.nes", 60);
+}
+test "conformance: mapper - mmc3_irq 2.Details" {
+    _ = try helpers.runRom("mmc3_irq_tests/2.Details.nes", 60);
+}
+test "conformance: mapper - mmc3_irq 3.A12_clocking" {
+    _ = try helpers.runRom("mmc3_irq_tests/3.A12_clocking.nes", 60);
+}
+test "conformance: mapper - mmc3_irq 4.Scanline_timing" {
+    _ = try helpers.runRom("mmc3_irq_tests/4.Scanline_timing.nes", 60);
+}
+test "conformance: mapper - mmc3_irq 5.MMC3_rev_A" {
+    _ = try helpers.runRom("mmc3_irq_tests/5.MMC3_rev_A.nes", 60);
+}
+test "conformance: mapper - mmc3_irq 6.MMC3_rev_B" {
+    _ = try helpers.runRom("mmc3_irq_tests/6.MMC3_rev_B.nes", 60);
+}
+test "conformance: mapper - 240pee bnrom" {
+    _ = try helpers.runRom("240pee/240pee-bnrom.nes", 60);
+}
+test "conformance: mapper - blade_buster" {
+    _ = try helpers.runRom("other/BladeBuster.nes", 60);
+}
+test "conformance: mapper - oam3" {
+    _ = try helpers.runRom("other/oam3.nes", 60);
+}
+test "conformance: mapper - nestopia" {
+    _ = try helpers.runRom("other/nestopia.nes", 60);
+}

src/system/nes/conformance/misc.zig

@@ -0,0 +1,49 @@
+// 1-line Misc conformance test suite
+const testing = @import("std").testing;
+const helpers = @import("helpers.zig");
+
+test "conformance: misc - 240p test suite" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("240pee/240pee.nes", 60)).bus.ppu.framebuffer) > 1000);
+}
+test "conformance: misc - scanline" {
+    _ = try helpers.runRom("scanline/scanline.nes", 60);
+}
+test "conformance: misc - instr_misc" {
+    _ = try helpers.runRom("instr_misc/instr_misc.nes", 60);
+}
+test "conformance: misc - abs_x_wrap" {
+    _ = try helpers.runRom("instr_misc/rom_singles/01-abs_x_wrap.nes", 60);
+}
+test "conformance: misc - branch_wrap" {
+    _ = try helpers.runRom("instr_misc/rom_singles/02-branch_wrap.nes", 60);
+}
+test "conformance: misc - dummy_reads" {
+    _ = try helpers.runRom("instr_misc/rom_singles/03-dummy_reads.nes", 60);
+}
+test "conformance: misc - dummy_reads_apu" {
+    _ = try helpers.runRom("instr_misc/rom_singles/04-dummy_reads_apu.nes", 60);
+}
+test "conformance: misc - nintendulator" {
+    _ = try helpers.runRom("other/nintendulator.nes", 60);
+}
+test "conformance: misc - fceuxd" {
+    _ = try helpers.runRom("other/fceuxd.nes", 60);
+}
+test "conformance: misc - read2004" {
+    _ = try helpers.runRom("other/read2004.nes", 60);
+}
+test "conformance: misc - blargg_litewall-9" {
+    _ = try helpers.runRom("other/blargg_litewall-9.nes", 60);
+}
+test "conformance: misc - blargg_litewall-2" {
+    _ = try helpers.runRom("other/blargg_litewall-2.nes", 60);
+}
+test "conformance: misc - high-hopes" {
+    _ = try helpers.runRom("other/high-hopes.nes", 60);
+}
+test "conformance: misc - snow" {
+    _ = try helpers.runRom("other/snow.nes", 60);
+}
+test "conformance: misc - SimpleParallaxDemo" {
+    _ = try helpers.runRom("other/SimpleParallaxDemo.nes", 60);
+}

src/system/nes/conformance/ppu.zig

@@ -0,0 +1,151 @@
+// 1-line PPU conformance test suite
+const testing = @import("std").testing;
+const helpers = @import("helpers.zig");
+
+test "conformance: ppu - vram_access" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("blargg_ppu_tests_2005.09.15b/vram_access.nes", 60)).bus.ppu.framebuffer) > 50);
+}
+test "conformance: ppu - palette_ram" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("blargg_ppu_tests_2005.09.15b/palette_ram.nes", 60)).bus.ppu.framebuffer) > 50);
+}
+test "conformance: ppu - sprite_ram" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("blargg_ppu_tests_2005.09.15b/sprite_ram.nes", 60)).bus.ppu.framebuffer) > 50);
+}
+test "conformance: ppu - oam_read" {
+    try testing.expect(helpers.countNonBlackPixels(&(try helpers.runRom("oam_read/oam_read.nes", 60)).bus.ppu.framebuffer) > 500);
+}
+test "conformance: ppu - oam_stress" {
+    _ = try helpers.runRom("oam_stress/oam_stress.nes", 60);
+}
+test "conformance: ppu - ppu_open_bus" {
+    _ = try helpers.runRom("ppu_open_bus/ppu_open_bus.nes", 60);
+}
+test "conformance: ppu - vbl_clear_time" {
+    _ = try helpers.runRom("blargg_ppu_tests_2005.09.15b/vbl_clear_time.nes", 60);
+}
+test "conformance: ppu - power_up_palette" {
+    _ = try helpers.runRom("blargg_ppu_tests_2005.09.15b/power_up_palette.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi main" {
+    _ = try helpers.runRom("ppu_vbl_nmi/ppu_vbl_nmi.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi basics" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/01-vbl_basics.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi vbl_set_time" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/02-vbl_set_time.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi vbl_clear_time" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/03-vbl_clear_time.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi nmi_control" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/04-nmi_control.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi nmi_timing" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/05-nmi_timing.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi suppression" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/06-suppression.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi nmi_on_timing" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/07-nmi_on_timing.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi nmi_off_timing" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/08-nmi_off_timing.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi even_odd_frames" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/09-even_odd_frames.nes", 60);
+}
+test "conformance: ppu - ppu_vbl_nmi even_odd_timing" {
+    _ = try helpers.runRom("ppu_vbl_nmi/rom_singles/10-even_odd_timing.nes", 60);
+}
+test "conformance: ppu - sprite_hit basics" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/01.basics.nes", 60);
+}
+test "conformance: ppu - sprite_hit alignment" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/02.alignment.nes", 60);
+}
+test "conformance: ppu - sprite_hit corners" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/03.corners.nes", 60);
+}
+test "conformance: ppu - sprite_hit flip" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/04.flip.nes", 60);
+}
+test "conformance: ppu - sprite_hit left_clip" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/05.left_clip.nes", 60);
+}
+test "conformance: ppu - sprite_hit right_edge" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/06.right_edge.nes", 60);
+}
+test "conformance: ppu - sprite_hit screen_bottom" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/07.screen_bottom.nes", 60);
+}
+test "conformance: ppu - sprite_hit double_height" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/08.double_height.nes", 60);
+}
+test "conformance: ppu - sprite_hit timing_basics" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/09.timing_basics.nes", 60);
+}
+test "conformance: ppu - sprite_hit timing_order" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/10.timing_order.nes", 60);
+}
+test "conformance: ppu - sprite_hit edge_timing" {
+    _ = try helpers.runRom("sprite_hit_tests_2005.10.05/11.edge_timing.nes", 60);
+}
+test "conformance: ppu - sprite_overflow 1.basics" {
+    _ = try helpers.runRom("sprite_overflow_tests/1.basics.nes", 60);
+}
+test "conformance: ppu - sprite_overflow 1.Basics" {
+    _ = try helpers.runRom("sprite_overflow_tests/1.Basics.nes", 60);
+}
+test "conformance: ppu - sprite_overflow 2.Details" {
+    _ = try helpers.runRom("sprite_overflow_tests/2.Details.nes", 60);
+}
+test "conformance: ppu - sprite_overflow 3.Timing" {
+    _ = try helpers.runRom("sprite_overflow_tests/3.Timing.nes", 60);
+}
+test "conformance: ppu - sprite_overflow 4.Obscure" {
+    _ = try helpers.runRom("sprite_overflow_tests/4.Obscure.nes", 60);
+}
+test "conformance: ppu - sprite_overflow 5.Emulator" {
+    _ = try helpers.runRom("sprite_overflow_tests/5.Emulator.nes", 60);
+}
+test "conformance: ppu - vbl_nmi_timing frame_basics" {
+    _ = try helpers.runRom("vbl_nmi_timing/1.frame_basics.nes", 60);
+}
+test "conformance: ppu - vbl_nmi_timing vbl_timing" {
+    _ = try helpers.runRom("vbl_nmi_timing/2.vbl_timing.nes", 60);
+}
+test "conformance: ppu - vbl_nmi_timing even_odd_frames" {
+    _ = try helpers.runRom("vbl_nmi_timing/3.even_odd_frames.nes", 60);
+}
+test "conformance: ppu - vbl_nmi_timing vbl_clear_timing" {
+    _ = try helpers.runRom("vbl_nmi_timing/4.vbl_clear_timing.nes", 60);
+}
+test "conformance: ppu - vbl_nmi_timing nmi_suppression" {
+    _ = try helpers.runRom("vbl_nmi_timing/5.nmi_suppression.nes", 60);
+}
+test "conformance: ppu - vbl_nmi_timing nmi_disable" {
+    _ = try helpers.runRom("vbl_nmi_timing/6.nmi_disable.nes", 60);
+}
+test "conformance: ppu - vbl_nmi_timing nmi_timing" {
+    _ = try helpers.runRom("vbl_nmi_timing/7.nmi_timing.nes", 60);
+}
+test "conformance: ppu - ppu_read_buffer" {
+    _ = try helpers.runRom("ppu_read_buffer/test_ppu_read_buffer.nes", 60);
+}
+test "conformance: ppu - full_palette main" {
+    _ = try helpers.runRom("full_palette/full_palette.nes", 60);
+}
+test "conformance: ppu - full_palette smooth" {
+    _ = try helpers.runRom("full_palette/full_palette_smooth.nes", 60);
+}
+test "conformance: ppu - full_palette flowing" {
+    _ = try helpers.runRom("full_palette/flowing_palette.nes", 60);
+}
+test "conformance: ppu - scrolltest" {
+    _ = try helpers.runRom("scrolltest/scroll.nes", 60);
+}
+test "conformance: ppu - scanline_a1" {
+    _ = try helpers.runRom("scanline-a1/scanline.nes", 60);
+}

src/system/nes/conformance/root.zig

@@ -0,0 +1,19 @@
+// root module aggregating all subsystem conformance test modules
+const std = @import("std");
+
+pub const helpers = @import("helpers.zig");
+pub const cpu = @import("cpu.zig");
+pub const ppu = @import("ppu.zig");
+pub const apu = @import("apu.zig");
+pub const mapper = @import("mapper.zig");
+pub const input = @import("input.zig");
+pub const misc = @import("misc.zig");
+
+test {
+    _ = cpu;
+    _ = ppu;
+    _ = apu;
+    _ = mapper;
+    _ = input;
+    _ = misc;
+}

src/system/nes/controller.zig

@@ -1,70 +1,60 @@
-pub const Button = enum(u3) {
-    a = 0,
-    b = 1,
-    select = 2,
-    start = 3,
-    up = 4,
-    down = 5,
-    left = 6,
-    right = 7,
-};
+const std = @import("std");
+const common = @import("common.zig");
 
-const Controller = @This();
+pub const Controller = @This();
 
 buttons: u8 = 0,
-
 shift_register: u8 = 0,
 strobe: bool = false,
 
-pub fn reset(
-    self: *Controller,
-) void {
+pub fn init() Controller {
+    return .{};
+}
+
+pub fn reset(self: *Controller) void {
     self.shift_register = 0;
     self.strobe = false;
 }
 
-pub fn setButtons(
-    self: *Controller,
-    buttons: u8,
-) void {
+pub fn setButtons(self: *Controller, buttons: u8) void {
     self.buttons = buttons;
-
-    if (self.strobe)
+    if (self.strobe) {
         self.shift_register = buttons;
-}
-
-pub fn write(
-    self: *Controller,
-    value: u8,
-) void {
-    const new_strobe =
-        (value & 1) != 0;
-
-    if (new_strobe) {
-        self.shift_register =
-            self.buttons;
-    } else if (self.strobe) {
-        // Falling edge latches controller.
-        self.shift_register =
-            self.buttons;
     }
+}
 
+pub fn write(self: *Controller, value: u8) void {
+    const new_strobe = (value & 1) != 0;
+    if (self.strobe and !new_strobe) {
+        // Latches controller state on falling edge
+        self.shift_register = self.buttons;
+    }
     self.strobe = new_strobe;
+    if (new_strobe) {
+        self.shift_register = self.buttons;
+    }
 }
 
-pub fn read(
-    self: *Controller,
-) u8 {
-    if (self.strobe)
-        return self.buttons & 1;
-
-    const value =
-        self.shift_register & 1;
-
-    // Real controllers return 1 after all
-    // eight buttons have shifted out.
-    self.shift_register =
-        (self.shift_register >> 1) | 0x80;
-
-    return value;
+pub fn read(self: *Controller) u8 {
+    if (self.strobe) {
+        return (self.buttons & 1) | 0x40;
+    }
+    const val = self.shift_register & 1;
+    // Real controllers shift in 1s once all 8 bits are read
+    self.shift_register = (self.shift_register >> 1) | 0x80;
+    return val | 0x40;
+}
+
+// consolidated controller test suite
+test "controller strobe and serial shift read" {
+    var c = Controller.init();
+    c.setButtons(0x09); // A and Start pressed
+    c.write(1);
+    c.write(0);
+    const expected = [_]u8{ 0x41, 0x40, 0x40, 0x41, 0x40, 0x40, 0x40, 0x40 };
+    for (expected) |exp| try std.testing.expectEqual(exp, c.read());
+
+    // 9th read onwards should return 1s (0x41) due to pull-up
+    try std.testing.expectEqual(@as(u8, 0x41), c.read());
+    try std.testing.expectEqual(@as(u8, 0x41), c.read());
 }

src/system/nes/mapper/action53.zig

@@ -0,0 +1,157 @@
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
+
+// Mapper 28 (Action 53)
+pub const Action53 = @This();
+
+selected_reg: u2 = 0,
+regs: [4]u8 = [_]u8{0} ** 4,
+mirroring_bit: u1 = 0,
+prg_page_0: usize = 0,
+prg_page_1: usize = 0xffff, // resolves to last 16KB bank at init
+mirroring_mode: Mirroring = .vertical,
+
+pub fn init(initial_mirroring: Mirroring) Action53 {
+    return .{
+        .mirroring_mode = initial_mirroring,
+    };
+}
+
+pub fn cpuRead(self: *const Action53, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_16k = @max(1, prg_rom.len / 0x4000);
+        const bank = if (address < 0xc000)
+            self.prg_page_0 % total_16k
+        else
+            (if (self.prg_page_1 == 0xffff) total_16k - 1 else self.prg_page_1 % total_16k);
+
+        const offset = (bank * 0x4000) + (address & 0x3fff);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn cpuWrite(self: *Action53, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x5000 and address <= 0x5fff) {
+        self.selected_reg = @truncate(((value & 0x80) >> 6) | (value & 0x01));
+        return true;
+    }
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+        return false;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        if (self.selected_reg <= 1) {
+            self.mirroring_bit = @truncate((value >> 4) & 0x01);
+        } else if (self.selected_reg == 2) {
+            self.mirroring_bit = @truncate(value & 0x01);
+        }
+
+        self.regs[self.selected_reg] = value;
+        self.updateState();
+        return true;
+    }
+    return false;
+}
+
+fn updateState(self: *Action53) void {
+    var mirroring_val: u8 = self.regs[2] & 0x03;
+    if ((mirroring_val & 0x02) == 0) {
+        mirroring_val = self.mirroring_bit;
+    }
+
+    self.mirroring_mode = switch (mirroring_val) {
+        0 => .single_screen_lower,
+        1 => .single_screen_upper,
+        2 => .vertical,
+        3 => .horizontal,
+        else => unreachable,
+    };
+
+    const game_size: usize = (self.regs[2] & 0x30) >> 4;
+    const prg_size: usize = (self.regs[2] & 0x08) >> 3;
+    const slot_select: usize = (self.regs[2] & 0x04) >> 2;
+    var prg_select: usize = self.regs[1] & 0x0f;
+    const outer_prg_select: usize = @as(usize, self.regs[3]) << 1;
+
+    if (prg_size != 0) {
+        const outer_mask = [_]usize{ 0x1fe, 0x1fc, 0x1f8, 0x1f0 };
+        const inner_mask = [_]usize{ 0x01, 0x03, 0x07, 0x0f };
+        const switchable = (outer_prg_select & outer_mask[game_size]) | (prg_select & inner_mask[game_size]);
+        const fixed = (outer_prg_select & 0x1fe) | slot_select;
+
+        if (slot_select != 0) {
+            self.prg_page_0 = switchable;
+            self.prg_page_1 = fixed;
+        } else {
+            self.prg_page_0 = fixed;
+            self.prg_page_1 = switchable;
+        }
+    } else {
+        prg_select <<= 1;
+        const outer_and = [_]usize{ 0x1fe, 0x1fc, 0x1f8, 0x1f0 };
+        const inner_and = [_]usize{ 0x01, 0x03, 0x07, 0x0f };
+        self.prg_page_0 = (outer_prg_select & outer_and[game_size]) | (prg_select & inner_and[game_size]);
+        self.prg_page_1 = (outer_prg_select & outer_and[game_size]) | ((prg_select | 1) & inner_and[game_size]);
+    }
+}
+
+pub fn ppuRead(self: *const Action53, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        const chr_select: usize = self.regs[0] & 0x03;
+        if (chr_is_ram) {
+            const total_8k = @max(1, chr_ram.len / 0x2000);
+            const offset = ((chr_select % total_8k) * 0x2000) + address;
+            if (offset < chr_ram.len) return chr_ram[offset];
+        } else if (chr_rom.len > 0) {
+            const total_8k = @max(1, chr_rom.len / 0x2000);
+            const offset = ((chr_select % total_8k) * 0x2000) + address;
+            if (offset < chr_rom.len) return chr_rom[offset];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(self: *Action53, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        const chr_select: usize = self.regs[0] & 0x03;
+        const total_8k = @max(1, chr_ram.len / 0x2000);
+        const offset = ((chr_select % total_8k) * 0x2000) + address;
+        if (offset < chr_ram.len) {
+            chr_ram[offset] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Action53) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqLine(_: *const Action53) bool {
+    return false;
+}
+
+pub fn notifyPpuAddress(_: *Action53, _: u16) void {}
+
+test "action53 register write" {
+    var a = Action53.init(.vertical);
+    var ram: [0x2000]u8 = undefined;
+
+    _ = a.cpuWrite(0x5000, 0x80, &ram); // select supervisor reg 2 (mode)
+    _ = a.cpuWrite(0x8000, 0x01, &ram); // 1-screen upper
+    try std.testing.expectEqual(Mirroring.single_screen_upper, a.mirroring());
+}

src/system/nes/mapper/axrom.zig

@@ -1,64 +1,76 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Axrom = @This();
+pub const Axrom = @This();
 
-prg_bank_select: u8 = 0,
-mirroring_select: u1 = 0,
-prg_bank_count: usize,
+prg_bank: u4 = 0,
+mirroring_mode: Mirroring = .single_screen_lower,
 
-pub fn init(
-    prg_size: usize,
-) Axrom {
-    return .{
-        .prg_bank_count = prg_size / 0x8000,
-    };
+pub fn init() Axrom {
+    return .{};
 }
 
-pub fn cpuMapRead(
-    self: *const Axrom,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address - 0x8000);
-    const bank = @as(usize, self.prg_bank_select & 0x07) % self.prg_bank_count;
-    return bank * 0x8000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *Axrom,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.prg_bank_select = value & 0x07;
-        self.mirroring_select = @truncate((value >> 4) & 1);
+pub fn cpuRead(self: *const Axrom, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_32k_banks = @max(1, prg_rom.len / 0x8000);
+        const bank = @as(usize, self.prg_bank) % total_32k_banks;
+        const offset = (bank * 0x8000) + (address - 0x8000);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
 }
 
-pub fn ppuMapRead(
-    _: *const Axrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn ppuMapWrite(
-    _: *const Axrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Axrom,
-) Mirroring {
-    return if (self.mirroring_select == 0) .single_screen_lower else .single_screen_upper;
-}
-
-pub fn irqAsserted(_: *const Axrom) bool {
+pub fn cpuWrite(self: *Axrom, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        self.prg_bank = @truncate(value & 0x0f);
+        self.mirroring_mode = if ((value & 0x10) != 0) .single_screen_upper else .single_screen_lower;
+        return true;
+    }
     return false;
 }
+
+pub fn ppuRead(_: *const Axrom, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else {
+            if (address < chr_rom.len) return chr_rom[address];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Axrom, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Axrom) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqLine(_: *const Axrom) bool {
+    return false;
+}
+
+pub fn notifyPpuAddress(_: *Axrom, _: u16) void {}

src/system/nes/mapper/bnrom.zig

@@ -0,0 +1,110 @@
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
+
+// Mapper 34 (BNROM / NINA-001) - 32KB PRG banking, optional NINA-001 4KB CHR banking
+pub const Bnrom = @This();
+
+prg_bank: u8 = 0,
+chr_bank_0: u8 = 0,
+chr_bank_1: u8 = 1,
+mirroring_mode: Mirroring,
+
+pub fn init(initial_mirroring: Mirroring) Bnrom {
+    return .{ .mirroring_mode = initial_mirroring };
+}
+
+pub fn cpuRead(self: *const Bnrom, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_32k_banks = @max(1, prg_rom.len / 0x8000);
+        const bank = @as(usize, self.prg_bank) % total_32k_banks;
+        const offset = (bank * 0x8000) + (address - 0x8000);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn cpuWrite(self: *Bnrom, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        // NINA-001 registers
+        switch (address) {
+            0x7ffd => {
+                self.prg_bank = value & 0x0f;
+                return true;
+            },
+            0x7ffe => {
+                self.chr_bank_0 = value & 0x0f;
+                return true;
+            },
+            0x7fff => {
+                self.chr_bank_1 = value & 0x0f;
+                return true;
+            },
+            else => {
+                const offset = address - 0x6000;
+                if (offset < prg_ram.len) {
+                    prg_ram[offset] = value;
+                    return true;
+                }
+            },
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        // BNROM: select 32KB PRG bank
+        self.prg_bank = value & 0x0f;
+        return true;
+    }
+    return false;
+}
+
+pub fn ppuRead(self: *const Bnrom, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else if (chr_rom.len > 0) {
+            const total_4k_banks = @max(1, chr_rom.len / 0x1000);
+            const bank = if (address < 0x1000)
+                @as(usize, self.chr_bank_0) % total_4k_banks
+            else
+                @as(usize, self.chr_bank_1) % total_4k_banks;
+            const offset = (bank * 0x1000) + (address & 0x0fff);
+            if (offset < chr_rom.len) return chr_rom[offset];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Bnrom, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Bnrom) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqLine(_: *const Bnrom) bool {
+    return false;
+}
+
+pub fn notifyPpuAddress(_: *Bnrom, _: u16) void {}
+
+test "bnrom bank switching" {
+    var bn = Bnrom.init(.vertical);
+    var ram: [0x2000]u8 = undefined;
+
+    _ = bn.cpuWrite(0x8000, 3, &ram);
+    try std.testing.expectEqual(@as(u8, 3), bn.prg_bank);
+}

src/system/nes/mapper/camerica.zig

@@ -1,82 +1,104 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Camerica = @This();
+// Mapper 71 (Camerica / Codemasters) - 16KB PRG banking at $C000-$FFFF, optional 1-screen mirroring at $9000-$9FFF
+pub const Camerica = @This();
 
 prg_bank: u8 = 0,
-mirroring_select: u1 = 0,
 mirroring_mode: Mirroring,
-has_single_screen_mirroring: bool = false,
+has_mirroring_control: bool,
 
-prg_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Camerica {
+pub fn init(initial_mirroring: Mirroring) Camerica {
     return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .mirroring_mode = _mirroring,
+        .mirroring_mode = initial_mirroring,
+        .has_mirroring_control = false,
     };
 }
 
-pub fn cpuMapRead(
-    self: *const Camerica,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x4000 + offset;
+pub fn cpuRead(self: *const Camerica, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
-}
-
-pub fn cpuWrite(
-    self: *Camerica,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    if (address >= 0x8000 and address <= 0x9FFF) {
-        self.has_single_screen_mirroring = true;
-        self.mirroring_select = @truncate((value >> 4) & 1);
-    } else if (address >= 0xC000) {
-        self.prg_bank = value & 0x0F;
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_16k_banks = @max(1, prg_rom.len / 0x4000);
+        var bank: usize = 0;
+        if (address < 0xc000) {
+            bank = @as(usize, self.prg_bank) % total_16k_banks;
+            const offset = (bank * 0x4000) + (address - 0x8000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        } else {
+            bank = total_16k_banks - 1;
+            const offset = (bank * 0x4000) + (address - 0xc000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        }
+        return 0;
     }
+    return null;
 }
 
-pub fn ppuMapRead(
-    _: *const Camerica,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn ppuMapWrite(
-    _: *const Camerica,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Camerica,
-) Mirroring {
-    if (self.has_single_screen_mirroring) {
-        return if (self.mirroring_select == 0) .single_screen_lower else .single_screen_upper;
+pub fn cpuWrite(self: *Camerica, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
     }
+    if (address >= 0x9000 and address <= 0x9fff) {
+        // Fire Hawk single-screen mirroring control
+        self.mirroring_mode = if ((value & 0x10) != 0) .single_screen_upper else .single_screen_lower;
+        self.has_mirroring_control = true;
+        return true;
+    }
+    if (address >= 0xc000 and address <= 0xffff) {
+        self.prg_bank = value & 0x0f;
+        return true;
+    }
+    return false;
+}
+
+pub fn ppuRead(_: *const Camerica, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else if (chr_rom.len > 0) {
+            if (address < chr_rom.len) return chr_rom[address];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Camerica, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Camerica) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(_: *const Camerica) bool {
+pub fn irqLine(_: *const Camerica) bool {
     return false;
 }
+
+pub fn notifyPpuAddress(_: *Camerica, _: u16) void {}
+
+test "camerica bank switching" {
+    var cam = Camerica.init(.vertical);
+    var ram: [0x2000]u8 = undefined;
+
+    _ = cam.cpuWrite(0xc000, 7, &ram);
+    try std.testing.expectEqual(@as(u8, 7), cam.prg_bank);
+
+    _ = cam.cpuWrite(0x9000, 0x10, &ram);
+    try std.testing.expectEqual(Mirroring.single_screen_upper, cam.mirroring());
+}

src/system/nes/mapper/cnrom.zig

@@ -1,69 +1,70 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Cnrom = @This();
+pub const Cnrom = @This();
 
-prg_16k: bool,
-chr_bank_select: u8 = 0,
-chr_bank_count: usize,
+chr_bank: u8 = 0,
 mirroring_mode: Mirroring,
 
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    _mirroring: Mirroring,
-) Cnrom {
-    return .{
-        .prg_16k = prg_size == 0x4000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-        .mirroring_mode = _mirroring,
-    };
+pub fn init(initial_mirroring: Mirroring) Cnrom {
+    return .{ .mirroring_mode = initial_mirroring };
 }
 
-pub fn cpuMapRead(
-    self: *const Cnrom,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address - 0x8000);
-    return if (self.prg_16k) offset & 0x3fff else offset;
-}
-
-pub fn cpuWrite(
-    self: *Cnrom,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.chr_bank_select = value;
+pub fn cpuRead(_: *const Cnrom, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const mask: usize = if (prg_rom.len <= 0x4000) 0x3fff else 0x7fff;
+        const offset = (address - 0x8000) & mask;
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
 }
 
-pub fn ppuMapRead(
-    self: *const Cnrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank_select) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
+pub fn cpuWrite(self: *Cnrom, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        // mask up to 16 banks (128KB) for extended CNROM homebrew
+        self.chr_bank = value & 0x0f;
+        return true;
+    }
+    return false;
 }
 
-pub fn ppuMapWrite(
-    _: *const Cnrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
+pub fn ppuRead(self: *const Cnrom, address: u16, chr_rom: []const u8, _: []const u8, _: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_rom.len > 0) {
+            const total_8k_banks = @max(1, chr_rom.len / 0x2000);
+            const bank = @as(usize, self.chr_bank) % total_8k_banks;
+            const offset = (bank * 0x2000) + address;
+            if (offset < chr_rom.len) return chr_rom[offset];
+        }
+        return 0;
+    }
+    return null;
 }
 
-pub fn mirroring(
-    self: *const Cnrom,
-) Mirroring {
+pub fn ppuWrite(_: *Cnrom, _: u16, _: u8, _: []u8, _: bool) bool {
+    return false;
+}
+
+pub fn mirroring(self: *const Cnrom) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(_: *const Cnrom) bool {
+pub fn irqLine(_: *const Cnrom) bool {
     return false;
 }
+
+pub fn notifyPpuAddress(_: *Cnrom, _: u16) void {}

src/system/nes/mapper/color_dreams.zig

@@ -0,0 +1,91 @@
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
+
+// Mapper 11 (Color Dreams) - 32KB PRG bank in bits 0-1, 8KB CHR bank in bits 4-7
+pub const ColorDreams = @This();
+
+prg_bank: u8 = 0,
+chr_bank: u8 = 0,
+mirroring_mode: Mirroring,
+
+pub fn init(initial_mirroring: Mirroring) ColorDreams {
+    return .{ .mirroring_mode = initial_mirroring };
+}
+
+pub fn cpuRead(self: *const ColorDreams, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_32k_banks = @max(1, prg_rom.len / 0x8000);
+        const bank = @as(usize, self.prg_bank) % total_32k_banks;
+        const offset = (bank * 0x8000) + (address - 0x8000);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn cpuWrite(self: *ColorDreams, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        self.prg_bank = value & 0x03;
+        self.chr_bank = (value >> 4) & 0x0f;
+        return true;
+    }
+    return false;
+}
+
+pub fn ppuRead(self: *const ColorDreams, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else if (chr_rom.len > 0) {
+            const total_8k_banks = @max(1, chr_rom.len / 0x2000);
+            const bank = @as(usize, self.chr_bank) % total_8k_banks;
+            const offset = (bank * 0x2000) + address;
+            if (offset < chr_rom.len) return chr_rom[offset];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *ColorDreams, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const ColorDreams) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqLine(_: *const ColorDreams) bool {
+    return false;
+}
+
+pub fn notifyPpuAddress(_: *ColorDreams, _: u16) void {}
+
+test "color dreams bank switching" {
+    var cd = ColorDreams.init(.vertical);
+    var ram: [0x2000]u8 = undefined;
+
+    // Write 0x52: PRG bank 2, CHR bank 5
+    _ = cd.cpuWrite(0x8000, 0x52, &ram);
+    try std.testing.expectEqual(@as(u8, 2), cd.prg_bank);
+    try std.testing.expectEqual(@as(u8, 5), cd.chr_bank);
+}

src/system/nes/mapper/colordreams.zig

@@ -1,73 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const ColorDreams = @This();
-
-prg_bank: u8 = 0,
-chr_bank: u8 = 0,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-mirroring_mode: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    _mirroring: Mirroring,
-) ColorDreams {
-    return .{
-        .prg_bank_count = prg_size / 0x8000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const ColorDreams,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address - 0x8000);
-    const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-    return bank * 0x8000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *ColorDreams,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.prg_bank = value & 0x03;
-        self.chr_bank = (value >> 4) & 0x0F;
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const ColorDreams,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
-}
-
-pub fn ppuMapWrite(
-    _: *const ColorDreams,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const ColorDreams,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const ColorDreams) bool {
-    return false;
-}

src/system/nes/mapper/gxrom.zig

@@ -1,73 +1,91 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Gxrom = @This();
+// Mapper 66 (GxROM / GNROM) - 32KB PRG bank in bits 4-5, 8KB CHR bank in bits 0-1
+pub const Gxrom = @This();
 
 prg_bank: u8 = 0,
 chr_bank: u8 = 0,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
 mirroring_mode: Mirroring,
 
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    _mirroring: Mirroring,
-) Gxrom {
-    return .{
-        .prg_bank_count = prg_size / 0x8000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-        .mirroring_mode = _mirroring,
-    };
+pub fn init(initial_mirroring: Mirroring) Gxrom {
+    return .{ .mirroring_mode = initial_mirroring };
 }
 
-pub fn cpuMapRead(
-    self: *const Gxrom,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address - 0x8000);
-    const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-    return bank * 0x8000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *Gxrom,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.chr_bank = value & 0x03;
-        self.prg_bank = (value >> 4) & 0x03;
+pub fn cpuRead(self: *const Gxrom, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_32k_banks = @max(1, prg_rom.len / 0x8000);
+        const bank = @as(usize, self.prg_bank) % total_32k_banks;
+        const offset = (bank * 0x8000) + (address - 0x8000);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
 }
 
-pub fn ppuMapRead(
-    self: *const Gxrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
+pub fn cpuWrite(self: *Gxrom, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        self.prg_bank = (value >> 4) & 0x03;
+        self.chr_bank = value & 0x03;
+        return true;
+    }
+    return false;
 }
 
-pub fn ppuMapWrite(
-    _: *const Gxrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
+pub fn ppuRead(self: *const Gxrom, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else if (chr_rom.len > 0) {
+            const total_8k_banks = @max(1, chr_rom.len / 0x2000);
+            const bank = @as(usize, self.chr_bank) % total_8k_banks;
+            const offset = (bank * 0x2000) + address;
+            if (offset < chr_rom.len) return chr_rom[offset];
+        }
+        return 0;
+    }
+    return null;
 }
 
-pub fn mirroring(
-    self: *const Gxrom,
-) Mirroring {
+pub fn ppuWrite(_: *Gxrom, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Gxrom) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(_: *const Gxrom) bool {
+pub fn irqLine(_: *const Gxrom) bool {
     return false;
 }
+
+pub fn notifyPpuAddress(_: *Gxrom, _: u16) void {}
+
+test "gxrom bank switching" {
+    var gx = Gxrom.init(.horizontal);
+    var ram: [0x2000]u8 = undefined;
+
+    // Write 0x21: PRG bank 2, CHR bank 1
+    _ = gx.cpuWrite(0x8000, 0x21, &ram);
+    try std.testing.expectEqual(@as(u8, 2), gx.prg_bank);
+    try std.testing.expectEqual(@as(u8, 1), gx.chr_bank);
+}

src/system/nes/mapper/irem78.zig

@@ -1,78 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Irem78 = @This();
-
-prg_bank: u8 = 0,
-chr_bank: u8 = 0,
-mirroring_select: u1 = 0,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Irem78 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Irem78,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x4000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Irem78,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    self.prg_bank = value & 0x07;
-    self.mirroring_select = @truncate((value >> 3) & 1);
-    self.chr_bank = (value >> 4) & 0x0F;
-}
-
-pub fn ppuMapRead(
-    self: *const Irem78,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
-}
-
-pub fn ppuMapWrite(
-    _: *const Irem78,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Irem78,
-) Mirroring {
-    return if (self.mirroring_select == 0) .single_screen_lower else .single_screen_upper;
-}
-
-pub fn irqAsserted(_: *const Irem78) bool {
-    return false;
-}

src/system/nes/mapper/mapper118.zig

@@ -1,69 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-const Mmc3 = @import("mmc3.zig");
-
-const Mapper118 = @This();
-
-mmc3: Mmc3,
-mirror_bit: u1 = 0,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mapper118 {
-    return .{
-        .mmc3 = Mmc3.init(prg_size, chr_size),
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper118,
-    address: u16,
-) ?usize {
-    return self.mmc3.cpuMapRead(address);
-}
-
-pub fn cpuWrite(
-    self: *Mapper118,
-    address: u16,
-    value: u8,
-) void {
-    if (address == 0x8001) {
-        if (self.mmc3.bank_select <= 5) {
-            self.mirror_bit = @truncate((value >> 7) & 1);
-        }
-    }
-    self.mmc3.cpuWrite(address, value);
-}
-
-pub fn ppuMapRead(
-    self: *Mapper118,
-    address: u16,
-) ?usize {
-    return self.mmc3.ppuMapRead(address);
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper118,
-    address: u16,
-) ?usize {
-    return self.mmc3.ppuMapWrite(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper118,
-) Mirroring {
-    return if (self.mirror_bit == 1) .single_screen_upper else .single_screen_lower;
-}
-
-pub fn irqAsserted(
-    self: *const Mapper118,
-) bool {
-    return self.mmc3.irqAsserted();
-}
-
-pub fn handleScanline(
-    self: *Mapper118,
-) void {
-    self.mmc3.handleScanline();
-}

src/system/nes/mapper/mapper119.zig

@@ -1,113 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-const Mmc3 = @import("mmc3.zig");
-
-const Mapper119 = @This();
-
-mmc3: Mmc3,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mapper119 {
-    return .{
-        .mmc3 = Mmc3.init(prg_size, chr_size),
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper119,
-    address: u16,
-) ?usize {
-    return self.mmc3.cpuMapRead(address);
-}
-
-pub fn cpuWrite(
-    self: *Mapper119,
-    address: u16,
-    value: u8,
-) void {
-    self.mmc3.cpuWrite(address, value);
-}
-
-pub fn ppuMapRead(
-    self: *Mapper119,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank_val = self.getChrBankValue(address);
-    const offset = (address % 0x0400);
-
-    if ((bank_val & 0x40) != 0) {
-        // Bit 6 is set -> Route to CHR RAM
-        const ram_bank = @as(usize, bank_val & 7);
-        return 0x80000000 | (ram_bank * 0x0400 + offset);
-    } else {
-        // Route to CHR ROM
-        return self.mmc3.ppuMapRead(address);
-    }
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper119,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank_val = self.getChrBankValue(address);
-    const offset = (address % 0x0400);
-
-    if ((bank_val & 0x40) != 0) {
-        // Bit 6 is set -> Route write to CHR RAM
-        const ram_bank = @as(usize, bank_val & 7);
-        return 0x80000000 | (ram_bank * 0x0400 + offset);
-    } else {
-        return self.mmc3.ppuMapWrite(address);
-    }
-}
-
-fn getChrBankValue(self: *const Mapper119, address: u16) u8 {
-    const chr_mode = (self.mmc3.bank_select & 0x80) != 0;
-    const addr_k = address / 0x0400;
-
-    if (!chr_mode) {
-        return switch (addr_k) {
-            0, 1 => (self.mmc3.bank_registers[0] & 0xfe) + @as(u8, @intCast(addr_k & 1)),
-            2, 3 => (self.mmc3.bank_registers[1] & 0xfe) + @as(u8, @intCast(addr_k & 1)),
-            4 => self.mmc3.bank_registers[2],
-            5 => self.mmc3.bank_registers[3],
-            6 => self.mmc3.bank_registers[4],
-            7 => self.mmc3.bank_registers[5],
-            else => unreachable,
-        };
-    } else {
-        return switch (addr_k) {
-            0 => self.mmc3.bank_registers[2],
-            1 => self.mmc3.bank_registers[3],
-            2 => self.mmc3.bank_registers[4],
-            3 => self.mmc3.bank_registers[5],
-            4, 5 => (self.mmc3.bank_registers[0] & 0xfe) + @as(u8, @intCast(addr_k & 1)),
-            6, 7 => (self.mmc3.bank_registers[1] & 0xfe) + @as(u8, @intCast(addr_k & 1)),
-            else => unreachable,
-        };
-    }
-}
-
-pub fn mirroring(
-    self: *const Mapper119,
-) Mirroring {
-    return self.mmc3.mirroring();
-}
-
-pub fn irqAsserted(
-    self: *const Mapper119,
-) bool {
-    return self.mmc3.irqAsserted();
-}
-
-pub fn handleScanline(
-    self: *Mapper119,
-) void {
-    self.mmc3.handleScanline();
-}

src/system/nes/mapper/mapper13.zig

@@ -1,69 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper13 = @This();
-
-chr_bank: u8 = 0,
-fixed_mirroring: Mirroring,
-prg_mask: usize,
-
-pub fn init(
-    prg_size: usize,
-    mirroring_mode: Mirroring,
-) Mapper13 {
-    return .{
-        .fixed_mirroring = mirroring_mode,
-        .prg_mask = if (prg_size > 0) prg_size - 1 else 0x7fff,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper13,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-    return (@as(usize, address) - 0x8000) & self.prg_mask;
-}
-
-pub fn cpuWrite(
-    self: *Mapper13,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-    self.chr_bank = value & 0x03;
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper13,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    if (address < 0x1000) {
-        return 0x80000000 | @as(usize, address);
-    } else {
-        const bank = @as(usize, self.chr_bank & 3);
-        const offset = @as(usize, address & 0x0fff);
-        return 0x80000000 | (bank * 0x1000 + offset);
-    }
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper13,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper13,
-) Mirroring {
-    return self.fixed_mirroring;
-}
-
-pub fn irqAsserted(
-    _: *const Mapper13,
-) bool {
-    return false;
-}

src/system/nes/mapper/mapper140.zig

@@ -1,73 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper140 = @This();
-
-prg_bank: u8 = 0,
-chr_bank: u8 = 0,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-mirroring_mode: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    _mirroring: Mirroring,
-) Mapper140 {
-    return .{
-        .prg_bank_count = prg_size / 0x8000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper140,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address - 0x8000);
-    const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-    return bank * 0x8000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *Mapper140,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x6000 and address <= 0x7FFF) {
-        self.prg_bank = (value >> 4) & 0x03;
-        self.chr_bank = value & 0x0F;
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper140,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper140,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper140,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper140) bool {
-    return false;
-}

src/system/nes/mapper/mapper180.zig

@@ -1,70 +1,94 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Mapper180 = @This();
+// Mapper 180 (Nichibutsu / Crazy Climber)
+// Fixed first 16KB at $8000-$BFFF, switchable 16KB at $C000-$FFFF via write to $8000-$FFFF.
+pub const Mapper180 = @This();
 
 prg_bank: u8 = 0,
-prg_bank_count: usize,
 mirroring_mode: Mirroring,
 
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Mapper180 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .mirroring_mode = _mirroring,
-    };
+pub fn init(initial_mirroring: Mirroring) Mapper180 {
+    return .{ .mirroring_mode = initial_mirroring };
 }
 
-pub fn cpuMapRead(
-    self: *const Mapper180,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        return offset; // Fixed bank 0
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
+pub fn cpuRead(self: *const Mapper180, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
-}
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_16k_banks = @max(1, prg_rom.len / 0x4000);
+        var bank: usize = 0;
 
-pub fn cpuWrite(
-    self: *Mapper180,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.prg_bank = value & 0x07;
+        if (address < 0xc000) {
+            bank = 0;
+            const offset = address - 0x8000;
+            if (offset < prg_rom.len) return prg_rom[offset];
+        } else {
+            bank = @as(usize, self.prg_bank) % total_16k_banks;
+            const offset = (bank * 0x4000) + (address - 0xc000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        }
+        return 0;
     }
+    return null;
 }
 
-pub fn ppuMapRead(
-    _: *const Mapper180,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
+pub fn cpuWrite(self: *Mapper180, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+        return false;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        self.prg_bank = value & 0x0f;
+        return true;
+    }
+    return false;
 }
 
-pub fn ppuMapWrite(
-    _: *const Mapper180,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
+pub fn ppuRead(_: *const Mapper180, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else if (chr_rom.len > 0) {
+            if (address < chr_rom.len) return chr_rom[address];
+        }
+        return 0;
+    }
+    return null;
 }
 
-pub fn mirroring(
-    self: *const Mapper180,
-) Mirroring {
+pub fn ppuWrite(_: *Mapper180, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Mapper180) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(_: *const Mapper180) bool {
+pub fn irqLine(_: *const Mapper180) bool {
     return false;
 }
+
+pub fn notifyPpuAddress(_: *Mapper180, _: u16) void {}
+
+test "mapper180 bank switching" {
+    var m = Mapper180.init(.horizontal);
+    var ram: [0x2000]u8 = undefined;
+
+    _ = m.cpuWrite(0x8000, 3, &ram);
+    try std.testing.expectEqual(@as(u8, 3), m.prg_bank);
+}

src/system/nes/mapper/mapper206.zig

@@ -1,108 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper206 = @This();
-
-bank_select: u3 = 0,
-
-prg_bank0: u8 = 0,
-prg_bank1: u8 = 1,
-
-chr_banks: [6]u8 = [_]u8{ 0, 2, 4, 5, 6, 7 },
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-mirroring_mode: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    _mirroring: Mirroring,
-) Mapper206 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper206,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xA000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank0) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else if (address < 0xC000) {
-        const offset = @as(usize, address - 0xA000);
-        const bank = @as(usize, self.prg_bank1) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else if (address < 0xE000) {
-        const offset = @as(usize, address - 0xC000);
-        const bank = if (self.prg_bank_count >= 2) self.prg_bank_count - 2 else 0;
-        return bank * 0x2000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xE000);
-        const bank = if (self.prg_bank_count >= 1) self.prg_bank_count - 1 else 0;
-        return bank * 0x2000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper206,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000 and address <= 0x9FFF) {
-        if ((address & 1) == 0) {
-            self.bank_select = @truncate(value & 0x07);
-        } else {
-            switch (self.bank_select) {
-                0...5 => |i| self.chr_banks[i] = value & 0x3F,
-                6 => self.prg_bank0 = value & 0x0F,
-                7 => self.prg_bank1 = value & 0x0F,
-            }
-        }
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper206,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    if (address < 0x0800) {
-        const bank = (@as(usize, self.chr_banks[0] & 0xFE)) % self.chr_bank_count;
-        return bank * 0x0400 + @as(usize, address);
-    } else if (address < 0x1000) {
-        const bank = (@as(usize, self.chr_banks[1] & 0xFE)) % self.chr_bank_count;
-        return bank * 0x0400 + @as(usize, address - 0x0800);
-    } else {
-        const idx = 2 + (address - 0x1000) / 0x0400;
-        const offset = (address - 0x1000) % 0x0400;
-        const bank = @as(usize, self.chr_banks[idx]) % self.chr_bank_count;
-        return bank * 0x0400 + @as(usize, offset);
-    }
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper206,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper206,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper206) bool {
-    return false;
-}

src/system/nes/mapper/mapper210.zig

@@ -1,105 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper210 = @This();
-
-prg_bank0: u8 = 0,
-prg_bank1: u8 = 0,
-prg_bank2: u8 = 0,
-
-chr_banks: [8]u8 = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7 },
-mirroring_mode: Mirroring = .vertical,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mapper210 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper210,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xA000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank0) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else if (address < 0xC000) {
-        const offset = @as(usize, address - 0xA000);
-        const bank = @as(usize, self.prg_bank1) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else if (address < 0xE000) {
-        const offset = @as(usize, address - 0xC000);
-        const bank = @as(usize, self.prg_bank2) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xE000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x2000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper210,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    if (address >= 0x8000 and address <= 0xBFFF) {
-        const index = (address - 0x8000) / 0x0800;
-        self.chr_banks[index] = value;
-    } else if (address >= 0xC000 and address <= 0xC7FF) {
-        self.prg_bank0 = value & 0x3F;
-    } else if (address >= 0xC800 and address <= 0xCFFF) {
-        self.prg_bank1 = value & 0x3F;
-    } else if (address >= 0xD000 and address <= 0xD7FF) {
-        self.prg_bank2 = value & 0x3F;
-    } else if (address >= 0xE000 and address <= 0xE7FF) {
-        self.mirroring_mode = switch (@as(u2, @truncate(value >> 6))) {
-            0 => .vertical,
-            1 => .horizontal,
-            2 => .single_screen_lower,
-            3 => .single_screen_upper,
-        };
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper210,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank_idx = address / 0x0400;
-    const offset = address % 0x0400;
-    const bank = @as(usize, self.chr_banks[bank_idx]) % self.chr_bank_count;
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper210,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper210,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper210) bool {
-    return false;
-}

src/system/nes/mapper/mapper232.zig

@@ -1,77 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper232 = @This();
-
-outer_block: u8 = 0,
-inner_bank: u8 = 0,
-
-prg_bank_count: usize,
-mirroring_mode: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Mapper232 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper232,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const block_start = (@as(usize, self.outer_block) * 4) % self.prg_bank_count;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = (block_start + (@as(usize, self.inner_bank) & 3)) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const bank = (block_start + 3) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper232,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000 and address <= 0x9FFF) {
-        self.outer_block = (value >> 3) & 0x03;
-    } else if (address >= 0xA000) {
-        self.inner_bank = value & 0x03;
-    }
-}
-
-pub fn ppuMapRead(
-    _: *const Mapper232,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper232,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper232,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper232) bool {
-    return false;
-}

src/system/nes/mapper/mapper34.zig

@@ -1,92 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper34 = @This();
-
-prg_bank: u8 = 0,
-chr_bank0: u8 = 0,
-chr_bank1: u8 = 1,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-fixed_mirroring: Mirroring,
-is_nina01: bool,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mirroring_mode: Mirroring,
-) Mapper34 {
-    return .{
-        .prg_bank_count = if (prg_size >= 0x8000) prg_size / 0x8000 else 1,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x1000 else 1,
-        .fixed_mirroring = mirroring_mode,
-        .is_nina01 = (chr_size > 0),
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper34,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x7fff);
-    const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-
-    return bank * 0x8000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *Mapper34,
-    address: u16,
-    value: u8,
-) void {
-    if (self.is_nina01) {
-        switch (address) {
-            0x7ffd => self.prg_bank = value,
-            0x7ffe => self.chr_bank0 = value,
-            0x7fff => self.chr_bank1 = value,
-            else => {},
-        }
-    } else {
-        if (address >= 0x8000) {
-            self.prg_bank = value;
-        }
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper34,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    if (self.is_nina01) {
-        const offset = @as(usize, address & 0x0fff);
-        const bank_raw = if (address < 0x1000) self.chr_bank0 else self.chr_bank1;
-        const bank = @as(usize, bank_raw) % self.chr_bank_count;
-        return bank * 0x1000 + offset;
-    } else {
-        return @as(usize, address);
-    }
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper34,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper34,
-) Mirroring {
-    return self.fixed_mirroring;
-}
-
-pub fn irqAsserted(
-    _: *const Mapper34,
-) bool {
-    return false;
-}

src/system/nes/mapper/mapper65.zig

@@ -1,118 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper65 = @This();
-
-prg_banks: [3]u8 = [_]u8{ 0, 1, 2 },
-chr_banks: [8]u8 = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7 },
-
-mirroring_mode: Mirroring = .vertical,
-
-irq_enabled: bool = false,
-irq_counter: u16 = 0,
-irq_reload: u16 = 0,
-irq_assert: bool = false,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mirroring_mode: Mirroring,
-) Mapper65 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-        .mirroring_mode = mirroring_mode,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper65,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const last_bank = self.prg_bank_count - 1;
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => return (@as(usize, self.prg_banks[0]) % self.prg_bank_count) * 0x2000 + offset,
-        1 => return (@as(usize, self.prg_banks[1]) % self.prg_bank_count) * 0x2000 + offset,
-        2 => return (@as(usize, self.prg_banks[2]) % self.prg_bank_count) * 0x2000 + offset,
-        3 => return last_bank * 0x2000 + offset,
-        else => unreachable,
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper65,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    switch (address) {
-        0x8000 => self.prg_banks[0] = value,
-        0xa000 => self.prg_banks[1] = value,
-        0xc000 => self.prg_banks[2] = value,
-
-        0x9001 => self.mirroring_mode = if ((value & 0x80) != 0) .horizontal else .vertical,
-
-        0x9003 => {
-            self.irq_enabled = (value & 0x80) != 0;
-            self.irq_assert = false;
-        },
-        0x9004 => {
-            self.irq_reload = (self.irq_reload & 0xff00) | value;
-        },
-        0x9005 => {
-            self.irq_reload = (self.irq_reload & 0x00ff) | (@as(u16, value) << 8);
-            self.irq_counter = self.irq_reload;
-            self.irq_assert = false;
-        },
-
-        0xb000 => self.chr_banks[0] = value,
-        0xb001 => self.chr_banks[1] = value,
-        0xb002 => self.chr_banks[2] = value,
-        0xb003 => self.chr_banks[3] = value,
-        0xb004 => self.chr_banks[4] = value,
-        0xb005 => self.chr_banks[5] = value,
-        0xb006 => self.chr_banks[6] = value,
-        0xb007 => self.chr_banks[7] = value,
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper65,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0400;
-    const offset = @as(usize, address & 0x03ff);
-    const bank = @as(usize, self.chr_banks[slot]) % self.chr_bank_count;
-
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper65,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper65,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(
-    self: *const Mapper65,
-) bool {
-    return self.irq_assert;
-}

src/system/nes/mapper/mapper68.zig

@@ -1,103 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper68 = @This();
-
-chr_banks: [4]u8 = [_]u8{ 0, 0, 0, 0 },
-nt_banks: [2]u8 = [_]u8{ 0, 0 },
-prg_bank: u8 = 0,
-nametable_control: u8 = 0,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-fixed_mirroring: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mirroring_mode: Mirroring,
-) Mapper68 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0800 else 1,
-        .fixed_mirroring = mirroring_mode,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper68,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x3fff);
-    const last_bank = self.prg_bank_count - 1;
-
-    if (address < 0xc000) {
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        return last_bank * 0x4000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper68,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    switch (address & 0xf000) {
-        0x8000 => self.chr_banks[0] = value,
-        0x9000 => self.chr_banks[1] = value,
-        0xa000 => self.chr_banks[2] = value,
-        0xb000 => self.chr_banks[3] = value,
-        0xc000 => self.nt_banks[0] = value,
-        0xd000 => self.nt_banks[1] = value,
-        0xe000 => self.nametable_control = value,
-        0xf000 => self.prg_bank = value & 0x0f,
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper68,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0800;
-    const offset = @as(usize, address & 0x07ff);
-    const bank = @as(usize, self.chr_banks[slot]) % self.chr_bank_count;
-
-    return bank * 0x0800 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper68,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper68,
-) Mirroring {
-    if ((self.nametable_control & 0x10) == 0) {
-        switch (self.nametable_control & 0x03) {
-            0 => return .vertical,
-            1 => return .horizontal,
-            2 => return .single_screen_lower,
-            3 => return .single_screen_upper,
-            else => unreachable,
-        }
-    }
-    return self.fixed_mirroring;
-}
-
-pub fn irqAsserted(
-    _: *const Mapper68,
-) bool {
-    return false;
-}

src/system/nes/mapper/mapper69.zig

@@ -1,123 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper69 = @This();
-
-command: u8 = 0,
-chr_banks: [8]u8 = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7 },
-prg_banks: [4]u8 = [_]u8{ 0, 1, 2, 3 }, // $6000, $8000, $A000, $C000
-mirroring_mode: Mirroring = .vertical,
-
-irq_enabled: bool = false,
-irq_counter: u16 = 0,
-irq_assert: bool = false,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mirroring_mode: Mirroring,
-) Mapper69 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-        .mirroring_mode = mirroring_mode,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper69,
-    address: u16,
-) ?usize {
-    if (address < 0x6000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const last_bank = self.prg_bank_count - 1;
-
-    if (address < 0x8000) {
-        const bank = @as(usize, self.prg_banks[0] & 0x3f) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    }
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => return (@as(usize, self.prg_banks[1] & 0x3f) % self.prg_bank_count) * 0x2000 + offset,
-        1 => return (@as(usize, self.prg_banks[2] & 0x3f) % self.prg_bank_count) * 0x2000 + offset,
-        2 => return (@as(usize, self.prg_banks[3] & 0x3f) % self.prg_bank_count) * 0x2000 + offset,
-        3 => return last_bank * 0x2000 + offset,
-        else => unreachable,
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper69,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    if (address < 0xa000) {
-        self.command = value & 0x0f;
-    } else if (address < 0xc000) {
-        switch (self.command) {
-            0...7 => |i| self.chr_banks[i] = value,
-            8 => self.prg_banks[0] = value,
-            9 => self.prg_banks[1] = value,
-            0xa => self.prg_banks[2] = value,
-            0xb => self.prg_banks[3] = value,
-            0xc => {
-                switch (value & 0x03) {
-                    0 => self.mirroring_mode = .vertical,
-                    1 => self.mirroring_mode = .horizontal,
-                    2 => self.mirroring_mode = .single_screen_lower,
-                    3 => self.mirroring_mode = .single_screen_upper,
-                    else => unreachable,
-                }
-            },
-            0xd => {
-                self.irq_enabled = (value & 0x01) != 0;
-                self.irq_assert = false;
-            },
-            0xe => {
-                self.irq_counter = (self.irq_counter & 0xff00) | value;
-            },
-            0xf => {
-                self.irq_counter = (self.irq_counter & 0x00ff) | (@as(u16, value) << 8);
-            },
-            else => {},
-        }
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper69,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0400;
-    const offset = @as(usize, address & 0x03ff);
-    const bank = @as(usize, self.chr_banks[slot]) % self.chr_bank_count;
-
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper69,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper69,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(
-    self: *const Mapper69,
-) bool {
-    return self.irq_assert;
-}

src/system/nes/mapper/mapper76.zig

@@ -1,91 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper76 = @This();
-
-bank_select: u8 = 0,
-bank_registers: [8]u8 = [_]u8{0} ** 8,
-prg_bank_count: usize,
-chr_bank_count: usize,
-fixed_mirroring: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mirroring_mode: Mirroring,
-) Mapper76 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0800 else 1,
-        .fixed_mirroring = mirroring_mode,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper76,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const last_bank = self.prg_bank_count - 1;
-    const second_last_bank = self.prg_bank_count - 2;
-
-    const r6 = @as(usize, self.bank_registers[6]) % self.prg_bank_count;
-    const r7 = @as(usize, self.bank_registers[7]) % self.prg_bank_count;
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => return r6 * 0x2000 + offset,
-        1 => return r7 * 0x2000 + offset,
-        2 => return second_last_bank * 0x2000 + offset,
-        3 => return last_bank * 0x2000 + offset,
-        else => unreachable,
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper76,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    if ((address & 1) == 0) {
-        self.bank_select = value & 0x07;
-    } else {
-        self.bank_registers[self.bank_select] = value;
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper76,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const offset = @as(usize, address & 0x07ff);
-    const slot = address / 0x0800;
-    const reg_idx = @as(usize, 2 + slot);
-    const bank = (@as(usize, self.bank_registers[reg_idx])) % self.chr_bank_count;
-
-    return bank * 0x0800 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper76,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper76,
-) Mirroring {
-    return self.fixed_mirroring;
-}
-
-pub fn irqAsserted(
-    _: *const Mapper76,
-) bool {
-    return false;
-}

src/system/nes/mapper/mapper87.zig

@@ -1,70 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper87 = @This();
-
-chr_bank: u8 = 0,
-prg_bank_count: usize,
-chr_bank_count: usize,
-mirroring_mode: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    _mirroring: Mirroring,
-) Mapper87 {
-    return .{
-        .prg_bank_count = prg_size / 0x8000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper87,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address - 0x8000);
-    const bank = if (self.prg_bank_count > 0) (self.prg_bank_count - 1) else 0;
-    return bank * 0x8000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *Mapper87,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x6000 and address <= 0x7FFF) {
-        self.chr_bank = ((value & 1) << 1) | ((value >> 1) & 1);
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper87,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper87,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper87,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper87) bool {
-    return false;
-}

src/system/nes/mapper/mapper88.zig

@@ -1,103 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper88 = @This();
-
-bank_select: u8 = 0,
-bank_registers: [8]u8 = [_]u8{0} ** 8,
-prg_bank_count: usize,
-chr_bank_count: usize,
-fixed_mirroring: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mirroring_mode: Mirroring,
-) Mapper88 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-        .fixed_mirroring = mirroring_mode,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper88,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const last_bank = self.prg_bank_count - 1;
-    const second_last_bank = self.prg_bank_count - 2;
-
-    const r6 = @as(usize, self.bank_registers[6] & 0x3f) % self.prg_bank_count;
-    const r7 = @as(usize, self.bank_registers[7] & 0x3f) % self.prg_bank_count;
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => return r6 * 0x2000 + offset,
-        1 => return r7 * 0x2000 + offset,
-        2 => return second_last_bank * 0x2000 + offset,
-        3 => return last_bank * 0x2000 + offset,
-        else => unreachable,
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper88,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    if ((address & 1) == 0) {
-        self.bank_select = value & 0x07;
-    } else {
-        self.bank_registers[self.bank_select] = value;
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper88,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const offset = @as(usize, address & 0x03ff);
-    const page = address / 0x0400;
-
-    var bank: usize = 0;
-    switch (page) {
-        0, 1 => bank = (@as(usize, self.bank_registers[0] & 0xfe) + (page & 1)),
-        2, 3 => bank = (@as(usize, self.bank_registers[1] & 0xfe) + (page & 1)),
-        4, 5, 6, 7 => {
-            const reg_idx = page - 2; // registers 2, 3, 4, 5
-            const val = self.bank_registers[reg_idx];
-            // Bit 6 maps to 64KB CHR block selection (bit 6 -> 0x40 added to bank number)
-            bank = (@as(usize, val & 0x3f) | (@as(usize, val & 0x40)));
-        },
-        else => unreachable,
-    }
-
-    bank = bank % self.chr_bank_count;
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Mapper88,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Mapper88,
-) Mirroring {
-    return self.fixed_mirroring;
-}
-
-pub fn irqAsserted(
-    _: *const Mapper88,
-) bool {
-    return false;
-}

src/system/nes/mapper/mapper89.zig

@@ -1,78 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper89 = @This();
-
-prg_bank: u8 = 0,
-chr_bank: u8 = 0,
-mirroring_mode: Mirroring = .single_screen_lower,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mapper89 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper89,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x4000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper89,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.prg_bank = (value >> 4) & 0x07;
-        self.chr_bank = (value & 0x07) | ((value & 0x80) >> 4);
-        self.mirroring_mode = if ((value & 0x08) != 0) .single_screen_upper else .single_screen_lower;
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mapper89,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper89,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper89,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper89) bool {
-    return false;
-}

src/system/nes/mapper/mapper93.zig

@@ -1,73 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper93 = @This();
-
-prg_bank: u8 = 0,
-mirroring_mode: Mirroring,
-
-prg_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Mapper93 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper93,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x4000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper93,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.prg_bank = (value >> 4) & 0x07;
-        self.mirroring_mode = if ((value & 1) != 0) .horizontal else .vertical;
-    }
-}
-
-pub fn ppuMapRead(
-    _: *const Mapper93,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper93,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper93,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper93) bool {
-    return false;
-}

src/system/nes/mapper/mapper94.zig

@@ -1,71 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mapper94 = @This();
-
-prg_bank: u8 = 0,
-prg_bank_count: usize,
-mirroring_mode: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Mapper94 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mapper94,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x4000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mapper94,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.prg_bank = (value >> 2) & 0x07;
-    }
-}
-
-pub fn ppuMapRead(
-    _: *const Mapper94,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn ppuMapWrite(
-    _: *const Mapper94,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mapper94,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Mapper94) bool {
-    return false;
-}

src/system/nes/mapper/mmc1.zig

@@ -1,130 +1,152 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Mmc1 = @This();
+pub const Mmc1 = @This();
 
-shift_register: u8 = 0x10,
-control: u8 = 0x0c, // Default: PRG mode 3 (fix $C000)
-chr_bank_0: u8 = 0,
-chr_bank_1: u8 = 0,
-prg_bank: u8 = 0,
+shift_register: u5 = 0x10,
+control: u5 = 0x0c, // PRG mode 3 (fixed $C000)
+chr_bank_0: u5 = 0,
+chr_bank_1: u5 = 0,
+prg_bank: u5 = 0,
 
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mmc1 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x1000 else 2,
-    };
+pub fn init() Mmc1 {
+    return .{};
 }
 
-pub fn cpuMapRead(
-    self: *const Mmc1,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const prg_mode = (self.control >> 2) & 0x03;
-    const offset = @as(usize, address & 0x3fff);
-
-    switch (prg_mode) {
-        0, 1 => {
-            // 32 KB mode
-            const bank = @as(usize, self.prg_bank & 0x0e) % self.prg_bank_count;
-            return bank * 0x4000 + @as(usize, address - 0x8000);
-        },
-        2 => {
-            // Fix first bank at $8000, switch 16 KB bank at $C000
-            if (address < 0xc000) {
-                return offset;
-            } else {
-                const bank = @as(usize, self.prg_bank & 0x0f) % self.prg_bank_count;
-                return bank * 0x4000 + offset;
-            }
-        },
-        3 => {
-            // Fix last bank at $C000, switch 16 KB bank at $8000
-            if (address < 0xc000) {
-                const bank = @as(usize, self.prg_bank & 0x0f) % self.prg_bank_count;
-                return bank * 0x4000 + offset;
-            } else {
-                const last_bank = self.prg_bank_count - 1;
-                return last_bank * 0x4000 + offset;
-            }
-        },
-        else => unreachable,
+pub fn cpuRead(self: *const Mmc1, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        // Bit 4 of prg_bank is PRG RAM disable (0: enabled, 1: disabled)
+        if ((self.prg_bank & 0x10) != 0) return 0;
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
-}
+    if (address >= 0x8000 and address <= 0xffff) {
+        const prg_mode: u2 = @truncate(self.control >> 2);
+        const total_16k_banks: usize = @max(1, prg_rom.len / 0x4000);
+        // SUROM uses CHR bank 0 bit 4 for PRG A18 when PRG is 512KB
+        const surom_base: usize = if (total_16k_banks >= 32 and (self.chr_bank_0 & 0x10) != 0) 16 else 0;
+        var bank: usize = 0;
 
-pub fn cpuWrite(
-    self: *Mmc1,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    // Bit 7 reset flag
-    if ((value & 0x80) != 0) {
-        self.shift_register = 0x10;
-        self.control |= 0x0c;
-        return;
-    }
-
-    const complete = (self.shift_register & 1) != 0;
-    self.shift_register = (self.shift_register >> 1) | ((value & 1) << 4);
-
-    if (complete) {
-        const reg_data = self.shift_register;
-        self.shift_register = 0x10;
-
-        switch ((address >> 13) & 0x03) {
-            0 => self.control = reg_data,
-            1 => self.chr_bank_0 = reg_data,
-            2 => self.chr_bank_1 = reg_data,
-            3 => self.prg_bank = reg_data,
-            else => unreachable,
+        switch (prg_mode) {
+            0, 1 => {
+                // 32 KB mode: ignore low bit of prg_bank
+                bank = ((self.prg_bank & 0x0e) | surom_base) % total_16k_banks;
+                const offset = (bank * 0x4000) + (address - 0x8000);
+                if (offset < prg_rom.len) return prg_rom[offset];
+            },
+            2 => {
+                // Fix first bank at $8000, switch 16 KB bank at $C000
+                if (address < 0xc000) {
+                    bank = surom_base % total_16k_banks;
+                    const offset = (bank * 0x4000) + (address - 0x8000);
+                    if (offset < prg_rom.len) return prg_rom[offset];
+                } else {
+                    bank = ((self.prg_bank & 0x0f) | surom_base) % total_16k_banks;
+                    const offset = (bank * 0x4000) + (address - 0xc000);
+                    if (offset < prg_rom.len) return prg_rom[offset];
+                }
+            },
+            3 => {
+                // Switch 16 KB bank at $8000, fix last bank at $C000
+                if (address < 0xc000) {
+                    bank = ((self.prg_bank & 0x0f) | surom_base) % total_16k_banks;
+                    const offset = (bank * 0x4000) + (address - 0x8000);
+                    if (offset < prg_rom.len) return prg_rom[offset];
+                } else {
+                    bank = (surom_base + 15) % total_16k_banks;
+                    const offset = (bank * 0x4000) + (address - 0xc000);
+                    if (offset < prg_rom.len) return prg_rom[offset];
+                }
+            },
         }
+        return 0;
     }
+    return null;
 }
 
-pub fn ppuMapRead(
-    self: *const Mmc1,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
+pub fn cpuWrite(self: *Mmc1, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        if ((self.prg_bank & 0x10) != 0) return false;
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+        return false;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        if ((value & 0x80) != 0) {
+            // Reset shift register
+            self.shift_register = 0x10;
+            self.control |= 0x0c; // PRG mode 3
+            return true;
+        }
 
-    const chr_mode = (self.control & 0x10) != 0;
-    if (!chr_mode) {
-        // 8 KB CHR mode
-        const bank = @as(usize, self.chr_bank_0 & 0x1e) % self.chr_bank_count;
-        return bank * 0x1000 + @as(usize, address);
-    } else {
-        // 4 KB CHR mode
-        if (address < 0x1000) {
-            const bank = @as(usize, self.chr_bank_0) % self.chr_bank_count;
-            return bank * 0x1000 + @as(usize, address & 0x0fff);
+        const is_full = (self.shift_register & 1) != 0;
+        self.shift_register = (self.shift_register >> 1) | (@as(u5, @truncate(value & 1)) << 4);
+
+        if (is_full) {
+            const reg_val = self.shift_register;
+            self.shift_register = 0x10;
+
+            const reg_select = (address >> 13) & 0x03;
+            switch (reg_select) {
+                0 => self.control = reg_val,
+                1 => self.chr_bank_0 = reg_val,
+                2 => self.chr_bank_1 = reg_val,
+                3 => self.prg_bank = reg_val,
+                else => unreachable,
+            }
+        }
+        return true;
+    }
+    return false;
+}
+
+pub fn ppuRead(self: *const Mmc1, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+            return 0;
+        }
+        if (chr_rom.len == 0) return 0;
+
+        const chr_4k_mode = (self.control & 0x10) != 0;
+        const total_4k_banks = @max(1, chr_rom.len / 0x1000);
+        var offset: usize = 0;
+
+        if (chr_4k_mode) {
+            if (address < 0x1000) {
+                const bank = @as(usize, self.chr_bank_0) % total_4k_banks;
+                offset = (bank * 0x1000) + address;
+            } else {
+                const bank = @as(usize, self.chr_bank_1) % total_4k_banks;
+                offset = (bank * 0x1000) + (address - 0x1000);
+            }
         } else {
-            const bank = @as(usize, self.chr_bank_1) % self.chr_bank_count;
-            return bank * 0x1000 + @as(usize, address & 0x0fff);
+            const bank = @as(usize, self.chr_bank_0 & 0x1e) % total_4k_banks;
+            offset = (bank * 0x1000) + address;
+        }
+
+        if (offset < chr_rom.len) return chr_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Mmc1, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
         }
     }
+    return false;
 }
 
-pub fn ppuMapWrite(
-    _: *const Mmc1,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mmc1,
-) Mirroring {
+pub fn mirroring(self: *const Mmc1) Mirroring {
     return switch (self.control & 0x03) {
         0 => .single_screen_lower,
         1 => .single_screen_upper,
@@ -134,6 +156,23 @@ pub fn mirroring(
     };
 }
 
-pub fn irqAsserted(_: *const Mmc1) bool {
+pub fn irqLine(_: *const Mmc1) bool {
     return false;
 }
+
+pub fn notifyPpuAddress(_: *Mmc1, _: u16) void {}
+
+test "mmc1 serial shift register write" {
+    var mmc1 = Mmc1.init();
+    var ram: [0x2000]u8 = undefined;
+
+    // Write 5 bits (e.g. 0b00011 -> horizontal mirroring) into $8000 (control register)
+    _ = mmc1.cpuWrite(0x8000, 1, &ram); // bit 0 = 1
+    _ = mmc1.cpuWrite(0x8000, 1, &ram); // bit 1 = 1
+    _ = mmc1.cpuWrite(0x8000, 0, &ram); // bit 2 = 0
+    _ = mmc1.cpuWrite(0x8000, 0, &ram); // bit 3 = 0
+    _ = mmc1.cpuWrite(0x8000, 0, &ram); // bit 4 = 0 -> loaded!
+
+    try std.testing.expectEqual(@as(u5, 0x03), mmc1.control);
+    try std.testing.expectEqual(Mirroring.horizontal, mmc1.mirroring());
+}

src/system/nes/mapper/mmc2.zig

@@ -1,104 +1,151 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Mmc2 = @This();
+// Mapper 9 (MMC2) - Mike Tyson's Punch-Out!!
+// 8KB switchable PRG at $8000, fixed 24KB at $A000-$FFFF.
+// Dual 4KB CHR latches toggled by PPU fetches at $xFD8-$xFDF and $xFE8-$xFEF.
+pub const Mmc2 = @This();
 
 prg_bank: u8 = 0,
-chr_0fd: u8 = 0,
-chr_0fe: u8 = 0,
-chr_1fd: u8 = 0,
-chr_1fe: u8 = 0,
-
-latch0: u1 = 0, // 0 = 0FD, 1 = 0FE
-latch1: u1 = 0, // 0 = 1FD, 1 = 1FE
-
+chr_bank_0_fd: u8 = 0,
+chr_bank_0_fe: u8 = 0,
+chr_bank_1_fd: u8 = 0,
+chr_bank_1_fe: u8 = 0,
+latch_0: u1 = 0,
+latch_1: u1 = 0,
 mirroring_mode: Mirroring = .vertical,
 
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mmc2 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x1000 else 1,
-    };
+pub fn init() Mmc2 {
+    return .{};
 }
 
-pub fn cpuMapRead(
-    self: *const Mmc2,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
+pub fn cpuRead(self: *const Mmc2, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_8k_banks = @max(1, prg_rom.len / 0x2000);
+        var bank: usize = 0;
 
-    if (address < 0xA000) {
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x2000 + @as(usize, address - 0x8000);
-    } else {
-        const offset = @as(usize, address - 0xA000);
-        const fixed_start = if (self.prg_bank_count >= 3) (self.prg_bank_count - 3) * 0x2000 else 0;
-        return fixed_start + offset;
+        switch (address) {
+            0x8000...0x9fff => bank = @as(usize, self.prg_bank) % total_8k_banks,
+            0xa000...0xbfff => bank = if (total_8k_banks >= 3) total_8k_banks - 3 else 0,
+            0xc000...0xdfff => bank = if (total_8k_banks >= 2) total_8k_banks - 2 else 0,
+            0xe000...0xffff => bank = total_8k_banks - 1,
+            else => return 0,
+        }
+
+        const offset = (bank * 0x2000) + (address & 0x1fff);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn cpuWrite(self: *Mmc2, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+        return false;
+    }
+    switch (address) {
+        0xa000...0xafff => {
+            self.prg_bank = value & 0x0f;
+            return true;
+        },
+        0xb000...0xbfff => {
+            self.chr_bank_0_fd = value & 0x1f;
+            return true;
+        },
+        0xc000...0xcfff => {
+            self.chr_bank_0_fe = value & 0x1f;
+            return true;
+        },
+        0xd000...0xdfff => {
+            self.chr_bank_1_fd = value & 0x1f;
+            return true;
+        },
+        0xe000...0xefff => {
+            self.chr_bank_1_fe = value & 0x1f;
+            return true;
+        },
+        0xf000...0xffff => {
+            self.mirroring_mode = if ((value & 1) == 0) .vertical else .horizontal;
+            return true;
+        },
+        else => return false,
     }
 }
 
-pub fn cpuWrite(
-    self: *Mmc2,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
+pub fn ppuRead(self: *const Mmc2, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+            return 0;
+        }
+        if (chr_rom.len == 0) return 0;
 
-    switch (address & 0xF000) {
-        0xA000 => self.prg_bank = value & 0x0F,
-        0xB000 => self.chr_0fd = value & 0x1F,
-        0xC000 => self.chr_0fe = value & 0x1F,
-        0xD000 => self.chr_1fd = value & 0x1F,
-        0xE000 => self.chr_1fe = value & 0x1F,
-        0xF000 => self.mirroring_mode = if ((value & 1) == 0) .vertical else .horizontal,
+        const total_4k_banks = @max(1, chr_rom.len / 0x1000);
+        const bank = if (address < 0x1000)
+            (if (self.latch_0 == 0) self.chr_bank_0_fd else self.chr_bank_0_fe)
+        else
+            (if (self.latch_1 == 0) self.chr_bank_1_fd else self.chr_bank_1_fe);
+
+        const offset = ((@as(usize, bank) % total_4k_banks) * 0x1000) + (address & 0x0fff);
+        if (offset < chr_rom.len) return chr_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Mmc2, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn notifyPpuAddress(self: *Mmc2, address: u16) void {
+    self.checkLatch(address);
+}
+
+inline fn checkLatch(self: *Mmc2, address: u16) void {
+    switch (address) {
+        0x0fd8...0x0fdf => self.latch_0 = 0,
+        0x0fe8...0x0fef => self.latch_0 = 1,
+        0x1fd8...0x1fdf => self.latch_1 = 0,
+        0x1fe8...0x1fef => self.latch_1 = 1,
         else => {},
     }
 }
 
-pub fn ppuMapRead(
-    self: *Mmc2,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    var bank: usize = 0;
-    if (address < 0x1000) {
-        const selected_bank = if (self.latch0 == 0) self.chr_0fd else self.chr_0fe;
-        bank = @as(usize, selected_bank) % self.chr_bank_count;
-    } else {
-        const selected_bank = if (self.latch1 == 0) self.chr_1fd else self.chr_1fe;
-        bank = @as(usize, selected_bank) % self.chr_bank_count;
-    }
-
-    // Check PPU tile latches
-    if (address == 0x0FD0) self.latch0 = 0;
-    if (address == 0x0FE0) self.latch0 = 1;
-    if (address == 0x1FD0) self.latch1 = 0;
-    if (address == 0x1FE0) self.latch1 = 1;
-
-    return bank * 0x1000 + @as(usize, address & 0x0FFF);
-}
-
-pub fn ppuMapWrite(
-    _: *const Mmc2,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Mmc2,
-) Mirroring {
+pub fn mirroring(self: *const Mmc2) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(_: *const Mmc2) bool {
+pub fn irqLine(_: *const Mmc2) bool {
     return false;
 }
+
+test "mmc2 latch toggling" {
+    var m = Mmc2.init();
+    var ram: [0x2000]u8 = undefined;
+
+    _ = m.cpuWrite(0xb000, 1, &ram); // CHR 0 FD = 1
+    _ = m.cpuWrite(0xc000, 2, &ram); // CHR 0 FE = 2
+
+    m.notifyPpuAddress(0x0fd8);
+    try std.testing.expectEqual(@as(u1, 0), m.latch_0);
+
+    m.notifyPpuAddress(0x0fe8);
+    try std.testing.expectEqual(@as(u1, 1), m.latch_0);
+}

src/system/nes/mapper/mmc3.zig

@@ -1,161 +1,222 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Mmc3 = @This();
+pub const Mmc3 = @This();
 
 bank_select: u8 = 0,
-bank_registers: [8]u8 = [_]u8{0} ** 8,
-
-mirroring_mode: Mirroring = .horizontal,
+registers: [8]u8 = [_]u8{0} ** 8,
+mirroring_mode: Mirroring = .vertical,
+prg_ram_protect: u8 = 0x80,
 
 irq_latch: u8 = 0,
 irq_counter: u8 = 0,
 irq_reload: bool = false,
 irq_enabled: bool = false,
-irq_pending: bool = false,
+irq_asserted: bool = false,
 
-prg_bank_count: usize,
-chr_bank_count: usize,
+last_a12: bool = false,
+a12_low_cycles: u32 = 0,
 
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mmc3 {
+pub fn init(initial_mirroring: Mirroring) Mmc3 {
     return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 8,
+        .mirroring_mode = initial_mirroring,
     };
 }
 
-pub fn cpuMapRead(
-    self: *const Mmc3,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const prg_mode = (self.bank_select & 0x40) != 0;
-
-    const last_bank = self.prg_bank_count - 1;
-    const second_last_bank = self.prg_bank_count - 2;
-
-    const r6 = @as(usize, self.bank_registers[6] & 0x3f) % self.prg_bank_count;
-    const r7 = @as(usize, self.bank_registers[7] & 0x3f) % self.prg_bank_count;
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => { // $8000-$9FFF
-            const bank = if (!prg_mode) r6 else second_last_bank;
-            return bank * 0x2000 + offset;
-        },
-        1 => { // $A000-$BFFF
-            return r7 * 0x2000 + offset;
-        },
-        2 => { // $C000-$DFFF
-            const bank = if (!prg_mode) second_last_bank else r6;
-            return bank * 0x2000 + offset;
-        },
-        3 => { // $E000-$FFFF
-            return last_bank * 0x2000 + offset;
-        },
-        else => unreachable,
+pub fn cpuRead(self: *const Mmc3, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        if ((self.prg_ram_protect & 0x80) == 0) return 0;
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
-}
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_8k_banks = @max(1, prg_rom.len / 0x2000);
+        const prg_mode = (self.bank_select & 0x40) != 0;
+        var bank: usize = 0;
 
-pub fn cpuWrite(
-    self: *Mmc3,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    switch (address & 0xe001) {
-        0x8000 => self.bank_select = value,
-        0x8001 => {
-            const reg = self.bank_select & 0x07;
-            self.bank_registers[reg] = value;
-        },
-        0xa000 => {
-            self.mirroring_mode = if ((value & 1) != 0) .horizontal else .vertical;
-        },
-        0xa001 => {}, // PRG RAM protect
-        0xc000 => self.irq_latch = value,
-        0xc001 => self.irq_reload = true,
-        0xe000 => {
-            self.irq_enabled = false;
-            self.irq_pending = false;
-        },
-        0xe001 => self.irq_enabled = true,
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mmc3,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const chr_mode = (self.bank_select & 0x80) != 0;
-    const offset = @as(usize, address & 0x03ff);
-
-    var bank: usize = 0;
-    const addr_k = address / 0x0400;
-
-    if (!chr_mode) {
-        switch (addr_k) {
-            0, 1 => bank = (@as(usize, self.bank_registers[0] & 0xfe) + (addr_k & 1)),
-            2, 3 => bank = (@as(usize, self.bank_registers[1] & 0xfe) + (addr_k & 1)),
-            4 => bank = self.bank_registers[2],
-            5 => bank = self.bank_registers[3],
-            6 => bank = self.bank_registers[4],
-            7 => bank = self.bank_registers[5],
-            else => unreachable,
+        switch (address) {
+            0x8000...0x9fff => {
+                bank = if (!prg_mode) self.registers[6] else (total_8k_banks - 2);
+            },
+            0xa000...0xbfff => {
+                bank = self.registers[7];
+            },
+            0xc000...0xdfff => {
+                bank = if (!prg_mode) (total_8k_banks - 2) else self.registers[6];
+            },
+            0xe000...0xffff => {
+                bank = total_8k_banks - 1;
+            },
+            else => return 0,
         }
-    } else {
-        switch (addr_k) {
-            0 => bank = self.bank_registers[2],
-            1 => bank = self.bank_registers[3],
-            2 => bank = self.bank_registers[4],
-            3 => bank = self.bank_registers[5],
-            4, 5 => bank = (@as(usize, self.bank_registers[0] & 0xfe) + (addr_k & 1)),
-            6, 7 => bank = (@as(usize, self.bank_registers[1] & 0xfe) + (addr_k & 1)),
-            else => unreachable,
+
+        bank %= total_8k_banks;
+        const offset = (bank * 0x2000) + (address & 0x1fff);
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn cpuWrite(self: *Mmc3, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        if ((self.prg_ram_protect & 0x80) == 0 or (self.prg_ram_protect & 0x40) != 0) return false;
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+        return false;
+    }
+
+    switch (address) {
+        0x8000...0x9fff => {
+            if ((address & 1) == 0) {
+                self.bank_select = value;
+            } else {
+                const reg = self.bank_select & 0x07;
+                self.registers[reg] = value;
+            }
+            return true;
+        },
+        0xa000...0xbfff => {
+            if ((address & 1) == 0) {
+                self.mirroring_mode = if ((value & 1) == 0) .vertical else .horizontal;
+            } else {
+                self.prg_ram_protect = value;
+            }
+            return true;
+        },
+        0xc000...0xdfff => {
+            if ((address & 1) == 0) {
+                self.irq_latch = value;
+            } else {
+                self.irq_reload = true;
+            }
+            return true;
+        },
+        0xe000...0xffff => {
+            if ((address & 1) == 0) {
+                self.irq_enabled = false;
+                self.irq_asserted = false;
+            } else {
+                self.irq_enabled = true;
+            }
+            return true;
+        },
+        else => return false,
+    }
+}
+
+pub fn ppuRead(self: *const Mmc3, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+            return 0;
+        }
+        if (chr_rom.len == 0) return 0;
+
+        const chr_inversion = (self.bank_select & 0x80) != 0;
+        const total_1k_banks = @max(1, chr_rom.len / 0x0400);
+        var bank: usize = 0;
+
+        if (!chr_inversion) {
+            switch (address) {
+                0x0000...0x07ff => bank = (self.registers[0] & 0xfe) +% ((address >> 10) & 1),
+                0x0800...0x0fff => bank = (self.registers[1] & 0xfe) +% ((address >> 10) & 1),
+                0x1000...0x13ff => bank = self.registers[2],
+                0x1400...0x17ff => bank = self.registers[3],
+                0x1800...0x1bff => bank = self.registers[4],
+                0x1c00...0x1fff => bank = self.registers[5],
+                else => unreachable,
+            }
+        } else {
+            switch (address) {
+                0x0000...0x03ff => bank = self.registers[2],
+                0x0400...0x07ff => bank = self.registers[3],
+                0x0800...0x0bff => bank = self.registers[4],
+                0x0c00...0x0fff => bank = self.registers[5],
+                0x1000...0x17ff => bank = (self.registers[0] & 0xfe) +% ((address >> 10) & 1),
+                0x1800...0x1fff => bank = (self.registers[1] & 0xfe) +% ((address >> 10) & 1),
+                else => unreachable,
+            }
+        }
+
+        bank %= total_1k_banks;
+        const offset = (bank * 0x0400) + (address & 0x03ff);
+        if (offset < chr_rom.len) return chr_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Mmc3, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
         }
     }
-
-    bank = bank % self.chr_bank_count;
-    return bank * 0x0400 + offset;
+    return false;
 }
 
-pub fn ppuMapWrite(
-    _: *const Mmc3,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
+pub fn notifyPpuAddress(self: *Mmc3, address: u16) void {
+    const a12 = (address & 0x1000) != 0;
+    if (a12) {
+        if (!self.last_a12 and self.a12_low_cycles >= 3) {
+            // Rising edge on A12
+            if (self.irq_counter == 0 or self.irq_reload) {
+                self.irq_counter = self.irq_latch;
+                self.irq_reload = false;
+            } else {
+                self.irq_counter -= 1;
+            }
 
-pub fn handleScanline(self: *Mmc3) void {
-    if (self.irq_counter == 0 or self.irq_reload) {
-        self.irq_counter = self.irq_latch;
-        self.irq_reload = false;
+            if (self.irq_counter == 0 and self.irq_enabled) {
+                self.irq_asserted = true;
+            }
+        }
+        self.a12_low_cycles = 0;
     } else {
-        self.irq_counter -= 1;
-    }
-
-    if (self.irq_counter == 0 and self.irq_enabled) {
-        self.irq_pending = true;
+        self.a12_low_cycles +|= 1;
     }
+    self.last_a12 = a12;
 }
 
-pub fn mirroring(
-    self: *const Mmc3,
-) Mirroring {
+pub fn mirroring(self: *const Mmc3) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(
-    self: *const Mmc3,
-) bool {
-    return self.irq_pending;
+pub fn irqLine(self: *const Mmc3) bool {
+    return self.irq_asserted;
+}
+
+test "mmc3 scanline counter rising a12" {
+    var mmc3 = Mmc3.init(.vertical);
+    var ram: [0x2000]u8 = undefined;
+
+    // Set IRQ latch to 2 and enable IRQ
+    _ = mmc3.cpuWrite(0xc000, 2, &ram); // latch = 2
+    _ = mmc3.cpuWrite(0xc001, 0, &ram); // reload
+    _ = mmc3.cpuWrite(0xe001, 0, &ram); // enable
+
+    try std.testing.expect(!mmc3.irqLine());
+
+    // Cycle A12: low for 4 cycles, then high
+    for (0..4) |_| mmc3.notifyPpuAddress(0x0000);
+    mmc3.notifyPpuAddress(0x1000); // 1st rising edge: reloaded to 2
+    try std.testing.expectEqual(@as(u8, 2), mmc3.irq_counter);
+    try std.testing.expect(!mmc3.irqLine());
+
+    for (0..4) |_| mmc3.notifyPpuAddress(0x0000);
+    mmc3.notifyPpuAddress(0x1000); // 2nd rising edge: 2 -> 1
+    try std.testing.expectEqual(@as(u8, 1), mmc3.irq_counter);
+    try std.testing.expect(!mmc3.irqLine());
+
+    for (0..4) |_| mmc3.notifyPpuAddress(0x0000);
+    mmc3.notifyPpuAddress(0x1000); // 3rd rising edge: 1 -> 0, IRQ asserted!
+    try std.testing.expectEqual(@as(u8, 0), mmc3.irq_counter);
+    try std.testing.expect(mmc3.irqLine());
 }

src/system/nes/mapper/mmc4.zig

@@ -1,117 +1,147 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Mmc4 = @This();
+// Mapper 10 (MMC4) - Fire Emblem & Fire Emblem Gaiden
+// 16KB switchable PRG at $8000, fixed last 16KB at $C000-$FFFF.
+// Dual 4KB CHR latches toggled by PPU fetches at $xFD8-$xFDF and $xFE8-$xFEF.
+pub const Mmc4 = @This();
 
 prg_bank: u8 = 0,
-
-chr_banks_fd: [2]u8 = [_]u8{ 0, 0 },
-chr_banks_fe: [2]u8 = [_]u8{ 0, 0 },
-
-latch: [2]u8 = [_]u8{ 0xfe, 0xfe },
-
+chr_bank_0_fd: u8 = 0,
+chr_bank_0_fe: u8 = 0,
+chr_bank_1_fd: u8 = 0,
+chr_bank_1_fe: u8 = 0,
+latch_0: u1 = 0,
+latch_1: u1 = 0,
 mirroring_mode: Mirroring = .vertical,
-prg_bank_count: usize,
-chr_bank_count: usize,
 
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mmc4 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x1000 else 2,
-    };
+pub fn init() Mmc4 {
+    return .{};
 }
 
-pub fn cpuMapRead(
-    self: *const Mmc4,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
+pub fn cpuRead(self: *const Mmc4, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_16k_banks = @max(1, prg_rom.len / 0x4000);
+        var bank: usize = 0;
 
-    const offset = @as(usize, address & 0x3fff);
-    const last_bank = self.prg_bank_count - 1;
+        if (address < 0xc000) {
+            bank = @as(usize, self.prg_bank) % total_16k_banks;
+            const offset = (bank * 0x4000) + (address - 0x8000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        } else {
+            bank = total_16k_banks - 1;
+            const offset = (bank * 0x4000) + (address - 0xc000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        }
+        return 0;
+    }
+    return null;
+}
 
-    if (address < 0xc000) {
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        return last_bank * 0x4000 + offset;
+pub fn cpuWrite(self: *Mmc4, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+        return false;
+    }
+    switch (address) {
+        0xa000...0xafff => {
+            self.prg_bank = value & 0x0f;
+            return true;
+        },
+        0xb000...0xbfff => {
+            self.chr_bank_0_fd = value & 0x1f;
+            return true;
+        },
+        0xc000...0xcfff => {
+            self.chr_bank_0_fe = value & 0x1f;
+            return true;
+        },
+        0xd000...0xdfff => {
+            self.chr_bank_1_fd = value & 0x1f;
+            return true;
+        },
+        0xe000...0xefff => {
+            self.chr_bank_1_fe = value & 0x1f;
+            return true;
+        },
+        0xf000...0xffff => {
+            self.mirroring_mode = if ((value & 1) == 0) .vertical else .horizontal;
+            return true;
+        },
+        else => return false,
     }
 }
 
-pub fn cpuWrite(
-    self: *Mmc4,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0xa000) return;
+pub fn ppuRead(self: *const Mmc4, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+            return 0;
+        }
+        if (chr_rom.len == 0) return 0;
 
-    switch (address & 0xf000) {
-        0xa000 => self.prg_bank = value & 0x0f,
-        0xb000 => self.chr_banks_fd[0] = value & 0x1f,
-        0xc000 => self.chr_banks_fe[0] = value & 0x1f,
-        0xd000 => self.chr_banks_fd[1] = value & 0x1f,
-        0xe000 => self.chr_banks_fe[1] = value & 0x1f,
-        0xf000 => self.mirroring_mode = if ((value & 1) != 0) .horizontal else .vertical,
+        const total_4k_banks = @max(1, chr_rom.len / 0x1000);
+        const bank = if (address < 0x1000)
+            (if (self.latch_0 == 0) self.chr_bank_0_fd else self.chr_bank_0_fe)
+        else
+            (if (self.latch_1 == 0) self.chr_bank_1_fd else self.chr_bank_1_fe);
+
+        const offset = ((@as(usize, bank) % total_4k_banks) * 0x1000) + (address & 0x0fff);
+        if (offset < chr_rom.len) return chr_rom[offset];
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(_: *Mmc4, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn notifyPpuAddress(self: *Mmc4, address: u16) void {
+    self.checkLatch(address);
+}
+
+inline fn checkLatch(self: *Mmc4, address: u16) void {
+    switch (address) {
+        0x0fd8...0x0fdf => self.latch_0 = 0,
+        0x0fe8...0x0fef => self.latch_0 = 1,
+        0x1fd8...0x1fdf => self.latch_1 = 0,
+        0x1fe8...0x1fef => self.latch_1 = 1,
         else => {},
     }
 }
 
-pub fn ppuMapRead(
-    self: *Mmc4,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x1000;
-    const offset = @as(usize, address & 0x0fff);
-
-    const bank_raw = if (self.latch[slot] == 0xfd)
-        self.chr_banks_fd[slot]
-    else
-        self.chr_banks_fe[slot];
-
-    const bank = @as(usize, bank_raw) % self.chr_bank_count;
-    const result = bank * 0x1000 + offset;
-
-    // Check PPU tile latch triggers ($0FD0-$0FDF, $0FE0-$0FEF, $1FD0-$1FDF, $1FE0-$1FEF)
-    if (address == 0x0fd0 or address == 0x0fe0) {
-        self.latch[0] = @truncate(address >> 4);
-    } else if (address == 0x1fd0 or address == 0x1fe0) {
-        self.latch[1] = @truncate(address >> 4);
-    }
-
-    return result;
-}
-
-pub fn ppuMapWrite(
-    self: *const Mmc4,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x1000;
-    const offset = @as(usize, address & 0x0fff);
-
-    const bank_raw = if (self.latch[slot] == 0xfd)
-        self.chr_banks_fd[slot]
-    else
-        self.chr_banks_fe[slot];
-
-    const bank = @as(usize, bank_raw) % self.chr_bank_count;
-    return bank * 0x1000 + offset;
-}
-
-pub fn mirroring(
-    self: *const Mmc4,
-) Mirroring {
+pub fn mirroring(self: *const Mmc4) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(
-    _: *const Mmc4,
-) bool {
+pub fn irqLine(_: *const Mmc4) bool {
     return false;
 }
+
+test "mmc4 latch toggling" {
+    var m = Mmc4.init();
+    var ram: [0x2000]u8 = undefined;
+
+    _ = m.cpuWrite(0xa000, 3, &ram); // PRG bank 3
+    try std.testing.expectEqual(@as(u8, 3), m.prg_bank);
+
+    m.notifyPpuAddress(0x1fe8);
+    try std.testing.expectEqual(@as(u1, 1), m.latch_1);
+}

src/system/nes/mapper/mmc5.zig

@@ -1,171 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Mmc5 = @This();
-
-prg_mode: u2 = 3,
-chr_mode: u2 = 0,
-
-prg_banks: [4]u8 = [_]u8{ 0, 0, 1, 2 }, // $6000, $8000, $A000, $C000
-chr_banks: [12]u16 = [_]u16{0} ** 12,
-
-mult_a: u8 = 0,
-mult_b: u8 = 0,
-
-irq_scanline: u8 = 0,
-irq_enabled: bool = false,
-irq_assert: bool = false,
-irq_in_frame: bool = false,
-current_scanline: u8 = 0,
-
-mirroring_mode: Mirroring = .vertical,
-ex_ram: [1024]u8 = [_]u8{0} ** 1024,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Mmc5 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Mmc5,
-    address: u16,
-) ?usize {
-    if (address < 0x5c00) return null;
-
-    if (address >= 0x5c00 and address <= 0x5fff) {
-        return 0x80000000 | @as(usize, address - 0x5c00);
-    }
-
-    if (address < 0x8000) {
-        const bank = @as(usize, self.prg_banks[0] & 0x7f) % self.prg_bank_count;
-        return bank * 0x2000 + (address & 0x1fff);
-    }
-
-    const last_bank = self.prg_bank_count - 1;
-    const offset = @as(usize, address & 0x1fff);
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => return (@as(usize, self.prg_banks[1] & 0x7f) % self.prg_bank_count) * 0x2000 + offset,
-        1 => return (@as(usize, self.prg_banks[2] & 0x7f) % self.prg_bank_count) * 0x2000 + offset,
-        2 => return (@as(usize, self.prg_banks[3] & 0x7f) % self.prg_bank_count) * 0x2000 + offset,
-        3 => return last_bank * 0x2000 + offset,
-        else => unreachable,
-    }
-}
-
-pub fn cpuWrite(
-    self: *Mmc5,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x5c00 and address <= 0x5fff) {
-        self.ex_ram[address - 0x5c00] = value;
-        return;
-    }
-
-    switch (address) {
-        0x5100 => self.prg_mode = @truncate(value & 3),
-        0x5101 => self.chr_mode = @truncate(value & 3),
-        0x5105 => {
-            switch (@as(u2, @truncate(value & 3))) {
-                0 => self.mirroring_mode = .vertical,
-                1 => self.mirroring_mode = .horizontal,
-                2 => self.mirroring_mode = .single_screen_lower,
-                3 => self.mirroring_mode = .single_screen_upper,
-            }
-        },
-        0x5114 => self.prg_banks[0] = value,
-        0x5115 => self.prg_banks[1] = value,
-        0x5116 => self.prg_banks[2] = value,
-        0x5117 => self.prg_banks[3] = value,
-        0x5120...0x512b => |addr| {
-            const idx = addr - 0x5120;
-            self.chr_banks[idx] = value;
-        },
-        0x5203 => self.irq_scanline = value,
-        0x5204 => {
-            self.irq_enabled = (value & 0x80) != 0;
-            self.irq_assert = false;
-        },
-        0x5205 => self.mult_a = value,
-        0x5206 => self.mult_b = value,
-        else => {},
-    }
-}
-
-pub fn readRegister(
-    self: *Mmc5,
-    address: u16,
-) ?u8 {
-    switch (address) {
-        0x5204 => {
-            const res: u8 = (if (self.irq_assert) @as(u8, 0x80) else 0) | (if (self.irq_in_frame) @as(u8, 0x40) else 0);
-            self.irq_assert = false;
-            return res;
-        },
-        0x5205 => {
-            const prod = @as(u16, self.mult_a) * @as(u16, self.mult_b);
-            return @truncate(prod);
-        },
-        0x5206 => {
-            const prod = @as(u16, self.mult_a) * @as(u16, self.mult_b);
-            return @truncate(prod >> 8);
-        },
-        else => return null,
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Mmc5,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0400;
-    const offset = @as(usize, address & 0x03ff);
-    const bank = @as(usize, self.chr_banks[slot]) % self.chr_bank_count;
-
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Mmc5,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn handleScanline(
-    self: *Mmc5,
-) void {
-    self.current_scanline += 1;
-    if (self.current_scanline == self.irq_scanline) {
-        if (self.irq_enabled) {
-            self.irq_assert = true;
-        }
-    }
-    if (self.current_scanline >= 240) {
-        self.current_scanline = 0;
-        self.irq_in_frame = false;
-    }
-}
-
-pub fn mirroring(
-    self: *const Mmc5,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(
-    self: *const Mmc5,
-) bool {
-    return self.irq_assert;
-}

src/system/nes/mapper/nina03.zig

@@ -1,73 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Nina03 = @This();
-
-prg_bank: u8 = 0,
-chr_bank: u8 = 0,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-mirroring_mode: Mirroring,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    _mirroring: Mirroring,
-) Nina03 {
-    return .{
-        .prg_bank_count = prg_size / 0x8000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Nina03,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address - 0x8000);
-    const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-    return bank * 0x8000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *Nina03,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x4100 and address <= 0x5FFF) {
-        self.prg_bank = @truncate((value >> 3) & 1);
-        self.chr_bank = value & 0x07;
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Nina03,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
-    return bank * 0x2000 + @as(usize, address);
-}
-
-pub fn ppuMapWrite(
-    _: *const Nina03,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Nina03,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Nina03) bool {
-    return false;
-}

src/system/nes/mapper/nrom.zig

@@ -1,71 +1,83 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Nrom = @This();
+pub const Nrom = @This();
 
-prg_16k: bool,
 mirroring_mode: Mirroring,
 
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Nrom {
-    return .{
-        .prg_16k = prg_size == 0x4000,
-        .mirroring_mode = _mirroring,
-    };
+pub fn init(initial_mirroring: Mirroring) Nrom {
+    return .{ .mirroring_mode = initial_mirroring };
 }
 
-pub fn cpuMapRead(
-    self: *const Nrom,
-    address: u16,
-) ?usize {
-    if (address < 0x8000)
-        return null;
-
-    const offset =
-        @as(usize, address) - 0x8000;
-
-    return if (self.prg_16k)
-        offset & 0x3fff
-    else
-        offset;
+pub fn cpuRead(_: *const Nrom, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const mask: usize = if (prg_rom.len <= 0x4000) 0x3fff else 0x7fff;
+        const offset = (address - 0x8000) & mask;
+        if (offset < prg_rom.len) return prg_rom[offset];
+        return 0;
+    }
+    return null;
 }
 
-pub fn cpuWrite(
-    _: *Nrom,
-    _: u16,
-    _: u8,
-) void {
-    // NROM has no mapper registers.
+pub fn cpuWrite(_: *Nrom, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    return false;
 }
 
-pub fn ppuMapRead(
-    _: *const Nrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000)
-        return null;
-
-    return address;
+pub fn ppuRead(_: *const Nrom, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else {
+            if (chr_rom.len > 0) return chr_rom[address & (chr_rom.len - 1)];
+        }
+        return 0;
+    }
+    return null;
 }
 
-pub fn ppuMapWrite(
-    _: *const Nrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000)
-        return null;
-
-    return address;
+pub fn ppuWrite(_: *Nrom, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
 }
 
-pub fn mirroring(
-    self: *const Nrom,
-) Mirroring {
+pub fn mirroring(self: *const Nrom) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(_: *const Nrom) bool {
+pub fn irqLine(_: *const Nrom) bool {
     return false;
 }
+
+pub fn notifyPpuAddress(_: *Nrom, _: u16) void {}
+
+test "nrom prg mirroring" {
+    var nrom = Nrom.init(.horizontal);
+    var prg_16k: [0x4000]u8 = undefined;
+    @memset(&prg_16k, 0x42);
+    prg_16k[0] = 0x11;
+
+    var ram: [0x2000]u8 = undefined;
+    @memset(&ram, 0);
+
+    // 16K PRG is mirrored at $8000 and $C000
+    try std.testing.expectEqual(@as(?u8, 0x11), nrom.cpuRead(0x8000, &prg_16k, &ram));
+    try std.testing.expectEqual(@as(?u8, 0x11), nrom.cpuRead(0xc000, &prg_16k, &ram));
+}

src/system/nes/mapper/root.zig

@@ -1,147 +1,97 @@
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
+
 pub const Nrom = @import("nrom.zig");
-pub const Uxrom = @import("uxrom.zig");
 pub const Mmc1 = @import("mmc1.zig");
+pub const Uxrom = @import("uxrom.zig");
 pub const Cnrom = @import("cnrom.zig");
 pub const Mmc3 = @import("mmc3.zig");
-pub const Mmc4 = @import("mmc4.zig");
-pub const Mmc5 = @import("mmc5.zig");
 pub const Axrom = @import("axrom.zig");
+pub const ColorDreams = @import("color_dreams.zig");
+pub const Unrom512 = @import("unrom512.zig");
+pub const Bnrom = @import("bnrom.zig");
 pub const Gxrom = @import("gxrom.zig");
-pub const Mmc2 = @import("mmc2.zig");
-pub const ColorDreams = @import("colordreams.zig");
 pub const Camerica = @import("camerica.zig");
-pub const Mapper13 = @import("mapper13.zig");
-pub const Mapper34 = @import("mapper34.zig");
-pub const Mapper65 = @import("mapper65.zig");
-pub const Mapper68 = @import("mapper68.zig");
-pub const Mapper69 = @import("mapper69.zig");
-pub const Nina03 = @import("nina03.zig");
-pub const Mapper76 = @import("mapper76.zig");
-pub const Irem78 = @import("irem78.zig");
-pub const Mapper87 = @import("mapper87.zig");
-pub const Mapper88 = @import("mapper88.zig");
-pub const Mapper89 = @import("mapper89.zig");
-pub const Mapper93 = @import("mapper93.zig");
-pub const Mapper94 = @import("mapper94.zig");
-pub const Mapper118 = @import("mapper118.zig");
-pub const Mapper119 = @import("mapper119.zig");
-pub const Mapper140 = @import("mapper140.zig");
+pub const Mmc2 = @import("mmc2.zig");
+pub const Mmc4 = @import("mmc4.zig");
+pub const Action53 = @import("action53.zig");
 pub const Mapper180 = @import("mapper180.zig");
-pub const Vrc1 = @import("vrc1.zig");
-pub const Vrc2 = @import("vrc2.zig");
-pub const Mapper232 = @import("mapper232.zig");
-pub const Vrc3 = @import("vrc3.zig");
-pub const Vrc4 = @import("vrc4.zig");
-pub const Vrc6 = @import("vrc6.zig");
-pub const Vrc7 = @import("vrc7.zig");
-pub const Mapper210 = @import("mapper210.zig");
-pub const Mapper206 = @import("mapper206.zig");
-
-const types = @import("types.zig");
-pub const Mirroring = types.Mirroring;
 
 pub const Mapper = union(enum) {
     nrom: Nrom,
-    uxrom: Uxrom,
     mmc1: Mmc1,
+    uxrom: Uxrom,
     cnrom: Cnrom,
     mmc3: Mmc3,
-    mmc4: Mmc4,
-    mmc5: Mmc5,
     axrom: Axrom,
-    gxrom: Gxrom,
-    mmc2: Mmc2,
     color_dreams: ColorDreams,
+    unrom512: Unrom512,
+    bnrom: Bnrom,
+    gxrom: Gxrom,
     camerica: Camerica,
-    mapper13: Mapper13,
-    mapper34: Mapper34,
-    mapper65: Mapper65,
-    mapper68: Mapper68,
-    mapper69: Mapper69,
-    nina03: Nina03,
-    mapper76: Mapper76,
-    irem78: Irem78,
-    mapper87: Mapper87,
-    mapper88: Mapper88,
-    mapper89: Mapper89,
-    mapper93: Mapper93,
-    mapper94: Mapper94,
-    mapper118: Mapper118,
-    mapper119: Mapper119,
-    mapper140: Mapper140,
+    mmc2: Mmc2,
+    mmc4: Mmc4,
+    action53: Action53,
     mapper180: Mapper180,
-    vrc1: Vrc1,
-    vrc2: Vrc2,
-    mapper232: Mapper232,
-    vrc3: Vrc3,
-    vrc4: Vrc4,
-    vrc6: Vrc6,
-    vrc7: Vrc7,
-    mapper210: Mapper210,
-    mapper206: Mapper206,
 
-    pub fn cpuMapRead(
-        self: *const Mapper,
-        address: u16,
-    ) ?usize {
+    pub fn cpuRead(self: *const Mapper, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
         return switch (self.*) {
-            inline else => |*mapper| mapper.cpuMapRead(address),
+            inline else => |*m| m.cpuRead(address, prg_rom, prg_ram),
         };
     }
 
-    pub fn cpuWrite(
-        self: *Mapper,
-        address: u16,
-        value: u8,
-    ) void {
+    pub fn cpuWrite(self: *Mapper, address: u16, value: u8, prg_ram: []u8) bool {
+        return switch (self.*) {
+            inline else => |*m| m.cpuWrite(address, value, prg_ram),
+        };
+    }
+
+    pub fn ppuRead(self: *const Mapper, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+        return switch (self.*) {
+            inline else => |*m| m.ppuRead(address, chr_rom, chr_ram, chr_is_ram),
+        };
+    }
+
+    pub fn ppuWrite(self: *Mapper, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+        return switch (self.*) {
+            inline else => |*m| m.ppuWrite(address, value, chr_ram, chr_is_ram),
+        };
+    }
+
+    pub fn mirroring(self: *const Mapper) Mirroring {
+        return switch (self.*) {
+            inline else => |*m| m.mirroring(),
+        };
+    }
+
+    pub fn irqLine(self: *const Mapper) bool {
+        return switch (self.*) {
+            inline else => |*m| m.irqLine(),
+        };
+    }
+
+    pub fn notifyPpuAddress(self: *Mapper, address: u16) void {
         switch (self.*) {
-            inline else => |*mapper| mapper.cpuWrite(address, value),
-        }
-    }
-
-    pub fn ppuMapRead(
-        self: *Mapper,
-        address: u16,
-    ) ?usize {
-        return switch (self.*) {
-            inline else => |*mapper| mapper.ppuMapRead(address),
-        };
-    }
-
-    pub fn ppuMapWrite(
-        self: *const Mapper,
-        address: u16,
-    ) ?usize {
-        return switch (self.*) {
-            inline else => |*mapper| mapper.ppuMapWrite(address),
-        };
-    }
-
-    pub fn mirroring(
-        self: *const Mapper,
-    ) Mirroring {
-        return switch (self.*) {
-            inline else => |*mapper| mapper.mirroring(),
-        };
-    }
-
-    pub fn irqAsserted(
-        self: *const Mapper,
-    ) bool {
-        return switch (self.*) {
-            inline else => |*mapper| mapper.irqAsserted(),
-        };
-    }
-
-    pub fn handleScanline(
-        self: *Mapper,
-    ) void {
-        switch (self.*) {
-            .mmc3 => |*mmc3| mmc3.handleScanline(),
-            .mmc5 => |*mmc5| mmc5.handleScanline(),
-            .mapper118 => |*m118| m118.handleScanline(),
-            .mapper119 => |*m119| m119.handleScanline(),
-            else => {},
+            inline else => |*m| m.notifyPpuAddress(address),
         }
     }
 };
+
+test {
+    _ = Nrom;
+    _ = Mmc1;
+    _ = Uxrom;
+    _ = Cnrom;
+    _ = Mmc3;
+    _ = Axrom;
+    _ = ColorDreams;
+    _ = Unrom512;
+    _ = Bnrom;
+    _ = Gxrom;
+    _ = Camerica;
+    _ = Mmc2;
+    _ = Mmc4;
+    _ = Action53;
+    _ = Mapper180;
+}

src/system/nes/mapper/types.zig

@@ -1,9 +0,0 @@
-pub const Mirroring = enum {
-    horizontal,
-    vertical,
-    four_screen,
-
-    // Needed by later mappers.
-    single_screen_lower,
-    single_screen_upper,
-};

src/system/nes/mapper/unrom512.zig

@@ -0,0 +1,112 @@
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
+
+// Mapper 30 (UNROM-512) - 16KB PRG bank (bits 0-4), 8KB CHR-RAM bank (bits 5-6), 1-screen mirroring (bit 7)
+pub const Unrom512 = @This();
+
+prg_bank: u8 = 0,
+chr_bank: u8 = 0,
+mirroring_mode: Mirroring,
+supports_mirroring_control: bool,
+
+pub fn init(initial_mirroring: Mirroring) Unrom512 {
+    return .{
+        .mirroring_mode = initial_mirroring,
+        .supports_mirroring_control = (initial_mirroring == .single_screen_lower or initial_mirroring == .single_screen_upper),
+    };
+}
+
+pub fn cpuRead(self: *const Unrom512, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_16k_banks = @max(1, prg_rom.len / 0x4000);
+        var bank: usize = 0;
+        if (address < 0xc000) {
+            bank = @as(usize, self.prg_bank) % total_16k_banks;
+            const offset = (bank * 0x4000) + (address - 0x8000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        } else {
+            bank = total_16k_banks - 1;
+            const offset = (bank * 0x4000) + (address - 0xc000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn cpuWrite(self: *Unrom512, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        self.prg_bank = value & 0x1f;
+        self.chr_bank = (value >> 5) & 0x03;
+        if (self.supports_mirroring_control) {
+            self.mirroring_mode = if ((value & 0x80) != 0) .single_screen_upper else .single_screen_lower;
+        }
+        return true;
+    }
+    return false;
+}
+
+pub fn ppuRead(self: *const Unrom512, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            const total_8k_banks = @max(1, chr_ram.len / 0x2000);
+            const bank = @as(usize, self.chr_bank) % total_8k_banks;
+            const offset = (bank * 0x2000) + address;
+            if (offset < chr_ram.len) return chr_ram[offset];
+        } else if (chr_rom.len > 0) {
+            const total_8k_banks = @max(1, chr_rom.len / 0x2000);
+            const bank = @as(usize, self.chr_bank) % total_8k_banks;
+            const offset = (bank * 0x2000) + address;
+            if (offset < chr_rom.len) return chr_rom[offset];
+        }
+        return 0;
+    }
+    return null;
+}
+
+pub fn ppuWrite(self: *Unrom512, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        const total_8k_banks = @max(1, chr_ram.len / 0x2000);
+        const bank = @as(usize, self.chr_bank) % total_8k_banks;
+        const offset = (bank * 0x2000) + address;
+        if (offset < chr_ram.len) {
+            chr_ram[offset] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Unrom512) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqLine(_: *const Unrom512) bool {
+    return false;
+}
+
+pub fn notifyPpuAddress(_: *Unrom512, _: u16) void {}
+
+test "unrom512 bank switching" {
+    var un = Unrom512.init(.single_screen_lower);
+    var ram: [0x2000]u8 = undefined;
+
+    // Write: PRG bank 5, CHR bank 2, Upper screen mirroring
+    _ = un.cpuWrite(0x8000, 0x80 | (2 << 5) | 5, &ram);
+    try std.testing.expectEqual(@as(u8, 5), un.prg_bank);
+    try std.testing.expectEqual(@as(u8, 2), un.chr_bank);
+    try std.testing.expectEqual(Mirroring.single_screen_upper, un.mirroring());
+}

src/system/nes/mapper/uxrom.zig

@@ -1,73 +1,83 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
+const std = @import("std");
+const common = @import("../common.zig");
+const Mirroring = common.Mirroring;
 
-const Uxrom = @This();
+pub const Uxrom = @This();
 
-prg_bank_select: u8 = 0,
-prg_bank_count: usize,
+prg_bank: u8 = 0,
 mirroring_mode: Mirroring,
 
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Uxrom {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .mirroring_mode = _mirroring,
-    };
+pub fn init(initial_mirroring: Mirroring) Uxrom {
+    return .{ .mirroring_mode = initial_mirroring };
 }
 
-pub fn cpuMapRead(
-    self: *const Uxrom,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x3fff);
-
-    if (address < 0xc000) {
-        // Switchable 16 KB bank
-        const bank = @as(usize, self.prg_bank_select) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        // Fixed to last 16 KB bank
-        const last_bank = self.prg_bank_count - 1;
-        return last_bank * 0x4000 + offset;
+pub fn cpuRead(self: *const Uxrom, address: u16, prg_rom: []const u8, prg_ram: []const u8) ?u8 {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) return prg_ram[offset];
+        return 0;
     }
-}
-
-pub fn cpuWrite(
-    self: *Uxrom,
-    address: u16,
-    value: u8,
-) void {
-    if (address >= 0x8000) {
-        self.prg_bank_select = value;
+    if (address >= 0x8000 and address <= 0xffff) {
+        const total_16k_banks = @max(1, prg_rom.len / 0x4000);
+        var bank: usize = 0;
+        if (address < 0xc000) {
+            bank = @as(usize, self.prg_bank) % total_16k_banks;
+            const offset = (bank * 0x4000) + (address - 0x8000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        } else {
+            bank = total_16k_banks - 1;
+            const offset = (bank * 0x4000) + (address - 0xc000);
+            if (offset < prg_rom.len) return prg_rom[offset];
+        }
+        return 0;
     }
+    return null;
 }
 
-pub fn ppuMapRead(
-    _: *const Uxrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
+pub fn cpuWrite(self: *Uxrom, address: u16, value: u8, prg_ram: []u8) bool {
+    if (address >= 0x6000 and address <= 0x7fff) {
+        const offset = address - 0x6000;
+        if (offset < prg_ram.len) {
+            prg_ram[offset] = value;
+            return true;
+        }
+    }
+    if (address >= 0x8000 and address <= 0xffff) {
+        // mask up to 32 banks (512KB) for UOROM / UNROM
+        self.prg_bank = value & 0x1f;
+        return true;
+    }
+    return false;
 }
 
-pub fn ppuMapWrite(
-    _: *const Uxrom,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
+pub fn ppuRead(_: *const Uxrom, address: u16, chr_rom: []const u8, chr_ram: []const u8, chr_is_ram: bool) ?u8 {
+    if (address < 0x2000) {
+        if (chr_is_ram) {
+            if (address < chr_ram.len) return chr_ram[address];
+        } else {
+            if (address < chr_rom.len) return chr_rom[address];
+        }
+        return 0;
+    }
+    return null;
 }
 
-pub fn mirroring(
-    self: *const Uxrom,
-) Mirroring {
+pub fn ppuWrite(_: *Uxrom, address: u16, value: u8, chr_ram: []u8, chr_is_ram: bool) bool {
+    if (address < 0x2000 and chr_is_ram) {
+        if (address < chr_ram.len) {
+            chr_ram[address] = value;
+            return true;
+        }
+    }
+    return false;
+}
+
+pub fn mirroring(self: *const Uxrom) Mirroring {
     return self.mirroring_mode;
 }
 
-pub fn irqAsserted(_: *const Uxrom) bool {
+pub fn irqLine(_: *const Uxrom) bool {
     return false;
 }
+
+pub fn notifyPpuAddress(_: *Uxrom, _: u16) void {}

src/system/nes/mapper/vrc1.zig

@@ -1,106 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Vrc1 = @This();
-
-prg_bank0: u8 = 0,
-prg_bank1: u8 = 0,
-prg_bank2: u8 = 0,
-
-chr_bank0: u8 = 0,
-chr_bank1: u8 = 0,
-
-mirroring_mode: Mirroring = .vertical,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Vrc1 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x1000 else 1,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Vrc1,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xA000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank0) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else if (address < 0xC000) {
-        const offset = @as(usize, address - 0xA000);
-        const bank = @as(usize, self.prg_bank1) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else if (address < 0xE000) {
-        const offset = @as(usize, address - 0xC000);
-        const bank = @as(usize, self.prg_bank2) % self.prg_bank_count;
-        return bank * 0x2000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xE000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x2000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Vrc1,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    switch (address & 0xF000) {
-        0x8000 => self.prg_bank0 = value & 0x0F,
-        0x9000 => {
-            self.mirroring_mode = if ((value & 1) != 0) .horizontal else .vertical;
-            self.chr_bank0 = (self.chr_bank0 & 0x0F) | ((value & 0x02) << 3);
-            self.chr_bank1 = (self.chr_bank1 & 0x0F) | ((value & 0x04) << 2);
-        },
-        0xA000 => self.prg_bank1 = value & 0x0F,
-        0xC000 => self.prg_bank2 = value & 0x0F,
-        0xE000 => self.chr_bank0 = (self.chr_bank0 & 0x10) | (value & 0x0F),
-        0xF000 => self.chr_bank1 = (self.chr_bank1 & 0x10) | (value & 0x0F),
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Vrc1,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    if (address < 0x1000) {
-        const bank = @as(usize, self.chr_bank0) % self.chr_bank_count;
-        return bank * 0x1000 + @as(usize, address);
-    } else {
-        const bank = @as(usize, self.chr_bank1) % self.chr_bank_count;
-        return bank * 0x1000 + @as(usize, address - 0x1000);
-    }
-}
-
-pub fn ppuMapWrite(
-    _: *const Vrc1,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Vrc1,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(_: *const Vrc1) bool {
-    return false;
-}

src/system/nes/mapper/vrc2.zig

@@ -1,127 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Vrc2 = @This();
-
-prg_banks: [2]u8 = [_]u8{ 0, 1 },
-chr_banks_lo: [8]u4 = [_]u4{0} ** 8,
-chr_banks_hi: [8]u5 = [_]u5{0} ** 8,
-
-mirroring_mode: Mirroring = .vertical,
-
-mapper_id: u8,
-shift_a: u3,
-shift_b: u3,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mapper_id: u8,
-) Vrc2 {
-    const shift_a: u3 = if (mapper_id == 22) 1 else 0;
-    const shift_b: u3 = if (mapper_id == 22) 0 else 1;
-
-    return .{
-        .mapper_id = mapper_id,
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-        .shift_a = shift_a,
-        .shift_b = shift_b,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Vrc2,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const last_bank = self.prg_bank_count - 1;
-    const second_last_bank = self.prg_bank_count - 2;
-
-    const r0 = @as(usize, self.prg_banks[0] & 0x1f) % self.prg_bank_count;
-    const r1 = @as(usize, self.prg_banks[1] & 0x1f) % self.prg_bank_count;
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => return r0 * 0x2000 + offset,
-        1 => return r1 * 0x2000 + offset,
-        2 => return second_last_bank * 0x2000 + offset,
-        3 => return last_bank * 0x2000 + offset,
-        else => unreachable,
-    }
-}
-
-pub fn cpuWrite(
-    self: *Vrc2,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    const reg_addr = address & 0xf000;
-    const a = @as(u1, @truncate((address >> self.shift_a) & 1));
-    const b = @as(u1, @truncate((address >> self.shift_b) & 1));
-    const line = (@as(u2, b) << 1) | a;
-
-    switch (reg_addr) {
-        0x8000 => self.prg_banks[0] = value & 0x1f,
-        0x9000 => {
-            self.mirroring_mode = if ((value & 0x01) != 0) .horizontal else .vertical;
-        },
-        0xa000 => self.prg_banks[1] = value & 0x1f,
-        0xb000, 0xc000, 0xd000, 0xe000 => {
-            const slot_base = ((reg_addr - 0xb000) / 0x1000) * 2;
-            const is_hi = (line & 2) != 0;
-            const is_odd = (line & 1) != 0;
-            const slot = slot_base + (if (is_odd) @as(usize, 1) else 0);
-
-            if (!is_hi) {
-                self.chr_banks_lo[slot] = @truncate(value & 0x0f);
-            } else {
-                self.chr_banks_hi[slot] = @truncate(value & 0x1f);
-            }
-        },
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Vrc2,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0400;
-    const offset = @as(usize, address & 0x03ff);
-
-    var bank_val = (@as(usize, self.chr_banks_hi[slot]) << 4) | @as(usize, self.chr_banks_lo[slot]);
-    if (self.mapper_id == 22) {
-        bank_val >>= 1;
-    }
-    const bank = bank_val % self.chr_bank_count;
-
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Vrc2,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Vrc2,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(
-    _: *const Vrc2,
-) bool {
-    return false;
-}

src/system/nes/mapper/vrc3.zig

@@ -1,118 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Vrc3 = @This();
-
-prg_bank: u8 = 0,
-prg_bank_count: usize,
-mirroring_mode: Mirroring,
-
-irq_latch: u16 = 0,
-irq_counter: u16 = 0,
-irq_enabled: bool = false,
-irq_mode_8bit: bool = false,
-irq_asserted_flag: bool = false,
-
-pub fn init(
-    prg_size: usize,
-    _mirroring: Mirroring,
-) Vrc3 {
-    return .{
-        .prg_bank_count = prg_size / 0x4000,
-        .mirroring_mode = _mirroring,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Vrc3,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    if (address < 0xC000) {
-        const offset = @as(usize, address - 0x8000);
-        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
-        return bank * 0x4000 + offset;
-    } else {
-        const offset = @as(usize, address - 0xC000);
-        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
-        return fixed_bank * 0x4000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Vrc3,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    switch (address & 0xF000) {
-        0x8000 => self.irq_latch = (self.irq_latch & 0xFFF0) | (value & 0x0F),
-        0x9000 => self.irq_latch = (self.irq_latch & 0xFF0F) | ((@as(u16, value) & 0x0F) << 4),
-        0xA000 => self.irq_latch = (self.irq_latch & 0xF0FF) | ((@as(u16, value) & 0x0F) << 8),
-        0xB000 => self.irq_latch = (self.irq_latch & 0x0FFF) | ((@as(u16, value) & 0x0F) << 12),
-        0xC000 => {
-            self.irq_enabled = (value & 0x02) != 0;
-            self.irq_mode_8bit = (value & 0x04) != 0;
-            self.irq_asserted_flag = false;
-            if (self.irq_enabled) {
-                self.irq_counter = self.irq_latch;
-            }
-        },
-        0xD000 => {
-            self.irq_asserted_flag = false;
-        },
-        0xF000 => {
-            self.prg_bank = value & 0x07;
-        },
-        else => {},
-    }
-}
-
-pub fn tickCycle(self: *Vrc3) void {
-    if (self.irq_enabled) {
-        if (self.irq_mode_8bit) {
-            const low = @as(u8, @truncate(self.irq_counter));
-            if (low == 0xFF) {
-                self.irq_counter = (self.irq_counter & 0xFF00) | @as(u16, @as(u8, @truncate(self.irq_latch)));
-                self.irq_asserted_flag = true;
-            } else {
-                self.irq_counter += 1;
-            }
-        } else {
-            if (self.irq_counter == 0xFFFF) {
-                self.irq_counter = self.irq_latch;
-                self.irq_asserted_flag = true;
-            } else {
-                self.irq_counter += 1;
-            }
-        }
-    }
-}
-
-pub fn ppuMapRead(
-    _: *const Vrc3,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn ppuMapWrite(
-    _: *const Vrc3,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-    return address;
-}
-
-pub fn mirroring(
-    self: *const Vrc3,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(self: *const Vrc3) bool {
-    return self.irq_asserted_flag;
-}

src/system/nes/mapper/vrc4.zig

@@ -1,179 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Vrc4 = @This();
-
-prg_banks: [2]u8 = [_]u8{ 0, 1 },
-chr_banks_lo: [8]u4 = [_]u4{0} ** 8,
-chr_banks_hi: [8]u5 = [_]u5{0} ** 8,
-
-prg_mode: u1 = 0,
-mirroring_mode: Mirroring = .vertical,
-
-irq_reload: u8 = 0,
-irq_counter: u8 = 0,
-irq_enabled: bool = false,
-irq_enable_on_ack: bool = false,
-irq_mode: u1 = 0, // 0 = cycle, 1 = scanline
-irq_prescaler: i16 = 341,
-irq_assert: bool = false,
-
-shift_a: u3,
-shift_b: u3,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mapper_id: u8,
-) Vrc4 {
-    const shift_a: u3 = if (mapper_id == 21) 1 else 0;
-    const shift_b: u3 = if (mapper_id == 21) 2 else 2;
-
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-        .shift_a = shift_a,
-        .shift_b = shift_b,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Vrc4,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const last_bank = self.prg_bank_count - 1;
-    const second_last_bank = self.prg_bank_count - 2;
-
-    const r0 = @as(usize, self.prg_banks[0] & 0x1f) % self.prg_bank_count;
-    const r1 = @as(usize, self.prg_banks[1] & 0x1f) % self.prg_bank_count;
-
-    const bank_idx = (address - 0x8000) / 0x2000;
-    var bank: usize = 0;
-
-    if (self.prg_mode == 0) {
-        switch (bank_idx) {
-            0 => bank = r0,
-            1 => bank = r1,
-            2 => bank = second_last_bank,
-            3 => bank = last_bank,
-            else => unreachable,
-        }
-    } else {
-        switch (bank_idx) {
-            0 => bank = second_last_bank,
-            1 => bank = r1,
-            2 => bank = r0,
-            3 => bank = last_bank,
-            else => unreachable,
-        }
-    }
-
-    return bank * 0x2000 + offset;
-}
-
-pub fn cpuWrite(
-    self: *Vrc4,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    const reg_addr = address & 0xf000;
-    const a = @as(u1, @truncate((address >> self.shift_a) & 1));
-    const b = @as(u1, @truncate((address >> self.shift_b) & 1));
-    const line = (@as(u2, b) << 1) | a;
-
-    switch (reg_addr) {
-        0x8000 => {
-            self.prg_banks[0] = value & 0x1f;
-        },
-        0x9000 => {
-            if (line == 0) {
-                switch (@as(u2, @truncate(value & 3))) {
-                    0 => self.mirroring_mode = .vertical,
-                    1 => self.mirroring_mode = .horizontal,
-                    2 => self.mirroring_mode = .single_screen_lower,
-                    3 => self.mirroring_mode = .single_screen_upper,
-                }
-            } else if (line == 1 or line == 2) {
-                self.prg_mode = @truncate((value >> 1) & 1);
-            }
-        },
-        0xa000 => {
-            self.prg_banks[1] = value & 0x1f;
-        },
-        0xb000, 0xc000, 0xd000, 0xe000 => {
-            const slot_base = ((reg_addr - 0xb000) / 0x1000) * 2;
-            const is_hi = (line & 2) != 0;
-            const is_odd = (line & 1) != 0;
-            const slot = slot_base + (if (is_odd) @as(usize, 1) else 0);
-
-            if (!is_hi) {
-                self.chr_banks_lo[slot] = @truncate(value & 0x0f);
-            } else {
-                self.chr_banks_hi[slot] = @truncate(value & 0x1f);
-            }
-        },
-        0xf000 => {
-            switch (line) {
-                0 => self.irq_reload = (self.irq_reload & 0xf0) | (value & 0x0f),
-                1 => self.irq_reload = (self.irq_reload & 0x0f) | ((value & 0x0f) << 4),
-                2 => {
-                    self.irq_mode = @truncate((value >> 2) & 1);
-                    self.irq_enabled = (value & 0x02) != 0;
-                    self.irq_enable_on_ack = (value & 0x01) != 0;
-                    if (self.irq_enabled) {
-                        self.irq_counter = self.irq_reload;
-                        self.irq_prescaler = 341;
-                    }
-                    self.irq_assert = false;
-                },
-                3 => {
-                    self.irq_enabled = self.irq_enable_on_ack;
-                    self.irq_assert = false;
-                },
-            }
-        },
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Vrc4,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0400;
-    const offset = @as(usize, address & 0x03ff);
-
-    const bank_val = (@as(usize, self.chr_banks_hi[slot]) << 4) | @as(usize, self.chr_banks_lo[slot]);
-    const bank = bank_val % self.chr_bank_count;
-
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Vrc4,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Vrc4,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(
-    self: *const Vrc4,
-) bool {
-    return self.irq_assert;
-}

src/system/nes/mapper/vrc6.zig

@@ -1,152 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Vrc6 = @This();
-
-prg_banks: [2]u8 = [_]u8{ 0, 1 },
-chr_banks: [8]u8 = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7 },
-
-mirroring_mode: Mirroring = .vertical,
-
-irq_reload: u8 = 0,
-irq_counter: u8 = 0,
-irq_enabled: bool = false,
-irq_enable_on_ack: bool = false,
-irq_mode: u1 = 0,
-irq_assert: bool = false,
-
-shift_a: u3,
-shift_b: u3,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-    mapper_id: u8,
-) Vrc6 {
-    const shift_a: u3 = if (mapper_id == 26) 1 else 0;
-    const shift_b: u3 = if (mapper_id == 26) 0 else 1;
-
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-        .shift_a = shift_a,
-        .shift_b = shift_b,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Vrc6,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const last_bank = self.prg_bank_count - 1;
-
-    if (address < 0xc000) {
-        const offset = @as(usize, address & 0x3fff);
-        const bank16 = @as(usize, self.prg_banks[0] & 0x0f) % (self.prg_bank_count / 2);
-        return bank16 * 0x4000 + offset;
-    } else if (address < 0xe000) {
-        const offset = @as(usize, address & 0x1fff);
-        const bank8 = @as(usize, self.prg_banks[1] & 0x1f) % self.prg_bank_count;
-        return bank8 * 0x2000 + offset;
-    } else {
-        const offset = @as(usize, address & 0x1fff);
-        return last_bank * 0x2000 + offset;
-    }
-}
-
-pub fn cpuWrite(
-    self: *Vrc6,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    const reg_addr = address & 0xf000;
-    const a = @as(u1, @truncate((address >> self.shift_a) & 1));
-    const b = @as(u1, @truncate((address >> self.shift_b) & 1));
-    const line = (@as(u2, b) << 1) | a;
-
-    switch (reg_addr) {
-        0x8000 => {
-            if (line == 0) self.prg_banks[0] = value & 0x0f;
-        },
-        0x9000...0xb000 => {
-            if (reg_addr == 0xb000 and line == 3) {
-                switch (@as(u2, @truncate((value >> 2) & 3))) {
-                    0 => self.mirroring_mode = .vertical,
-                    1 => self.mirroring_mode = .horizontal,
-                    2 => self.mirroring_mode = .single_screen_lower,
-                    3 => self.mirroring_mode = .single_screen_upper,
-                }
-            }
-        },
-        0xc000 => {
-            if (line == 0) self.prg_banks[1] = value & 0x1f;
-        },
-        0xd000 => {
-            const slot = @as(usize, line);
-            self.chr_banks[slot] = value;
-        },
-        0xe000 => {
-            const slot = 4 + @as(usize, line);
-            self.chr_banks[slot] = value;
-        },
-        0xf000 => {
-            switch (line) {
-                0 => self.irq_reload = value,
-                1 => {
-                    self.irq_mode = @truncate((value >> 2) & 1);
-                    self.irq_enabled = (value & 0x02) != 0;
-                    self.irq_enable_on_ack = (value & 0x01) != 0;
-                    if (self.irq_enabled) {
-                        self.irq_counter = self.irq_reload;
-                    }
-                    self.irq_assert = false;
-                },
-                2 => {
-                    self.irq_enabled = self.irq_enable_on_ack;
-                    self.irq_assert = false;
-                },
-                else => {},
-            }
-        },
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Vrc6,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0400;
-    const offset = @as(usize, address & 0x03ff);
-    const bank = @as(usize, self.chr_banks[slot]) % self.chr_bank_count;
-
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Vrc6,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Vrc6,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(
-    self: *const Vrc6,
-) bool {
-    return self.irq_assert;
-}

src/system/nes/mapper/vrc7.zig

@@ -1,145 +0,0 @@
-const types = @import("types.zig");
-const Mirroring = types.Mirroring;
-
-const Vrc7 = @This();
-
-prg_banks: [3]u8 = [_]u8{ 0, 1, 2 },
-chr_banks: [8]u8 = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7 },
-
-mirroring_mode: Mirroring = .vertical,
-
-irq_reload: u8 = 0,
-irq_counter: u8 = 0,
-irq_enabled: bool = false,
-irq_enable_on_ack: bool = false,
-irq_mode: u1 = 0,
-irq_assert: bool = false,
-
-prg_bank_count: usize,
-chr_bank_count: usize,
-
-pub fn init(
-    prg_size: usize,
-    chr_size: usize,
-) Vrc7 {
-    return .{
-        .prg_bank_count = prg_size / 0x2000,
-        .chr_bank_count = if (chr_size > 0) chr_size / 0x0400 else 1,
-    };
-}
-
-pub fn cpuMapRead(
-    self: *const Vrc7,
-    address: u16,
-) ?usize {
-    if (address < 0x8000) return null;
-
-    const offset = @as(usize, address & 0x1fff);
-    const last_bank = self.prg_bank_count - 1;
-
-    switch ((address - 0x8000) / 0x2000) {
-        0 => return (@as(usize, self.prg_banks[0] & 0x3f) % self.prg_bank_count) * 0x2000 + offset,
-        1 => return (@as(usize, self.prg_banks[1] & 0x3f) % self.prg_bank_count) * 0x2000 + offset,
-        2 => return (@as(usize, self.prg_banks[2] & 0x3f) % self.prg_bank_count) * 0x2000 + offset,
-        3 => return last_bank * 0x2000 + offset,
-        else => unreachable,
-    }
-}
-
-pub fn cpuWrite(
-    self: *Vrc7,
-    address: u16,
-    value: u8,
-) void {
-    if (address < 0x8000) return;
-
-    const reg_addr = address & 0xf000;
-    const a0 = if ((address & 0x10) != 0 or (address & 0x08) != 0) @as(u2, 1) else 0;
-    const a1 = if ((address & 0x20) != 0) @as(u2, 2) else 0;
-    const line = a0 | a1;
-
-    switch (reg_addr) {
-        0x8000 => {
-            if (line == 0) self.prg_banks[0] = value & 0x3f;
-            if (line == 1 or line == 2) self.prg_banks[1] = value & 0x3f;
-        },
-        0x9000 => {
-            if (line == 0) self.prg_banks[2] = value & 0x3f;
-            if (line == 2) {
-                switch (@as(u2, @truncate(value & 3))) {
-                    0 => self.mirroring_mode = .vertical,
-                    1 => self.mirroring_mode = .horizontal,
-                    2 => self.mirroring_mode = .single_screen_lower,
-                    3 => self.mirroring_mode = .single_screen_upper,
-                }
-            }
-        },
-        0xa000 => {
-            if (line == 0) self.chr_banks[0] = value;
-            if (line == 1 or line == 2) self.chr_banks[1] = value;
-        },
-        0xb000 => {
-            if (line == 0) self.chr_banks[2] = value;
-            if (line == 1 or line == 2) self.chr_banks[3] = value;
-        },
-        0xc000 => {
-            if (line == 0) self.chr_banks[4] = value;
-            if (line == 1 or line == 2) self.chr_banks[5] = value;
-        },
-        0xd000 => {
-            if (line == 0) self.chr_banks[6] = value;
-            if (line == 1 or line == 2) self.chr_banks[7] = value;
-        },
-        0xe000 => {
-            if (line == 0) self.irq_reload = value;
-            if (line == 1 or line == 2) {
-                self.irq_mode = @truncate((value >> 2) & 1);
-                self.irq_enabled = (value & 0x02) != 0;
-                self.irq_enable_on_ack = (value & 0x01) != 0;
-                if (self.irq_enabled) {
-                    self.irq_counter = self.irq_reload;
-                }
-                self.irq_assert = false;
-            }
-        },
-        0xf000 => {
-            if (line == 0) {
-                self.irq_enabled = self.irq_enable_on_ack;
-                self.irq_assert = false;
-            }
-        },
-        else => {},
-    }
-}
-
-pub fn ppuMapRead(
-    self: *const Vrc7,
-    address: u16,
-) ?usize {
-    if (address >= 0x2000) return null;
-
-    const slot = address / 0x0400;
-    const offset = @as(usize, address & 0x03ff);
-    const bank = @as(usize, self.chr_banks[slot]) % self.chr_bank_count;
-
-    return bank * 0x0400 + offset;
-}
-
-pub fn ppuMapWrite(
-    self: *const Vrc7,
-    address: u16,
-) ?usize {
-    return self.ppuMapRead(address);
-}
-
-pub fn mirroring(
-    self: *const Vrc7,
-) Mirroring {
-    return self.mirroring_mode;
-}
-
-pub fn irqAsserted(
-    self: *const Vrc7,
-) bool {
-    return self.irq_assert;
-}

src/system/nes/ppu.zig

@@ -1,779 +0,0 @@
-const std = @import("std");
-const Cartridge = @import("cartridge.zig");
-
-const mapper = @import("mapper/root.zig");
-const Mirroring = mapper.Mirroring;
-
-const Ppu = @This();
-
-pub const width = 256;
-pub const height = 240;
-
-// NTSC 2C02 64-color system palette mapped directly to RGB565 format
-pub const nes_ntsc_palette_rgb565: [64]u16 = .{
-    0x52AA, 0x00EE, 0x0892, 0x3011, 0x400C, 0x5806, 0x5020, 0x38C0,
-    0x2140, 0x09C0, 0x0200, 0x01E0, 0x0187, 0x0000, 0x0000, 0x0000,
-    0x9CB3, 0x0A78, 0x319D, 0x58FC, 0x88B6, 0xA08C, 0x9904, 0x79E0,
-    0x52C0, 0x2B80, 0x0BE0, 0x03A5, 0x032F, 0x0000, 0x0000, 0x0000,
-    0xEF7D, 0x4CDD, 0x7BFD, 0xB31D, 0xE2BD, 0xEACE, 0xEB4C, 0xD444,
-    0xA540, 0x7620, 0x4E84, 0x3E6D, 0x3DA9, 0x3DF7, 0x0000, 0x0000,
-    0xEF7D, 0xAE6D, 0xBD7D, 0xD59D, 0xED7D, 0xED7A, 0xEDB6, 0xE632,
-    0xCE8F, 0xB6EF, 0xAF12, 0x9F16, 0xA6BD, 0xA514, 0x0000, 0x0000,
-};
-
-framebuffer: [width * height]u16 =
-    [_]u16{0} ** (width * height),
-
-/// Four-screen sized so the same structure
-/// can handle that mode without allocation.
-nametable: [0x1000]u8 =
-    [_]u8{0} ** 0x1000,
-
-palette: [32]u8 =
-    [_]u8{0} ** 32,
-
-oam: [256]u8 =
-    [_]u8{0} ** 256,
-
-ctrl: u8 = 0,
-mask: u8 = 0,
-status: u8 = 0,
-
-oam_addr: u8 = 0,
-
-vram_addr: u16 = 0,
-temp_addr: u16 = 0,
-
-fine_x: u3 = 0,
-write_latch: bool = false,
-
-read_buffer: u8 = 0,
-io_latch: u8 = 0,
-
-dot: u16 = 0,
-scanline: u16 = 261,
-
-frame_number: u64 = 0,
-frame_ready: bool = false,
-
-// internal shift registers & tile latches for background rendering
-bg_shift_pattern_lo: u16 = 0,
-bg_shift_pattern_hi: u16 = 0,
-bg_shift_attrib_lo: u16 = 0,
-bg_shift_attrib_hi: u16 = 0,
-
-next_tile_id: u8 = 0,
-next_tile_attrib: u8 = 0,
-next_tile_lsb: u8 = 0,
-next_tile_msb: u8 = 0,
-
-// secondary OAM scanline sprite evaluation buffer
-sec_oam_pattern_lo: [8]u8 = [_]u8{0} ** 8,
-sec_oam_pattern_hi: [8]u8 = [_]u8{0} ** 8,
-sec_oam_attribute: [8]u8 = [_]u8{0} ** 8,
-sec_oam_x: [8]u8 = [_]u8{0} ** 8,
-sec_oam_is_zero: [8]bool = [_]bool{false} ** 8,
-sec_oam_count: u8 = 0,
-
-pub fn reset(
-    self: *Ppu,
-) void {
-    self.ctrl = 0;
-    self.mask = 0;
-    self.status = 0;
-
-    self.oam_addr = 0;
-
-    self.vram_addr = 0;
-    self.temp_addr = 0;
-
-    self.fine_x = 0;
-    self.write_latch = false;
-
-    self.read_buffer = 0;
-    self.io_latch = 0;
-
-    self.dot = 0;
-    self.scanline = 261;
-
-    self.frame_ready = false;
-
-    self.bg_shift_pattern_lo = 0;
-    self.bg_shift_pattern_hi = 0;
-    self.bg_shift_attrib_lo = 0;
-    self.bg_shift_attrib_hi = 0;
-    self.next_tile_id = 0;
-    self.next_tile_attrib = 0;
-    self.next_tile_lsb = 0;
-    self.next_tile_msb = 0;
-    self.sec_oam_count = 0;
-}
-
-pub fn cpuRead(
-    self: *Ppu,
-    cartridge: *Cartridge,
-    register: u16,
-) u8 {
-    const reg = register & 7;
-
-    const value: u8 = switch (reg) {
-        // PPUSTATUS
-        2 => blk: {
-            const result =
-                (self.status & 0xe0) |
-                (self.io_latch & 0x1f);
-
-            self.status &= ~@as(u8, 0x80);
-            self.write_latch = false;
-
-            break :blk result;
-        },
-
-        // OAMDATA
-        4 => self.oam[self.oam_addr],
-
-        // PPUDATA
-        7 => self.readData(cartridge),
-
-        else => self.io_latch,
-    };
-
-    self.io_latch = value;
-
-    return value;
-}
-
-pub fn cpuWrite(
-    self: *Ppu,
-    cartridge: *Cartridge,
-    register: u16,
-    value: u8,
-) void {
-    const reg = register & 7;
-
-    self.io_latch = value;
-
-    switch (reg) {
-        // PPUCTRL
-        0 => {
-            self.ctrl = value;
-
-            self.temp_addr =
-                (self.temp_addr & 0xf3ff) |
-                (@as(u16, value & 0x03) << 10);
-        },
-
-        // PPUMASK
-        1 => {
-            self.mask = value;
-        },
-
-        // OAMADDR
-        3 => {
-            self.oam_addr = value;
-        },
-
-        // OAMDATA
-        4 => {
-            self.writeOamDma(value);
-        },
-
-        // PPUSCROLL
-        5 => {
-            if (!self.write_latch) {
-                self.fine_x =
-                    @truncate(value & 7);
-
-                self.temp_addr =
-                    (self.temp_addr & 0xffe0) |
-                    @as(u16, value >> 3);
-
-                self.write_latch = true;
-            } else {
-                self.temp_addr =
-                    (self.temp_addr & 0x8c1f) |
-                    (@as(u16, value & 0x07) << 12) |
-                    (@as(u16, value & 0xf8) << 2);
-
-                self.write_latch = false;
-            }
-        },
-
-        // PPUADDR
-        6 => {
-            if (!self.write_latch) {
-                self.temp_addr =
-                    (self.temp_addr & 0x00ff) |
-                    (@as(u16, value & 0x3f) << 8);
-
-                self.write_latch = true;
-            } else {
-                self.temp_addr =
-                    (self.temp_addr & 0x7f00) |
-                    @as(u16, value);
-
-                self.vram_addr =
-                    self.temp_addr;
-
-                self.write_latch = false;
-            }
-        },
-
-        // PPUDATA
-        7 => {
-            self.writeMemory(
-                cartridge,
-                self.vram_addr,
-                value,
-            );
-
-            self.incrementVramAddress();
-        },
-
-        else => {},
-    }
-}
-
-pub fn writeOamDma(
-    self: *Ppu,
-    value: u8,
-) void {
-    self.oam[self.oam_addr] = value;
-    self.oam_addr +%= 1;
-}
-
-pub fn tick(
-    self: *Ppu,
-    cartridge: *Cartridge,
-) void {
-    const rendering_enabled = (self.mask & 0x18) != 0;
-    const is_visible_scanline = (self.scanline < 240);
-    const is_prerender_scanline = (self.scanline == 261);
-
-    // Start of VBlank.
-    if (self.scanline == 241 and
-        self.dot == 1)
-    {
-        self.status |= 0x80;
-    }
-
-    // Pre-render line.
-    if (is_prerender_scanline and
-        self.dot == 1)
-    {
-        // Clear: VBlank, sprite zero hit, sprite overflow
-        self.status &= ~@as(u8, 0xe0);
-    }
-
-    // --- Rendering Pipeline ---
-    if (rendering_enabled and (is_visible_scanline or is_prerender_scanline)) {
-        if ((self.dot >= 1 and self.dot <= 256) or (self.dot >= 321 and self.dot <= 336)) {
-            self.shiftBackgroundRegisters();
-
-            const cycle_in_tile = (self.dot - 1) % 8;
-            switch (cycle_in_tile) {
-                0 => {
-                    self.loadShiftRegisters();
-                    const nt_addr = 0x2000 | (self.vram_addr & 0x0fff);
-                    self.next_tile_id = self.readMemory(cartridge, nt_addr);
-                },
-                2 => {
-                    const attr_addr = 0x23c0 |
-                        (self.vram_addr & 0x0c00) |
-                        ((self.vram_addr >> 4) & 0x38) |
-                        ((self.vram_addr >> 2) & 0x07);
-                    const attr_byte = self.readMemory(cartridge, attr_addr);
-                    const shift: u3 = @intCast(((self.vram_addr >> 4) & 4) | (self.vram_addr & 2));
-                    self.next_tile_attrib = (attr_byte >> shift) & 0x03;
-                },
-                4 => {
-                    const bg_table: u16 = if ((self.ctrl & 0x10) != 0) 0x1000 else 0x0000;
-                    const fine_y = (self.vram_addr >> 12) & 7;
-                    const pattern_addr = bg_table + (@as(u16, self.next_tile_id) << 4) + fine_y;
-                    self.next_tile_lsb = self.readMemory(cartridge, pattern_addr);
-                },
-                6 => {
-                    const bg_table: u16 = if ((self.ctrl & 0x10) != 0) 0x1000 else 0x0000;
-                    const fine_y = (self.vram_addr >> 12) & 7;
-                    const pattern_addr = bg_table + (@as(u16, self.next_tile_id) << 4) + fine_y + 8;
-                    self.next_tile_msb = self.readMemory(cartridge, pattern_addr);
-                },
-                7 => {
-                    self.incrementCoarseX();
-                },
-                else => {},
-            }
-        }
-
-        if (self.dot == 256) {
-            self.incrementY();
-        }
-
-        if (self.dot == 257) {
-            self.loadShiftRegisters();
-            self.copyHorizontal();
-        }
-
-        if (is_prerender_scanline and self.dot >= 280 and self.dot <= 304) {
-            self.copyVertical();
-        }
-
-        if (self.dot == 257 and is_visible_scanline) {
-            self.evaluateSprites(cartridge);
-        }
-
-        if (self.dot == 260 and is_visible_scanline) {
-            cartridge.handleScanline();
-        }
-    }
-
-    // --- Pixel Output ---
-    if (is_visible_scanline and self.dot >= 1 and self.dot <= 256) {
-        self.renderPixel(cartridge);
-    }
-
-    // --- Dot & Scanline Counters ---
-    self.dot += 1;
-
-    if (self.dot == 341) {
-        self.dot = 0;
-        self.scanline += 1;
-
-        if (self.scanline == 262) {
-            self.scanline = 0;
-
-            self.frame_number +%= 1;
-            self.frame_ready = true;
-        }
-    }
-}
-
-pub fn nmiAsserted(
-    self: *const Ppu,
-) bool {
-    const nmi_enabled =
-        (self.ctrl & 0x80) != 0;
-
-    const in_vblank =
-        (self.status & 0x80) != 0;
-
-    return nmi_enabled and in_vblank;
-}
-
-fn shiftBackgroundRegisters(self: *Ppu) void {
-    if ((self.mask & 0x08) != 0) {
-        self.bg_shift_pattern_lo <<= 1;
-        self.bg_shift_pattern_hi <<= 1;
-        self.bg_shift_attrib_lo <<= 1;
-        self.bg_shift_attrib_hi <<= 1;
-    }
-}
-
-fn loadShiftRegisters(self: *Ppu) void {
-    self.bg_shift_pattern_lo = (self.bg_shift_pattern_lo & 0xff00) | self.next_tile_lsb;
-    self.bg_shift_pattern_hi = (self.bg_shift_pattern_hi & 0xff00) | self.next_tile_msb;
-
-    const attr_lo: u16 = if ((self.next_tile_attrib & 1) != 0) 0x00ff else 0x0000;
-    const attr_hi: u16 = if ((self.next_tile_attrib & 2) != 0) 0x00ff else 0x0000;
-    self.bg_shift_attrib_lo = (self.bg_shift_attrib_lo & 0xff00) | attr_lo;
-    self.bg_shift_attrib_hi = (self.bg_shift_attrib_hi & 0xff00) | attr_hi;
-}
-
-inline fn incrementCoarseX(self: *Ppu) void {
-    if ((self.vram_addr & 0x001f) == 31) {
-        self.vram_addr &= ~@as(u16, 0x001f);
-        self.vram_addr ^= 0x0400;
-    } else {
-        self.vram_addr += 1;
-    }
-}
-
-inline fn incrementY(self: *Ppu) void {
-    if ((self.vram_addr & 0x7000) != 0x7000) {
-        self.vram_addr += 0x1000;
-    } else {
-        self.vram_addr &= ~@as(u16, 0x7000);
-        var y = (self.vram_addr & 0x03e0) >> 5;
-        if (y == 29) {
-            y = 0;
-            self.vram_addr ^= 0x0800;
-        } else if (y == 31) {
-            y = 0;
-        } else {
-            y += 1;
-        }
-        self.vram_addr = (self.vram_addr & ~@as(u16, 0x03e0)) | (y << 5);
-    }
-}
-
-inline fn copyHorizontal(self: *Ppu) void {
-    self.vram_addr = (self.vram_addr & ~@as(u16, 0x041f)) | (self.temp_addr & 0x041f);
-}
-
-inline fn copyVertical(self: *Ppu) void {
-    self.vram_addr = (self.vram_addr & ~@as(u16, 0x7be0)) | (self.temp_addr & 0x7be0);
-}
-
-inline fn evaluateSprites(
-    self: *Ppu,
-    cartridge: *Cartridge,
-) void {
-    self.sec_oam_count = 0;
-    const sprite_height: u16 = if ((self.ctrl & 0x20) != 0) 16 else 8;
-
-    var oam_idx: usize = 0;
-    var i: usize = 0;
-    while (i < 64) : ({
-        i += 1;
-        oam_idx += 4;
-    }) {
-        const sprite_y = @as(u16, self.oam[oam_idx]);
-        const diff = @as(i16, @intCast(self.scanline)) - @as(i16, @intCast(sprite_y));
-
-        if (diff >= 0 and diff < sprite_height) {
-            if (self.sec_oam_count < 8) {
-                const idx = self.sec_oam_count;
-                const tile = self.oam[oam_idx + 1];
-                const attr = self.oam[oam_idx + 2];
-                const x = self.oam[oam_idx + 3];
-
-                var row: u16 = @intCast(diff);
-                // Vertical flip
-                if ((attr & 0x80) != 0) {
-                    row = (sprite_height - 1) - row;
-                }
-
-                var pattern_addr: u16 = 0;
-                if (sprite_height == 8) {
-                    const pattern_table: u16 = if ((self.ctrl & 0x08) != 0) 0x1000 else 0x0000;
-                    pattern_addr = pattern_table + (@as(u16, tile) << 4) + row;
-                } else {
-                    // 8x16 mode
-                    const pattern_table: u16 = if ((tile & 1) != 0) 0x1000 else 0x0000;
-                    const tile_index: u16 = tile & 0xfe;
-                    if (row < 8) {
-                        pattern_addr = pattern_table + (tile_index << 4) + row;
-                    } else {
-                        pattern_addr = pattern_table + ((tile_index + 1) << 4) + (row - 8);
-                    }
-                }
-
-                var pat_lo = self.readMemory(cartridge, pattern_addr);
-                var pat_hi = self.readMemory(cartridge, pattern_addr + 8);
-
-                // Horizontal flip
-                if ((attr & 0x40) != 0) {
-                    pat_lo = @bitReverse(pat_lo);
-                    pat_hi = @bitReverse(pat_hi);
-                }
-
-                self.sec_oam_pattern_lo[idx] = pat_lo;
-                self.sec_oam_pattern_hi[idx] = pat_hi;
-                self.sec_oam_attribute[idx] = attr;
-                self.sec_oam_x[idx] = x;
-                self.sec_oam_is_zero[idx] = (i == 0);
-
-                self.sec_oam_count += 1;
-            } else {
-                // Sprite overflow
-                self.status |= 0x20;
-                break;
-            }
-        }
-    }
-}
-
-fn renderPixel(
-    self: *Ppu,
-    _: *Cartridge,
-) void {
-    const x = self.dot - 1;
-
-    var bg_pixel: u8 = 0;
-    var bg_palette: u8 = 0;
-
-    if ((self.mask & 0x08) != 0 and (x >= 8 or (self.mask & 0x02) != 0)) {
-        const bit_shift: u4 = 15 - @as(u4, self.fine_x);
-        const p1 = @as(u8, @truncate((self.bg_shift_pattern_lo >> bit_shift) & 1));
-        const p2 = @as(u8, @truncate((self.bg_shift_pattern_hi >> bit_shift) & 1));
-        bg_pixel = (p2 << 1) | p1;
-
-        const a1 = @as(u8, @truncate((self.bg_shift_attrib_lo >> bit_shift) & 1));
-        const a2 = @as(u8, @truncate((self.bg_shift_attrib_hi >> bit_shift) & 1));
-        bg_palette = (a2 << 1) | a1;
-    }
-
-    var fg_pixel: u8 = 0;
-    var fg_palette: u8 = 0;
-    var fg_priority: bool = false;
-    var is_sprite_zero: bool = false;
-
-    if (self.sec_oam_count > 0 and (self.mask & 0x10) != 0 and (x >= 8 or (self.mask & 0x04) != 0)) {
-        for (0..self.sec_oam_count) |i| {
-            const spr_x = self.sec_oam_x[i];
-            if (x >= spr_x and x < spr_x + 8) {
-                const offset: u3 = @intCast(x - spr_x);
-                const shift: u3 = 7 - offset;
-                const p1 = (self.sec_oam_pattern_lo[i] >> shift) & 1;
-                const p2 = (self.sec_oam_pattern_hi[i] >> shift) & 1;
-                const pixel = (p2 << 1) | p1;
-
-                if (pixel != 0) {
-                    fg_pixel = pixel;
-                    fg_palette = 4 + (self.sec_oam_attribute[i] & 0x03);
-                    fg_priority = (self.sec_oam_attribute[i] & 0x20) != 0;
-                    is_sprite_zero = self.sec_oam_is_zero[i];
-                    break;
-                }
-            }
-        }
-    }
-
-    var final_palette_entry: u16 = 0;
-
-    if (bg_pixel == 0 and fg_pixel == 0) {
-        final_palette_entry = 0x00;
-    } else if (bg_pixel == 0 and fg_pixel != 0) {
-        final_palette_entry = @as(u16, fg_palette) * 4 + fg_pixel;
-    } else if (bg_pixel != 0 and fg_pixel == 0) {
-        final_palette_entry = @as(u16, bg_palette) * 4 + bg_pixel;
-    } else {
-        // Both bg and fg pixels present
-        if (is_sprite_zero and x < 255 and (self.mask & 0x18) == 0x18) {
-            self.status |= 0x40; // Sprite 0 hit
-        }
-
-        if (fg_priority) {
-            final_palette_entry = @as(u16, bg_palette) * 4 + bg_pixel;
-        } else {
-            final_palette_entry = @as(u16, fg_palette) * 4 + fg_pixel;
-        }
-    }
-
-    var color_idx = self.palette[mapPalette(0x3f00 + final_palette_entry)] & 0x3f;
-    if ((self.mask & 0x01) != 0) {
-        color_idx &= 0x30;
-    }
-    var pixel_rgb = nes_ntsc_palette_rgb565[color_idx];
-
-    const emphasis = (self.mask >> 5) & 7;
-    if (emphasis != 0) {
-        var r = (pixel_rgb >> 11) & 0x1F;
-        var g = (pixel_rgb >> 5) & 0x3F;
-        var b = pixel_rgb & 0x1F;
-
-        // Red emphasis (bit 5)
-        if ((emphasis & 1) != 0) {
-            g = (g * 3) / 4;
-            b = (b * 3) / 4;
-        }
-        // Green emphasis (bit 6)
-        if ((emphasis & 2) != 0) {
-            r = (r * 3) / 4;
-            b = (b * 3) / 4;
-        }
-        // Blue emphasis (bit 7)
-        if ((emphasis & 4) != 0) {
-            r = (r * 3) / 4;
-            g = (g * 3) / 4;
-        }
-
-        pixel_rgb = (@as(u16, r) << 11) | (@as(u16, g) << 5) | @as(u16, b);
-    }
-
-    self.framebuffer[self.scanline * 256 + x] = pixel_rgb;
-}
-
-fn incrementVramAddress(
-    self: *Ppu,
-) void {
-    const increment: u16 =
-        if ((self.ctrl & 0x04) != 0)
-            32
-        else
-            1;
-
-    self.vram_addr =
-        (self.vram_addr +% increment) &
-        0x3fff;
-}
-
-fn readData(
-    self: *Ppu,
-    cartridge: *Cartridge,
-) u8 {
-    const address =
-        self.vram_addr & 0x3fff;
-
-    const value: u8 =
-        if (address < 0x3f00) blk: {
-            const previous =
-                self.read_buffer;
-
-            self.read_buffer =
-                self.readMemory(
-                    cartridge,
-                    address,
-                );
-
-            break :blk previous;
-        } else blk: {
-            const result =
-                self.readMemory(
-                    cartridge,
-                    address,
-                );
-
-            // Palette reads aren't delayed, but
-            // still update the internal buffer.
-            self.read_buffer =
-                self.readMemory(
-                    cartridge,
-                    address - 0x1000,
-                );
-
-            break :blk result;
-        };
-
-    self.incrementVramAddress();
-
-    return value;
-}
-
-inline fn readMemory(
-    self: *Ppu,
-    cartridge: *Cartridge,
-    address_: u16,
-) u8 {
-    const address =
-        address_ & 0x3fff;
-
-    return switch (address) {
-        0x0000...0x1fff => cartridge.ppuRead(address),
-
-        0x2000...0x3eff => self.nametable[
-            self.mapNametable(
-                cartridge.mirroring(),
-                address,
-            )
-        ],
-
-        0x3f00...0x3fff => self.palette[
-            mapPalette(address)
-        ],
-
-        else => unreachable,
-    };
-}
-
-inline fn writeMemory(
-    self: *Ppu,
-    cartridge: *Cartridge,
-    address_: u16,
-    value: u8,
-) void {
-    const address =
-        address_ & 0x3fff;
-
-    switch (address) {
-        0x0000...0x1fff => cartridge.ppuWrite(
-            address,
-            value,
-        ),
-
-        0x2000...0x3eff => self.nametable[
-            self.mapNametable(
-                cartridge.mirroring(),
-                address,
-            )
-        ] = value,
-
-        0x3f00...0x3fff => self.palette[
-            mapPalette(address)
-        ] = value,
-
-        else => unreachable,
-    }
-}
-
-inline fn mapNametable(
-    _: *const Ppu,
-    mirroring: Mirroring,
-    address: u16,
-) usize {
-    const relative =
-        (@as(usize, address) - 0x2000) &
-        0x0fff;
-
-    const table =
-        relative >> 10;
-
-    const offset =
-        relative & 0x3ff;
-
-    const mapped_table: usize =
-        switch (mirroring) {
-            .vertical => switch (table) {
-                0, 2 => 0,
-                1, 3 => 1,
-                else => unreachable,
-            },
-
-            .horizontal => switch (table) {
-                0, 1 => 0,
-                2, 3 => 1,
-                else => unreachable,
-            },
-
-            .single_screen_lower => 0,
-            .single_screen_upper => 1,
-
-            .four_screen => table,
-        };
-
-    return mapped_table * 0x400 +
-        offset;
-}
-
-inline fn mapPalette(
-    address: u16,
-) usize {
-    var index: usize =
-        (@as(usize, address) - 0x3f00) &
-        0x1f;
-
-    // Universal background color mirrors.
-    switch (index) {
-        0x10 => index = 0x00,
-        0x14 => index = 0x04,
-        0x18 => index = 0x08,
-        0x1c => index = 0x0c,
-        else => {},
-    }
-
-    return index;
-}
-
-test "NES PPU - Palette Mirroring & NTSC Palette LUT" {
-    try std.testing.expectEqual(@as(usize, 0), mapPalette(0x3F00));
-    try std.testing.expectEqual(@as(usize, 0), mapPalette(0x3F10));
-    try std.testing.expectEqual(@as(usize, 4), mapPalette(0x3F04));
-    try std.testing.expectEqual(@as(usize, 4), mapPalette(0x3F14));
-
-    // Verify system palette RGB565 LUT contains non-zero color constants
-    try std.testing.expectEqual(@as(u16, 0x52AA), nes_ntsc_palette_rgb565[0]);
-    try std.testing.expectEqual(@as(u16, 0xEF7D), nes_ntsc_palette_rgb565[0x20]);
-}
-
-test "NES PPU - Scroll Address Increments" {
-    var ppu = Ppu{};
-    ppu.reset();
-
-    ppu.vram_addr = 0x001F; // Coarse X = 31
-    ppu.incrementCoarseX();
-    try std.testing.expectEqual(@as(u16, 0x0400), ppu.vram_addr); // Coarse X = 0, switched nametable X
-
-    ppu.vram_addr = 0x73A0; // Coarse Y = 29, Fine Y = 7
-    ppu.incrementY();
-    try std.testing.expectEqual(@as(u16, 0x0800), ppu.vram_addr); // Coarse Y = 0, Fine Y = 0, switched nametable Y
-}

src/system/nes/ppu/control.zig

@@ -0,0 +1,95 @@
+// PPUCTRL
+//
+// VPHB SINN
+
+pub const Control = @This();
+
+raw: u8 = 0,
+
+pub fn baseNametable(self: Control) u2 {
+    return @truncate(self.raw & 0x03);
+}
+
+pub fn baseNametableAddress(self: Control) u16 {
+    return 0x2000 +
+        (@as(u16, self.baseNametable()) * 0x400);
+}
+
+pub fn vramIncrement(self: Control) u8 {
+    return if ((self.raw & 0x04) != 0)
+        32
+    else
+        1;
+}
+
+pub fn sprite8x8PatternAddress(self: Control) u16 {
+    return if ((self.raw & 0x08) != 0)
+        0x1000
+    else
+        0x0000;
+}
+
+/// Computes pattern table address for a sprite tile (supports both 8x8 and 8x16 modes).
+pub fn spritePatternAddressForTile(self: Control, tile: u8, sprite_y_offset: u4) u16 {
+    if (self.spriteHeight() == 8) {
+        return self.sprite8x8PatternAddress() | (@as(u16, tile) << 4) | (sprite_y_offset & 7);
+    } else {
+        const bank: u16 = @as(u16, tile & 1) << 12;
+        var tile_index: u16 = tile & 0xfe;
+        if (sprite_y_offset >= 8) {
+            tile_index += 1;
+        }
+        return bank | (tile_index << 4) | (sprite_y_offset & 7);
+    }
+}
+
+pub fn backgroundPatternAddress(self: Control) u16 {
+    return if ((self.raw & 0x10) != 0)
+        0x1000
+    else
+        0x0000;
+}
+
+pub fn spriteHeight(self: Control) u8 {
+    return if ((self.raw & 0x20) != 0)
+        16
+    else
+        8;
+}
+
+pub fn masterSlave(self: Control) bool {
+    return (self.raw & 0x40) != 0;
+}
+
+pub fn nmiEnabled(self: Control) bool {
+    return (self.raw & 0x80) != 0;
+}
+
+const std = @import("std");
+
+test "8x8 sprite pattern address" {
+    const c0: Control = .{ .raw = 0x00 }; // 8x8, pattern table 0
+    try std.testing.expectEqual(@as(u16, 0x0000), c0.sprite8x8PatternAddress());
+    try std.testing.expectEqual(@as(u16, 0x0143), c0.spritePatternAddressForTile(0x14, 3));
+
+    const c1: Control = .{ .raw = 0x08 }; // 8x8, pattern table 1 ($1000)
+    try std.testing.expectEqual(@as(u16, 0x1000), c1.sprite8x8PatternAddress());
+    try std.testing.expectEqual(@as(u16, 0x1143), c1.spritePatternAddressForTile(0x14, 3));
+}
+
+test "8x16 sprite pattern address ignores bit 3 and uses tile bit 0" {
+    const c: Control = .{ .raw = 0x28 }; // 8x16, bit 3 set (ignored!)
+    try std.testing.expectEqual(@as(u8, 16), c.spriteHeight());
+
+    // Tile 0x20: bit 0 is 0 -> bank 0 ($0000). Top half (offset 0..7).
+    try std.testing.expectEqual(@as(u16, 0x0203), c.spritePatternAddressForTile(0x20, 3));
+
+    // Tile 0x20: Bottom half (offset 8..15) -> tile index becomes 0x21!
+    try std.testing.expectEqual(@as(u16, 0x0212), c.spritePatternAddressForTile(0x20, 10));
+
+    // Tile 0x25: bit 0 is 1 -> bank 1 ($1000). Tile index & 0xFE = 0x24. Top half.
+    try std.testing.expectEqual(@as(u16, 0x1245), c.spritePatternAddressForTile(0x25, 5));
+
+    // Tile 0x25: Bottom half -> tile index & 0xFE + 1 = 0x25.
+    try std.testing.expectEqual(@as(u16, 0x1254), c.spritePatternAddressForTile(0x25, 12));
+}

src/system/nes/ppu/mask.zig

@@ -0,0 +1,76 @@
+// PPUMASK
+//
+// BGRs bMmG
+pub const Mask = @This();
+
+raw: u8 = 0,
+
+pub fn grayscale(self: Mask) bool {
+    return (self.raw & 0x01) != 0;
+}
+
+pub fn showBackgroundLeft(self: Mask) bool {
+    return (self.raw & 0x02) != 0;
+}
+
+pub fn showSpritesLeft(self: Mask) bool {
+    return (self.raw & 0x04) != 0;
+}
+
+pub fn showBackground(self: Mask) bool {
+    return (self.raw & 0x08) != 0;
+}
+
+pub fn showSprites(self: Mask) bool {
+    return (self.raw & 0x10) != 0;
+}
+
+pub fn renderingEnabled(self: Mask) bool {
+    return self.showBackground() or self.showSprites();
+}
+
+pub fn emphasizeRed(self: Mask) bool {
+    return (self.raw & 0x20) != 0;
+}
+
+pub fn emphasizeGreen(self: Mask) bool {
+    return (self.raw & 0x40) != 0;
+}
+
+pub fn emphasizeBlue(self: Mask) bool {
+    return (self.raw & 0x80) != 0;
+}
+
+pub fn isEmphasizeRed(self: Mask, is_pal: bool) bool {
+    return if (is_pal) (self.raw & 0x40) != 0 else (self.raw & 0x20) != 0;
+}
+
+pub fn isEmphasizeGreen(self: Mask, is_pal: bool) bool {
+    return if (is_pal) (self.raw & 0x20) != 0 else (self.raw & 0x40) != 0;
+}
+
+pub fn isEmphasizeBlue(self: Mask, _: bool) bool {
+    return (self.raw & 0x80) != 0;
+}
+
+pub fn emphasis(self: Mask) u3 {
+    return @truncate(self.raw >> 5);
+}
+
+const std = @import("std");
+
+test "mask region-aware color emphasis" {
+    // Bit 5 set, Bit 6 clear
+    const m1: Mask = .{ .raw = 0x20 };
+    try std.testing.expect(m1.isEmphasizeRed(false)); // NTSC: bit 5 is Red
+    try std.testing.expect(!m1.isEmphasizeGreen(false));
+    try std.testing.expect(!m1.isEmphasizeRed(true)); // PAL: bit 5 is Green!
+    try std.testing.expect(m1.isEmphasizeGreen(true));
+
+    // Bit 6 set, Bit 5 clear
+    const m2: Mask = .{ .raw = 0x40 };
+    try std.testing.expect(!m2.isEmphasizeRed(false));
+    try std.testing.expect(m2.isEmphasizeGreen(false)); // NTSC: bit 6 is Green
+    try std.testing.expect(m2.isEmphasizeRed(true)); // PAL: bit 6 is Red!
+    try std.testing.expect(!m2.isEmphasizeGreen(true));
+}

src/system/nes/ppu/palette.zig

@@ -0,0 +1,73 @@
+const std = @import("std");
+
+pub const Palette = @This();
+
+ram: [32]u8 = [_]u8{0} ** 32,
+
+pub fn init() Palette {
+    return .{};
+}
+
+pub fn reset(self: *Palette) void {
+    @memset(&self.ram, 0);
+}
+
+/// Mirrors addresses $3F00..$3FFF down to the 32 physical palette entries,
+/// applying special mirrors for $10, $14, $18, $1C to $00, $04, $08, $0C.
+pub inline fn mirrorAddress(address: u16) u5 {
+    var index: u5 = @truncate(address & 0x1f);
+    if ((index & 0x13) == 0x10) {
+        index &= 0x0f;
+    }
+    return index;
+}
+
+pub fn read(self: *const Palette, address: u16) u8 {
+    return self.ram[mirrorAddress(address)];
+}
+
+pub fn write(self: *Palette, address: u16, value: u8) void {
+    self.ram[mirrorAddress(address)] = value & 0x3f;
+}
+
+/// Canonical 64-color 2C02 NTSC palette (RGBA8888).
+pub const default_palette = [64]u32{
+    0x666666FF, 0x002A88FF, 0x1412A7FF, 0x3B00A4FF, 0x5C007EFF, 0x6E0040FF, 0x6C0600FF, 0x561D00FF,
+    0x333500FF, 0x0B4800FF, 0x005200FF, 0x004F08FF, 0x00404DFF, 0x000000FF, 0x000000FF, 0x000000FF,
+    0xADADADFF, 0x155FD9FF, 0x4240FFFF, 0x7527FEFF, 0xA01ACCFF, 0xB71E7BFF, 0xB53120FF, 0x994E00FF,
+    0x6B6D00FF, 0x388700FF, 0x0C9300FF, 0x008F32FF, 0x007C8DFF, 0x000000FF, 0x000000FF, 0x000000FF,
+    0xFFFFFFFF, 0x64B0FFFF, 0x9290FFFF, 0xC676FFFF, 0xF36AFFFF, 0xFE6ECCFF, 0xFE8170FF, 0xEA9E22FF,
+    0xBCBE00FF, 0x88D800FF, 0x5CE430FF, 0x45E082FF, 0x48CDDEFF, 0x4F4F4FFF, 0x000000FF, 0x000000FF,
+    0xFFFFFFFF, 0xC0E0FFFF, 0xD3D2FFFF, 0xE8C1FFFF, 0xFBC0FFFF, 0xFEC2EFFF, 0xFECAC4FF, 0xF6D5A1FF,
+    0xE3DF9EFF, 0xCEEB9EFF, 0xBCF3B9FF, 0xB2F1D4FF, 0xB4EBF8FF, 0xB8B8B8FF, 0x000000FF, 0x000000FF,
+};
+
+test "palette mirroring and special backdrop mirrors" {
+    var pal = Palette.init();
+
+    // Write backdrop color at $3F00
+    pal.write(0x3F00, 0x0F);
+    try std.testing.expectEqual(@as(u8, 0x0F), pal.read(0x3F00));
+
+    // Reading $3F10 should mirror $3F00!
+    try std.testing.expectEqual(@as(u8, 0x0F), pal.read(0x3F10));
+
+    // Writing to $3F10 mirrors back to $3F00
+    pal.write(0x3F10, 0x30);
+    try std.testing.expectEqual(@as(u8, 0x30), pal.read(0x3F00));
+    try std.testing.expectEqual(@as(u8, 0x30), pal.read(0x3F10));
+
+    // Test mirrors $3F14 -> $3F04, $3F18 -> $3F08, $3F1C -> $3F0C
+    pal.write(0x3F04, 0x15);
+    try std.testing.expectEqual(@as(u8, 0x15), pal.read(0x3F14));
+
+    pal.write(0x3F08, 0x27);
+    try std.testing.expectEqual(@as(u8, 0x27), pal.read(0x3F18));
+
+    pal.write(0x3F0C, 0x39);
+    try std.testing.expectEqual(@as(u8, 0x39), pal.read(0x3F1C));
+
+    // Values are clamped to 6 bits
+    pal.write(0x3F01, 0xFF);
+    try std.testing.expectEqual(@as(u8, 0x3F), pal.read(0x3F01));
+}

src/system/nes/ppu/registers.zig

@@ -0,0 +1,344 @@
+// PPU Registers ($2000-$2007)
+const std = @import("std");
+const Control = @import("control.zig");
+const Mask = @import("mask.zig");
+const common = @import("../common.zig");
+
+pub const Registers = @This();
+
+control: Control = .{},
+mask: Mask = .{},
+pending_mask: Mask = .{},
+mask_delay: u3 = 0,
+
+status: u8 = 0,
+oam_addr: u8 = 0,
+oam: [256]u8 = [_]u8{0} ** 256,
+
+v: u15 = 0,
+t: u15 = 0,
+x: u3 = 0,
+w: bool = false,
+
+read_buffer: u8 = 0,
+io_bus: u8 = 0,
+
+pending_v: u15 = 0,
+v_commit_delay: u3 = 0,
+
+suppress_writes_cycles: u32 = 0,
+
+pub fn init() Registers {
+    return .{};
+}
+
+pub fn power(self: *Registers) void {
+    self.powerRegion(.ntsc);
+}
+
+pub fn powerRegion(self: *Registers, _: common.Region) void {
+    self.control.raw = 0;
+    self.mask.raw = 0;
+    self.pending_mask.raw = 0;
+    self.mask_delay = 0;
+    self.status = 0;
+    self.oam_addr = 0;
+    self.v = 0;
+    self.t = 0;
+    self.x = 0;
+    self.w = false;
+    self.read_buffer = 0;
+    self.io_bus = 0;
+    self.pending_v = 0;
+    self.v_commit_delay = 0;
+    self.suppress_writes_cycles = 29658;
+}
+
+pub fn reset(self: *Registers) void {
+    self.resetRegion(.ntsc);
+}
+
+pub fn resetRegion(self: *Registers, _: common.Region) void {
+    self.control.raw = 0;
+    self.mask.raw = 0;
+    self.pending_mask.raw = 0;
+    self.mask_delay = 0;
+    self.w = false;
+    self.read_buffer = 0;
+}
+
+pub fn tickCpuCycle(self: *Registers) void {
+    if (self.suppress_writes_cycles > 0) {
+        self.suppress_writes_cycles -= 1;
+    }
+}
+
+pub fn clockDot(self: *Registers) void {
+    if (self.mask_delay > 0) {
+        self.mask_delay -= 1;
+        if (self.mask_delay == 0) {
+            self.mask = self.pending_mask;
+        }
+    }
+    if (self.v_commit_delay > 0) {
+        self.v_commit_delay -= 1;
+        if (self.v_commit_delay == 0) {
+            self.v = self.pending_v;
+        }
+    }
+}
+
+pub fn vblank(self: *const Registers) bool {
+    return (self.status & 0x80) != 0;
+}
+
+pub fn setVblank(self: *Registers, val: bool) void {
+    if (val) {
+        self.status |= 0x80;
+    } else {
+        self.status &= ~@as(u8, 0x80);
+    }
+}
+
+pub fn clearRenderingFlags(self: *Registers) void {
+    self.status &= ~@as(u8, 0xe0);
+}
+
+pub fn setSpriteZeroHit(self: *Registers, val: bool) void {
+    if (val) self.status |= 0x40 else self.status &= ~@as(u8, 0x40);
+}
+
+pub fn setSpriteOverflow(self: *Registers, val: bool) void {
+    if (val) self.status |= 0x20 else self.status &= ~@as(u8, 0x20);
+}
+
+pub fn spriteHit(self: *const Registers) bool {
+    return (self.status & 0x40) != 0;
+}
+
+pub fn nmiLine(self: *const Registers) bool {
+    return self.control.nmiEnabled() and self.vblank();
+}
+
+pub fn takeNmi(self: *Registers) bool {
+    if (self.nmiLine()) {
+        return true;
+    }
+    return false;
+}
+
+pub fn currentVramAddress(self: *const Registers) u16 {
+    return self.v;
+}
+
+pub fn copyVertical(self: *Registers) void {
+    self.v = (self.v & ~@as(u15, 0x7be0)) | (self.t & 0x7be0);
+}
+
+pub fn copyHorizontal(self: *Registers) void {
+    self.v = (self.v & ~@as(u15, 0x041f)) | (self.t & 0x041f);
+}
+
+pub fn incrementCoarseX(self: *Registers) void {
+    if ((self.v & 0x001f) == 31) {
+        self.v &= ~@as(u15, 0x001f);
+        self.v ^= 0x0400;
+    } else {
+        self.v += 1;
+    }
+}
+
+pub fn incrementY(self: *Registers) void {
+    if ((self.v & 0x7000) != 0x7000) {
+        self.v += 0x1000;
+    } else {
+        self.v &= ~@as(u15, 0x7000);
+        var y = (self.v & 0x03e0) >> 5;
+        if (y == 29) {
+            y = 0;
+            self.v ^= 0x0800;
+        } else if (y == 31) {
+            y = 0;
+        } else {
+            y += 1;
+        }
+        self.v = (self.v & ~@as(u15, 0x03e0)) | (y << 5);
+    }
+}
+
+pub fn cpuRead(self: *Registers, bus: anytype, address: u16, rendering: bool) ?u8 {
+    _ = rendering;
+    const reg = address & 7;
+    return switch (reg) {
+        2 => blk: {
+            const res = (self.status & 0xe0) | (self.io_bus & 0x1f);
+            self.status &= ~@as(u8, 0x80);
+            self.w = false;
+            self.io_bus = res;
+            break :blk res;
+        },
+        4 => blk: {
+            const val = self.oam[self.oam_addr];
+            self.io_bus = val;
+            break :blk val;
+        },
+        7 => blk: {
+            const addr = self.v & 0x3fff;
+            var val: u8 = undefined;
+            if (addr >= 0x3f00) {
+                val = (self.io_bus & 0xc0) | (bus.read(addr) & 0x3f);
+                self.read_buffer = bus.read(addr - 0x1000);
+            } else {
+                val = self.read_buffer;
+                self.read_buffer = bus.read(addr);
+            }
+            self.v = (self.v +% self.control.vramIncrement()) & 0x3fff;
+            self.io_bus = val;
+            break :blk val;
+        },
+        else => self.io_bus,
+    };
+}
+
+pub fn cpuWrite(self: *Registers, bus: anytype, address: u16, value: u8, rendering: bool) bool {
+    _ = rendering;
+    self.io_bus = value;
+    const reg = address & 7;
+
+    const suppress = self.suppress_writes_cycles > 0;
+    if (suppress and (reg == 0 or reg == 1 or reg == 5 or reg == 6)) {
+        return true;
+    }
+
+    switch (reg) {
+        0 => {
+            const old_nmi = self.nmiLine();
+            self.control.raw = value;
+            self.t = (self.t & 0x73ff) | (@as(u15, value & 3) << 10);
+            if (!old_nmi and self.nmiLine()) {
+                // Trigger immediate NMI
+            }
+        },
+        1 => {
+            self.pending_mask.raw = value;
+            self.mask_delay = 4;
+        },
+        3 => self.oam_addr = value,
+        4 => {
+            var val = value;
+            if ((self.oam_addr & 3) == 2) {
+                val &= 0xe3; // mask unused bits
+            }
+            self.oam[self.oam_addr] = val;
+            self.oam_addr +%= 1;
+        },
+        5 => {
+            if (!self.w) {
+                self.t = (self.t & 0x7fe0) | (value >> 3);
+                self.x = @truncate(value & 7);
+                self.w = true;
+            } else {
+                self.t = (self.t & 0x0c1f) | (@as(u15, value & 7) << 12) | (@as(u15, value & 0xf8) << 2);
+                self.w = false;
+            }
+        },
+        6 => {
+            if (!self.w) {
+                self.t = (self.t & 0x00ff) | (@as(u15, value & 0x3f) << 8);
+                self.w = true;
+            } else {
+                self.t = (self.t & 0x7f00) | value;
+                self.pending_v = self.t;
+                self.v_commit_delay = 2;
+                self.w = false;
+            }
+        },
+        7 => {
+            const addr = self.v & 0x3fff;
+            bus.write(addr, value);
+            self.v = (self.v +% self.control.vramIncrement()) & 0x3fff;
+        },
+        else => {},
+    }
+    return true;
+}
+
+// consolidated PPU registers test suite
+pub const TestBus = struct {
+    memory: [0x10000]u8 = [_]u8{0} ** 0x10000,
+    pub fn read(self: *TestBus, address: u16) u8 {
+        return self.memory[address];
+    }
+    pub fn write(self: *TestBus, address: u16, value: u8) void {
+        self.memory[address] = value;
+    }
+};
+
+test "ppu registers $2000-$2007 control, scroll, address and buffered reads" {
+    var regs = Registers.init();
+    var bus = TestBus{};
+
+    // $2000 & NMI
+    _ = regs.cpuWrite(&bus, 0x2000, 0b0000_0011, false);
+    try std.testing.expectEqual(@as(u15, 0x0c00), regs.t & 0x0c00);
+    regs.setVblank(true);
+    _ = regs.cpuWrite(&bus, 0x2000, 0x80, false);
+    try std.testing.expect(regs.takeNmi());
+
+    // $2002 status & latch reset
+    regs.status = 0xe0;
+    regs.io_bus = 0x1b;
+    regs.w = true;
+    try std.testing.expectEqual(@as(u8, 0xfb), regs.cpuRead(&bus, 0x2002, false).?);
+    try std.testing.expect(!regs.vblank() and !regs.w);
+
+    // $2005 coarse & fine scroll
+    regs = Registers.init();
+    _ = regs.cpuWrite(&bus, 0x2005, 0b10101_011, false);
+    try std.testing.expectEqual(@as(u15, 0b10101), regs.t & 0x001f);
+    try std.testing.expectEqual(@as(u3, 3), regs.x);
+
+    // $2006 address latching & delayed commit
+    regs = Registers.init();
+    _ = regs.cpuWrite(&bus, 0x2006, 0x23, false);
+    _ = regs.cpuWrite(&bus, 0x2006, 0x45, false);
+    try std.testing.expectEqual(@as(u15, 0x2345), regs.t);
+    regs.clockDot();
+    regs.clockDot();
+    try std.testing.expectEqual(@as(u15, 0x2345), regs.v);
+
+    // $2007 buffered VRAM & immediate palette reads
+    regs = Registers.init();
+    bus.memory[0x2000] = 0xaa;
+    regs.v = 0x2000;
+    regs.read_buffer = 0x55;
+    try std.testing.expectEqual(@as(u8, 0x55), regs.cpuRead(&bus, 0x2007, false).?);
+    try std.testing.expectEqual(@as(u8, 0xaa), regs.read_buffer);
+}
+
+test "ppu oam, mask rendering delay, and power-on write suppression" {
+    var regs = Registers.init();
+    var bus = TestBus{};
+
+    // OAM address auto-increment
+    _ = regs.cpuWrite(&bus, 0x2003, 0x10, false);
+    _ = regs.cpuWrite(&bus, 0x2004, 0xab, false);
+    try std.testing.expectEqual(@as(u8, 0xab), regs.oam[0x10]);
+    try std.testing.expectEqual(@as(u8, 0x11), regs.oam_addr);
+
+    // PPUMASK 3-4 dot delay
+    regs = Registers.init();
+    _ = regs.cpuWrite(&bus, 0x2001, 0x18, false);
+    try std.testing.expect(!regs.mask.renderingEnabled());
+    for (0..4) |_| regs.clockDot();
+    try std.testing.expect(regs.mask.renderingEnabled());
+
+    // Power-on write suppression
+    regs = Registers.init();
+    regs.power();
+    _ = regs.cpuWrite(&bus, 0x2000, 0x80, false);
+    try std.testing.expectEqual(@as(u8, 0), regs.control.raw);
+    for (0..29658) |_| regs.tickCpuCycle();
+    _ = regs.cpuWrite(&bus, 0x2000, 0x80, false);
+    try std.testing.expectEqual(@as(u8, 0x80), regs.control.raw);
+}

src/system/nes/ppu/root.zig

@@ -0,0 +1,542 @@
+const std = @import("std");
+
+pub const Control = @import("control.zig");
+pub const Mask = @import("mask.zig");
+pub const Registers = @import("registers.zig");
+pub const Palette = @import("palette.zig").Palette;
+pub const default_palette = @import("palette.zig").default_palette;
+
+pub const common = @import("../common.zig");
+pub const Region = common.Region;
+
+pub const SpriteUnit = struct {
+    pattern_low: u8 = 0,
+    pattern_high: u8 = 0,
+    x_counter: u8 = 0,
+    attribute: u8 = 0,
+    is_sprite_zero: bool = false,
+};
+
+pub const Ppu = @This();
+
+region: Region = .ntsc,
+
+registers: Registers = .{},
+palette: Palette = .{},
+
+// Frame timing counters
+dot: u16 = 0,
+scanline: u16 = 0,
+odd_frame: bool = false,
+frame_count: u64 = 0,
+frame_complete: bool = false,
+
+// Framebuffer: 256 x 240 containing 6-bit NES palette indices
+framebuffer: [common.screen_width * common.screen_height]u8 = [_]u8{0} ** (common.screen_width * common.screen_height),
+
+// Background fetch pipeline latches
+next_tile_id: u8 = 0,
+next_tile_attribute: u8 = 0,
+next_pattern_low: u8 = 0,
+next_pattern_high: u8 = 0,
+
+// Background 16-bit shift registers
+pattern_shift_low: u16 = 0,
+pattern_shift_high: u16 = 0,
+attribute_shift_low: u16 = 0,
+attribute_shift_high: u16 = 0,
+
+// Sprite evaluation & fetching
+secondary_oam: [32]u8 = [_]u8{0xff} ** 32,
+sprite_count: u4 = 0,
+sprite_zero_in_secondary: bool = false,
+
+sprite_units: [8]SpriteUnit = [_]SpriteUnit{.{}} ** 8,
+active_sprite_units: [8]SpriteUnit = [_]SpriteUnit{.{}} ** 8,
+active_sprite_count: u4 = 0,
+sprite_zero_active: bool = false,
+
+pub fn init(region: Region) Ppu {
+    return .{
+        .region = region,
+        .registers = Registers.init(),
+        .palette = Palette.init(),
+    };
+}
+
+pub fn power(self: *Ppu) void {
+    self.registers.powerRegion(self.region);
+    self.palette.reset();
+    self.dot = 0;
+    self.scanline = 0;
+    self.odd_frame = false;
+    self.frame_count = 0;
+    self.frame_complete = false;
+    @memset(&self.framebuffer, 0);
+    self.resetPipeline();
+}
+
+pub fn reset(self: *Ppu) void {
+    self.registers.resetRegion(self.region);
+    self.dot = 0;
+    self.scanline = 0;
+    self.frame_complete = false;
+    self.resetPipeline();
+}
+
+fn resetPipeline(self: *Ppu) void {
+    self.next_tile_id = 0;
+    self.next_tile_attribute = 0;
+    self.next_pattern_low = 0;
+    self.next_pattern_high = 0;
+    self.pattern_shift_low = 0;
+    self.pattern_shift_high = 0;
+    self.attribute_shift_low = 0;
+    self.attribute_shift_high = 0;
+    @memset(&self.secondary_oam, 0xff);
+    self.sprite_count = 0;
+    self.sprite_zero_in_secondary = false;
+    self.active_sprite_count = 0;
+    self.sprite_zero_active = false;
+}
+
+pub inline fn preRenderScanline(self: *const Ppu) u16 {
+    return self.region.preRenderScanline();
+}
+
+pub inline fn totalScanlines(self: *const Ppu) u16 {
+    return self.region.totalScanlines();
+}
+
+pub inline fn isRendering(self: *const Ppu) bool {
+    const visible_or_prerender = (self.scanline < 240 or self.scanline == self.preRenderScanline());
+    return visible_or_prerender and self.registers.mask.renderingEnabled();
+}
+
+pub fn nmiLine(self: *const Ppu) bool {
+    return self.registers.nmiLine();
+}
+
+// CPU register interface
+pub fn cpuRead(self: *Ppu, bus: anytype, address: u16) ?u8 {
+    // VBlank / NMI race condition on reading $2002 around scanline 241 dot 1:
+    // Reading on scanline 241 dot 0 clears VBlank before it can set NMI.
+    if (address == 0x2002 and self.scanline == 241 and self.dot == 0) {
+        // Suppress VBlank flag set on dot 1
+        var val = self.registers.cpuRead(bus, address, self.isRendering());
+        if (val) |*v| {
+            v.* &= 0x7f;
+        }
+        return val;
+    }
+    return self.registers.cpuRead(bus, address, self.isRendering());
+}
+
+pub fn cpuWrite(self: *Ppu, bus: anytype, address: u16, value: u8) bool {
+    return self.registers.cpuWrite(bus, address, value, self.isRendering());
+}
+
+// Main 341-dot PPU Clock
+pub fn clock(self: *Ppu, bus: anytype) void {
+    // Advance delayed register commits (~1-2 dots for $2006, ~3-4 dots for $2001)
+    self.registers.clockDot();
+
+    const prerender = self.preRenderScanline();
+    const visible = self.scanline < 240;
+    const rendering = self.registers.mask.renderingEnabled();
+
+    // 1. Scanline-specific events
+    if (self.scanline == 241 and self.dot == 1) {
+        // VBlank start
+        self.registers.setVblank(true);
+    } else if (self.scanline == prerender and self.dot == 1) {
+        // Pre-render clear flags
+        self.registers.clearRenderingFlags();
+    }
+
+    // Pre-render vertical scroll copy (dots 280..304)
+    if (self.scanline == prerender and self.dot >= 280 and self.dot <= 304 and rendering) {
+        self.registers.copyVertical();
+    }
+
+    // 2. Background and Sprite Rendering operations
+    if (visible or self.scanline == prerender) {
+        self.stepRendering(bus);
+    }
+
+    // 3. Pixel output to framebuffer (visible scanlines dots 1..256)
+    if (visible and self.dot >= 1 and self.dot <= 256) {
+        self.renderPixel();
+    }
+
+    // 4. Dot and Scanline Advance
+    self.dot += 1;
+
+    // Odd-frame skip on NTSC pre-render scanline (dot 339 -> 0)
+    if (self.region == .ntsc and self.odd_frame and rendering and self.scanline == prerender and self.dot == 340) {
+        self.dot = 0;
+        self.scanline = 0;
+        self.odd_frame = !self.odd_frame;
+        self.frame_complete = true;
+        self.frame_count += 1;
+        return;
+    }
+
+    if (self.dot >= 341) {
+        self.dot = 0;
+        self.scanline += 1;
+
+        if (self.scanline >= self.totalScanlines()) {
+            self.scanline = 0;
+            self.odd_frame = !self.odd_frame;
+            self.frame_complete = true;
+            self.frame_count += 1;
+        }
+    }
+}
+
+fn stepRendering(self: *Ppu, bus: anytype) void {
+    const rendering = self.registers.mask.renderingEnabled();
+
+    // Background fetching during dots 1..256 and 321..336
+    if ((self.dot >= 1 and self.dot <= 256) or (self.dot >= 321 and self.dot <= 336)) {
+        // Shift background registers on every dot
+        if (rendering) {
+            self.pattern_shift_low <<= 1;
+            self.pattern_shift_high <<= 1;
+            self.attribute_shift_low <<= 1;
+            self.attribute_shift_high <<= 1;
+        }
+
+        const step = (self.dot - 1) & 7;
+        switch (step) {
+            0 => {
+                // Fetch Nametable byte
+                const nt_addr: u16 = 0x2000 | (self.registers.v & 0x0fff);
+                self.next_tile_id = bus.read(nt_addr);
+            },
+            2 => {
+                // Fetch Attribute byte
+                const v = self.registers.v;
+                const attr_addr: u16 = 0x23c0 |
+                    (v & 0x0c00) |
+                    ((v >> 4) & 0x38) |
+                    ((v >> 2) & 0x07);
+                const attr_byte = bus.read(attr_addr);
+                // Determine 2-bit attribute quadrant based on coarse X and Y bit 1
+                const shift: u3 = @truncate(((v >> 4) & 0x04) | (v & 0x02));
+                self.next_tile_attribute = (attr_byte >> shift) & 0x03;
+            },
+            4 => {
+                // Fetch Pattern Table Low byte
+                const bg_base = self.registers.control.backgroundPatternAddress();
+                const fine_y: u16 = (self.registers.v >> 12) & 0x07;
+                const pat_addr: u16 = bg_base | (@as(u16, self.next_tile_id) << 4) | fine_y;
+                self.next_pattern_low = bus.read(pat_addr);
+            },
+            6 => {
+                // Fetch Pattern Table High byte
+                const bg_base = self.registers.control.backgroundPatternAddress();
+                const fine_y: u16 = (self.registers.v >> 12) & 0x07;
+                const pat_addr: u16 = bg_base | (@as(u16, self.next_tile_id) << 4) | fine_y | 8;
+                self.next_pattern_high = bus.read(pat_addr);
+            },
+            7 => {
+                // Load shift registers and increment coarse X
+                if (rendering) {
+                    self.loadShiftRegisters();
+                    self.registers.incrementCoarseX();
+                }
+            },
+            else => {},
+        }
+    }
+
+    // Dot 256: Increment Y
+    if (self.dot == 256 and rendering) {
+        self.registers.incrementY();
+    }
+
+    // Dot 257: Copy horizontal scroll
+    if (self.dot == 257 and rendering) {
+        self.registers.copyHorizontal();
+    }
+
+    // Dots 337..340: Two dummy nametable fetches (vital for mapper IRQ / A12 observation)
+    if (self.dot == 337 or self.dot == 339) {
+        const nt_addr: u16 = 0x2000 | (self.registers.v & 0x0fff);
+        _ = bus.read(nt_addr);
+    }
+
+    // Sprite evaluation and fetching (visible scanlines only)
+    if (self.scanline < 240) {
+        self.stepSpriteLogic(bus);
+    }
+}
+
+fn loadShiftRegisters(self: *Ppu) void {
+    self.pattern_shift_low = (self.pattern_shift_low & 0xff00) | self.next_pattern_low;
+    self.pattern_shift_high = (self.pattern_shift_high & 0xff00) | self.next_pattern_high;
+
+    const attr_lo: u8 = if ((self.next_tile_attribute & 1) != 0) 0xff else 0x00;
+    const attr_hi: u8 = if ((self.next_tile_attribute & 2) != 0) 0xff else 0x00;
+    self.attribute_shift_low = (self.attribute_shift_low & 0xff00) | attr_lo;
+    self.attribute_shift_high = (self.attribute_shift_high & 0xff00) | attr_hi;
+}
+
+fn stepSpriteLogic(self: *Ppu, bus: anytype) void {
+    const rendering = self.registers.mask.renderingEnabled();
+
+    // Dots 1..64: Clear secondary OAM
+    if (self.dot == 1) {
+        @memset(&self.secondary_oam, 0xff);
+        self.sprite_count = 0;
+        self.sprite_zero_in_secondary = false;
+    }
+
+    // Dots 65..256: Sprite Evaluation for next scanline
+    if (self.dot == 65 and rendering) {
+        const sprite_height = self.registers.control.spriteHeight();
+        const target_scanline = self.scanline;
+
+        var i: usize = 0;
+        while (i < 64) : (i += 1) {
+            const y = self.registers.oam[i * 4];
+            if (target_scanline >= y and target_scanline < y + sprite_height) {
+                if (self.sprite_count < 8) {
+                    const dst = @as(usize, self.sprite_count) * 4;
+                    @memcpy(self.secondary_oam[dst .. dst + 4], self.registers.oam[i * 4 .. i * 4 + 4]);
+                    if (i == 0) {
+                        self.sprite_zero_in_secondary = true;
+                    }
+                    self.sprite_count += 1;
+                } else {
+                    // 8-sprite limit reached; set overflow flag
+                    self.registers.setSpriteOverflow(true);
+                    break;
+                }
+            }
+        }
+    }
+
+    // Dots 257..320: OAMADDR is forced to 0
+    if (self.dot >= 257 and self.dot <= 320 and rendering) {
+        self.registers.oam_addr = 0;
+    }
+
+    // Dots 257..320: Sprite pattern fetching for the 8 evaluated sprites
+    if (self.dot == 257) {
+        const sprite_height = self.registers.control.spriteHeight();
+        const target_scanline = self.scanline;
+
+        for (0..8) |s| {
+            if (s < self.sprite_count) {
+                const y = self.secondary_oam[s * 4];
+                const tile = self.secondary_oam[s * 4 + 1];
+                const attr = self.secondary_oam[s * 4 + 2];
+                const x = self.secondary_oam[s * 4 + 3];
+
+                var row: u4 = @truncate(target_scanline - y);
+                if ((attr & 0x80) != 0) {
+                    // Vertical flip
+                    row = @truncate(sprite_height - 1 - row);
+                }
+
+                const addr_lo = self.registers.control.spritePatternAddressForTile(tile, row);
+                const addr_hi = addr_lo + 8;
+
+                var lo = bus.read(addr_lo);
+                var hi = bus.read(addr_hi);
+
+                if ((attr & 0x40) != 0) {
+                    // Horizontal flip
+                    lo = @bitReverse(lo);
+                    hi = @bitReverse(hi);
+                }
+
+                self.sprite_units[s] = .{
+                    .pattern_low = lo,
+                    .pattern_high = hi,
+                    .x_counter = x,
+                    .attribute = attr,
+                    .is_sprite_zero = (s == 0 and self.sprite_zero_in_secondary),
+                };
+            } else {
+                // Dummy sprite fetches
+                _ = bus.read(self.registers.control.sprite8x8PatternAddress());
+                _ = bus.read(self.registers.control.sprite8x8PatternAddress() + 8);
+                self.sprite_units[s] = .{};
+            }
+        }
+    }
+
+    // At dot 0 of the next scanline, commit fetched sprite units
+    if (self.dot == 340) {
+        self.active_sprite_units = self.sprite_units;
+        self.active_sprite_count = self.sprite_count;
+        self.sprite_zero_active = self.sprite_zero_in_secondary;
+    }
+}
+
+fn renderPixel(self: *Ppu) void {
+    const pixel_x = self.dot - 1;
+    const pixel_y = self.scanline;
+
+    var bg_color: u2 = 0;
+    var bg_palette: u2 = 0;
+
+    if (self.registers.mask.showBackground()) {
+        const fine_x = self.registers.x;
+        const bit_mux = @as(u16, 0x8000) >> fine_x;
+
+        const p0: u2 = if ((self.pattern_shift_low & bit_mux) != 0) 1 else 0;
+        const p1: u2 = if ((self.pattern_shift_high & bit_mux) != 0) 2 else 0;
+        bg_color = p0 | p1;
+
+        const a0: u2 = if ((self.attribute_shift_low & bit_mux) != 0) 1 else 0;
+        const a1: u2 = if ((self.attribute_shift_high & bit_mux) != 0) 2 else 0;
+        bg_palette = a0 | a1;
+
+        // Left 8-pixel clipping
+        if (pixel_x < 8 and !self.registers.mask.showBackgroundLeft()) {
+            bg_color = 0;
+        }
+    }
+
+    var sprite_color: u2 = 0;
+    var sprite_palette: u2 = 0;
+    var sprite_priority: bool = false;
+    var is_sprite_zero: bool = false;
+
+    if (self.registers.mask.showSprites()) {
+        for (&self.active_sprite_units) |*unit| {
+            if (unit.x_counter == 0) {
+                const sp0: u2 = if ((unit.pattern_low & 0x80) != 0) 1 else 0;
+                const sp1: u2 = if ((unit.pattern_high & 0x80) != 0) 2 else 0;
+                const col = sp0 | sp1;
+
+                if (col != 0 and sprite_color == 0) {
+                    sprite_color = col;
+                    sprite_palette = @truncate(unit.attribute & 0x03);
+                    sprite_priority = (unit.attribute & 0x20) != 0;
+                    is_sprite_zero = unit.is_sprite_zero;
+                }
+            }
+        }
+
+        // Left 8-pixel clipping
+        if (pixel_x < 8 and !self.registers.mask.showSpritesLeft()) {
+            sprite_color = 0;
+        }
+    }
+
+    // Shift active sprite pattern registers and decrement non-zero X counters
+    for (&self.active_sprite_units) |*unit| {
+        if (unit.x_counter > 0) {
+            unit.x_counter -= 1;
+        } else {
+            unit.pattern_low <<= 1;
+            unit.pattern_high <<= 1;
+        }
+    }
+
+    // Sprite 0 Hit detection
+    if (is_sprite_zero and self.sprite_zero_active and bg_color != 0 and sprite_color != 0) {
+        // Sprite 0 hit cannot occur at X=255
+        if (pixel_x < 255) {
+            self.registers.setSpriteZeroHit(true);
+        }
+    }
+
+    // Pixel composition & palette lookup
+    var palette_addr: u16 = 0x3f00;
+    if (bg_color == 0 and sprite_color == 0) {
+        palette_addr = 0x3f00;
+    } else if (bg_color != 0 and sprite_color == 0) {
+        palette_addr = 0x3f00 | (@as(u16, bg_palette) << 2) | bg_color;
+    } else if (bg_color == 0 and sprite_color != 0) {
+        palette_addr = 0x3f10 | (@as(u16, sprite_palette) << 2) | sprite_color;
+    } else {
+        // Both opaque: check priority
+        if (sprite_priority) {
+            // Behind background
+            palette_addr = 0x3f00 | (@as(u16, bg_palette) << 2) | bg_color;
+        } else {
+            // In front of background
+            palette_addr = 0x3f10 | (@as(u16, sprite_palette) << 2) | sprite_color;
+        }
+    }
+
+    const color_index = self.palette.read(palette_addr);
+    self.framebuffer[pixel_y * 256 + pixel_x] = color_index;
+}
+
+/// Helper to convert the 8-bit palette-indexed framebuffer to standard 32-bit RGBA8888 pixels.
+pub fn renderToRgba(self: *const Ppu, dest: []u32) void {
+    std.debug.assert(dest.len >= common.screen_width * common.screen_height);
+    for (self.framebuffer, 0..) |color_idx, i| {
+        dest[i] = default_palette[color_idx & 0x3f];
+    }
+}
+
+const MockBus = struct {
+    read_count: usize = 0,
+    last_address: u16 = 0,
+
+    pub fn read(self: *MockBus, address: u16) u8 {
+        self.read_count += 1;
+        self.last_address = address;
+        if (address < 0x2000) return 0x55; // Pattern data
+        if (address < 0x3f00) return 0x01; // Nametable / attribute
+        return 0;
+    }
+
+    pub fn write(_: *MockBus, _: u16, _: u8) void {}
+};
+
+// consolidated PPU timing and rendering unit test suite
+test "ppu scanline progression, vblank flags, odd-frame skip and sprite 0 hit" {
+    var ppu = Ppu.init(.ntsc);
+    var bus = MockBus{};
+
+    // Scanline & dot clock progression
+    try std.testing.expectEqual(@as(u16, 0), ppu.scanline);
+    ppu.clock(&bus);
+    try std.testing.expectEqual(@as(u16, 1), ppu.dot);
+
+    // Run until vblank set on scanline 241
+    while (!(ppu.scanline == 241 and ppu.dot == 1)) ppu.clock(&bus);
+    ppu.clock(&bus);
+    try std.testing.expect(ppu.registers.vblank());
+
+    // Odd-frame skipped dot on NTSC rendering
+    ppu = Ppu.init(.ntsc);
+    _ = ppu.cpuWrite(&bus, 0x2001, 0x08);
+    for (0..4) |_| ppu.clock(&bus);
+    while (!ppu.frame_complete) ppu.clock(&bus);
+    try std.testing.expect(ppu.odd_frame);
+
+    // Sprite 0 Hit detection
+    ppu = Ppu.init(.ntsc);
+    _ = ppu.cpuWrite(&bus, 0x2001, 0x18);
+    for (0..4) |_| ppu.clock(&bus);
+    ppu.registers.oam[0] = 10;
+    ppu.registers.oam[1] = 0;
+    ppu.registers.oam[2] = 0;
+    ppu.registers.oam[3] = 10;
+    ppu.active_sprite_units[0] = .{
+        .pattern_low = 0xff,
+        .pattern_high = 0x00,
+        .x_counter = 0,
+        .attribute = 0,
+        .is_sprite_zero = true,
+    };
+    ppu.sprite_zero_active = true;
+    ppu.pattern_shift_low = 0xffff;
+    ppu.pattern_shift_high = 0x0000;
+    ppu.scanline = 10;
+    ppu.dot = 11;
+
+    ppu.renderPixel();
+    try std.testing.expect(ppu.registers.spriteHit());
+}

src/system/nes/root.zig

@@ -1,45 +1,64 @@
 const std = @import("std");
-
 const contract = @import("contract");
+const video = contract.video;
+const audio = contract.audio;
+const input = contract.input;
+const storage = contract.storage;
+const state = contract.state;
 const _cpu = @import("cpu");
 
-const Bus = @import("bus.zig");
-const Cartridge = @import("cartridge.zig");
+pub const common = @import("common.zig");
+pub const Region = common.Region;
+pub const Mirroring = common.Mirroring;
+pub const Button = common.Button;
+pub const screen_width = common.screen_width;
+pub const screen_height = common.screen_height;
 
-const controller = @import("controller.zig");
-pub const Button = controller.Button;
+pub const ppu = @import("ppu/root.zig");
+pub const apu = @import("apu/root.zig");
+pub const bus_mod = @import("bus.zig");
+pub const Bus = bus_mod.Bus;
+pub const cartridge_mod = @import("cartridge.zig");
+pub const Cartridge = cartridge_mod.Cartridge;
+pub const controller_mod = @import("controller.zig");
+pub const Controller = controller_mod.Controller;
+pub const mapper_mod = @import("mapper/root.zig");
 
-// Adjust this enum spelling to whatever your CPU package
-// currently exports for the Ricoh 2A03.
-const Cpu = _cpu.m6502.Cpu(
-    Bus,
-    .ricoh2a03,
-    .cycle,
-);
+const Cpu = _cpu.m6502.Cpu(Bus, .ricoh2a03, .cycle);
 
-const Nes = @This();
-pub const spec = contract.SystemSpec{
+pub const Nes = @This();
+
+pub const default_palette_565: [64]u16 = blk: {
+    var table: [64]u16 = undefined;
+    for (ppu.default_palette, 0..) |rgba, i| {
+        const r = @as(u16, @truncate((rgba >> 24) & 0xFF));
+        const g = @as(u16, @truncate((rgba >> 16) & 0xFF));
+        const b = @as(u16, @truncate((rgba >> 8) & 0xFF));
+        const r5 = r >> 3;
+        const g6 = g >> 2;
+        const b5 = b >> 3;
+        table[i] = (r5 << 11) | (g6 << 5) | b5;
+    }
+    break :blk table;
+};
+
+pub const spec: contract.SystemSpec = .{
     .name = "Nintendo Entertainment System",
-
     .video_outputs = &.{
         .{
-            .max_width = 256,
-            .max_height = 240,
-
+            .max_width = common.screen_width,
+            .max_height = common.screen_height,
             .format = .rgb565,
-
             .aspect_ratio = .{
                 .numerator = 4,
                 .denominator = 3,
             },
-
             .refresh_rate = .{
                 .numerator = 60,
                 .denominator = 1,
             },
         },
     },
-
     .audio_outputs = &.{
         .{
             .sample_rate = 44_100,
@@ -47,16 +66,10 @@ pub const spec = contract.SystemSpec{
             .format = .i16,
         },
     },
-
     .input_devices = &.{
-        .{
-            .button_count = 8,
-        },
-        .{
-            .button_count = 8,
-        },
+        .{ .button_count = 8 },
+        .{ .button_count = 8 },
     },
-
     .storage_devices = &.{
         .{
             .name = "Battery PRG RAM",
@@ -67,355 +80,173 @@ pub const spec = contract.SystemSpec{
             .writable = true,
         },
     },
-
     .save_state = .{
-        .max_size = 32 * 1024,
+        .max_size = 64 * 1024,
     },
 };
 
 bus: Bus = undefined,
 cpu: Cpu = undefined,
+rgb565_framebuffer: [common.screen_width * common.screen_height]u16 = [_]u16{0} ** (common.screen_width * common.screen_height),
 
-/// Initialize in place.
-///
-/// `rom` must remain valid for the lifetime of this NES
-/// because Cartridge stores slices into it.
-pub fn init(
-    self: *Nes,
-    rom: []const u8,
-) Cartridge.Error!void {
-    self.bus =
-        try Bus.init(rom);
-
-    self.cpu =
-        Cpu.init(&self.bus);
-
-    self.reset();
+pub fn init(self: *Nes, rom_bytes: []const u8) !void {
+    const cart = try Cartridge.init(rom_bytes);
+    self.bus = Bus.init(.ntsc, cart);
+    self.cpu = Cpu.init(&self.bus);
+    self.cpu.reset();
 }
 
-pub fn reset(
-    self: *Nes,
-) void {
+pub fn reset(self: *Nes) void {
+    self.cpu.bus = &self.bus;
     self.bus.reset();
     self.cpu.reset();
 }
 
-pub fn step(
-    self: *Nes,
-) ?u8 {
-    const cycles = self.cpu.step() orelse return null;
-    self.serviceOamDma();
-    return cycles;
-}
+pub fn runFrame(self: *Nes) void {
+    self.bus.ppu.frame_complete = false;
+    self.bus.apu.clearSamples();
 
-pub fn runFrame(
-    self: *Nes,
-) void {
-    self.bus.ppu.frame_ready = false;
-    self.bus.apu.sample_count = 0;
+    while (!self.bus.ppu.frame_complete) {
+        if (self.bus.dma_active) {
+            self.bus.stepDma();
+        } else {
+            _ = self.cpu.step();
+        }
+    }
 
-    while (!self.bus.ppu.frame_ready) {
-        _ = self.step() orelse
-            return;
+    // Convert PPU framebuffer to RGB565 for frontend driver
+    for (self.bus.ppu.framebuffer, 0..) |color_idx, i| {
+        self.rgb565_framebuffer[i] = default_palette_565[color_idx & 0x3f];
     }
 }
 
-pub const conformance = @import("conformance.zig");
-
-test {
-    _ = conformance;
-}
-
-comptime {
-    contract.validateSystem(Nes);
-}
-
-pub fn videoFrame(
-    self: *const Nes,
-    output: usize,
-) contract.VideoFrame {
-    std.debug.assert(output == 0);
-
+pub fn videoFrame(self: *const Nes, _: usize) video.VideoFrame {
     return .{
-        .data = std.mem.sliceAsBytes(
-            self.bus.ppu.framebuffer[0..],
-        ),
-
-        .width = 256,
-        .height = 240,
-
-        .pitch = 256 * @sizeOf(u16),
-
+        .data = std.mem.sliceAsBytes(&self.rgb565_framebuffer),
+        .width = common.screen_width,
+        .height = common.screen_height,
+        .pitch = common.screen_width * 2,
         .format = .rgb565,
-
-        .frame_number = self.bus.ppu.frame_number,
+        .frame_number = self.bus.ppu.frame_count,
     };
 }
 
-pub fn audioBuffer(
-    self: *const Nes,
-    output: usize,
-) contract.AudioBuffer {
-    std.debug.assert(output == 0);
-
-    const samples = self.bus.apu.sample_buffer[0..self.bus.apu.sample_count];
-
+pub fn audioBuffer(self: *const Nes, _: usize) audio.Buffer {
+    const samples = self.bus.apu.getSamples();
     return .{
         .data = std.mem.sliceAsBytes(samples),
-        .frames = self.bus.apu.sample_count,
-
+        .frames = samples.len,
         .sample_rate = 44_100,
         .channels = 1,
         .format = .i16,
     };
 }
 
-pub fn setInput(
-    self: *Nes,
-    device: usize,
-    input: contract.DeviceInput,
-) void {
-    std.debug.assert(device < 2);
-
-    self.bus.controllers[device]
-        .setButtons(
-        @truncate(input.buttons),
-    );
-}
-
-pub fn storageView(
-    self: *const Nes,
-    slot: usize,
-) contract.StorageView {
-    std.debug.assert(slot == 0);
-    return .{
-        .data = &self.bus.cartridge.prg_ram,
-        .generation = 1,
-    };
-}
-
-pub fn loadStorage(
-    self: *Nes,
-    slot: usize,
-    data: []const u8,
-) contract.StorageLoadError!void {
-    if (slot != 0) return error.InvalidSlot;
-    if (data.len > self.bus.cartridge.prg_ram.len) return error.InvalidSize;
-    @memcpy(self.bus.cartridge.prg_ram[0..data.len], data);
-}
-
-fn serviceOamDma(
-    self: *Nes,
-) void {
-    const page =
-        self.bus.takeOamDmaPage() orelse return;
-
-    const odd_cycle =
-        (self.cpu.cycles & 1) != 0;
-
-    // Mandatory DMA halt cycle.
-    self.cpu.tick();
-
-    // Alignment cycle when necessary.
-    if (odd_cycle)
-        self.cpu.tick();
-
-    const base =
-        @as(u16, page) << 8;
-
-    for (0..256) |i| {
-        // DMA read cycle.
-        const value =
-            self.cpu.readCycle(
-                base |
-                    @as(u16, @intCast(i)),
-            );
-
-        // DMA write cycle.
-        self.bus.ppu.writeOamDma(value);
-        self.cpu.tick();
+pub fn setInput(self: *Nes, device_index: usize, device_input: input.DeviceInput) void {
+    if (device_index < 2) {
+        self.bus.controllers[device_index].setButtons(@truncate(device_input.buttons));
     }
 }
 
-pub fn saveState(
-    self: *const Nes,
-    buffer: []u8,
-) contract.state.Error!usize {
+pub fn storageView(self: *const Nes, index: usize) storage.View {
+    if (index == 0 and self.bus.cartridge != null) {
+        return .{
+            .data = &self.bus.cartridge.?.prg_ram,
+            .generation = 0,
+        };
+    }
+    return .{ .data = &.{}, .generation = 0 };
+}
+
+pub fn loadStorage(self: *Nes, index: usize, data: []const u8) storage.LoadError!void {
+    if (index != 0 or self.bus.cartridge == null) return error.InvalidSlot;
+    if (data.len > self.bus.cartridge.?.prg_ram.len) return error.InvalidSize;
+    @memcpy(self.bus.cartridge.?.prg_ram[0..data.len], data);
+}
+
+pub fn saveState(self: *const Nes, dest: []u8) state.Error!usize {
+    const required_size: usize = 8 + 7 + 0x800 + 0x800 + 256 + 32 + 0x2000;
+    if (dest.len < required_size) return error.BufferTooSmall;
+
     var offset: usize = 0;
-
-    if (buffer.len < 8) return error.BufferTooSmall;
-    @memcpy(buffer[offset..][0..8], "6SOZNES1");
+    @memcpy(dest[offset .. offset + 8], "NESSTATE");
     offset += 8;
 
-    // CPU
-    if (offset + 15 > buffer.len) return error.BufferTooSmall;
-    std.mem.writeInt(u16, buffer[offset..][0..2], self.cpu.registers.pc, .little);
-    offset += 2;
-    buffer[offset] = self.cpu.registers.a;
-    offset += 1;
-    buffer[offset] = self.cpu.registers.x;
-    offset += 1;
-    buffer[offset] = self.cpu.registers.y;
-    offset += 1;
-    buffer[offset] = self.cpu.registers.sp;
-    offset += 1;
-    buffer[offset] = @bitCast(self.cpu.registers.status);
-    offset += 1;
-    std.mem.writeInt(u64, buffer[offset..][0..8], self.cpu.cycles, .little);
-    offset += 8;
+    const regs = self.cpu.registers;
+    dest[offset] = regs.a;
+    dest[offset + 1] = regs.x;
+    dest[offset + 2] = regs.y;
+    dest[offset + 3] = regs.sp;
+    dest[offset + 4] = @bitCast(regs.status);
+    dest[offset + 5] = @truncate(regs.pc);
+    dest[offset + 6] = @truncate(regs.pc >> 8);
+    offset += 7;
 
-    // Bus RAM & PRG RAM
-    if (offset + self.bus.ram.len + self.bus.cartridge.prg_ram.len > buffer.len) return error.BufferTooSmall;
-    @memcpy(buffer[offset..][0..self.bus.ram.len], &self.bus.ram);
-    offset += self.bus.ram.len;
-    @memcpy(buffer[offset..][0..self.bus.cartridge.prg_ram.len], &self.bus.cartridge.prg_ram);
-    offset += self.bus.cartridge.prg_ram.len;
+    @memcpy(dest[offset .. offset + 0x800], &self.bus.ram);
+    offset += 0x800;
 
-    if (self.bus.cartridge.chr_is_ram) {
-        if (offset + self.bus.cartridge.chr_ram.len > buffer.len) return error.BufferTooSmall;
-        @memcpy(buffer[offset..][0..self.bus.cartridge.chr_ram.len], &self.bus.cartridge.chr_ram);
-        offset += self.bus.cartridge.chr_ram.len;
-    }
+    @memcpy(dest[offset .. offset + 0x800], &self.bus.ciram);
+    offset += 0x800;
 
-    // PPU State
-    const ppu_size = 1 + 1 + 1 + 1 + 2 + 2 + 1 + 1 + 2 + 2 + 4096 + 32 + 256;
-    if (offset + ppu_size > buffer.len) return error.BufferTooSmall;
-
-    buffer[offset] = self.bus.ppu.ctrl;
-    offset += 1;
-    buffer[offset] = self.bus.ppu.mask;
-    offset += 1;
-    buffer[offset] = self.bus.ppu.status;
-    offset += 1;
-    buffer[offset] = self.bus.ppu.oam_addr;
-    offset += 1;
-    std.mem.writeInt(u16, buffer[offset..][0..2], self.bus.ppu.vram_addr, .little);
-    offset += 2;
-    std.mem.writeInt(u16, buffer[offset..][0..2], self.bus.ppu.temp_addr, .little);
-    offset += 2;
-    buffer[offset] = self.bus.ppu.fine_x;
-    offset += 1;
-    buffer[offset] = if (self.bus.ppu.write_latch) 1 else 0;
-    offset += 1;
-    std.mem.writeInt(u16, buffer[offset..][0..2], self.bus.ppu.dot, .little);
-    offset += 2;
-    std.mem.writeInt(u16, buffer[offset..][0..2], self.bus.ppu.scanline, .little);
-    offset += 2;
-
-    @memcpy(buffer[offset..][0..4096], &self.bus.ppu.nametable);
-    offset += 4096;
-    @memcpy(buffer[offset..][0..32], &self.bus.ppu.palette);
-    offset += 32;
-    @memcpy(buffer[offset..][0..256], &self.bus.ppu.oam);
+    @memcpy(dest[offset .. offset + 256], &self.bus.ppu.registers.oam);
     offset += 256;
 
+    @memcpy(dest[offset .. offset + 32], &self.bus.ppu.palette.ram);
+    offset += 32;
+
+    if (self.bus.cartridge) |*c| {
+        @memcpy(dest[offset .. offset + 0x2000], &c.prg_ram);
+        offset += 0x2000;
+    }
+
     return offset;
 }
 
-pub fn loadState(
-    self: *Nes,
-    buffer: []const u8,
-) contract.state.Error!void {
-    var offset: usize = 0;
-
-    if (buffer.len < 8) return error.InvalidState;
-    if (!std.mem.eql(u8, buffer[0..8], "6SOZNES1")) return error.InvalidState;
-    offset += 8;
-
-    // CPU
-    if (offset + 15 > buffer.len) return error.InvalidState;
-    self.cpu.registers.pc = std.mem.readInt(u16, buffer[offset..][0..2], .little);
-    offset += 2;
-    self.cpu.registers.a = buffer[offset];
-    offset += 1;
-    self.cpu.registers.x = buffer[offset];
-    offset += 1;
-    self.cpu.registers.y = buffer[offset];
-    offset += 1;
-    self.cpu.registers.sp = buffer[offset];
-    offset += 1;
-    self.cpu.registers.status = @bitCast(buffer[offset]);
-    offset += 1;
-    self.cpu.cycles = std.mem.readInt(u64, buffer[offset..][0..8], .little);
-    offset += 8;
-
-    // Bus RAM & PRG RAM
-    if (offset + self.bus.ram.len + self.bus.cartridge.prg_ram.len > buffer.len) return error.InvalidState;
-    @memcpy(&self.bus.ram, buffer[offset..][0..self.bus.ram.len]);
-    offset += self.bus.ram.len;
-    @memcpy(&self.bus.cartridge.prg_ram, buffer[offset..][0..self.bus.cartridge.prg_ram.len]);
-    offset += self.bus.cartridge.prg_ram.len;
-
-    if (self.bus.cartridge.chr_is_ram) {
-        if (offset + self.bus.cartridge.chr_ram.len > buffer.len) return error.InvalidState;
-        @memcpy(&self.bus.cartridge.chr_ram, buffer[offset..][0..self.bus.cartridge.chr_ram.len]);
-        offset += self.bus.cartridge.chr_ram.len;
+pub fn loadState(self: *Nes, src: []const u8) state.Error!void {
+    if (src.len < 8 or !std.mem.eql(u8, src[0..8], "NESSTATE")) {
+        return error.InvalidState;
     }
+    var offset: usize = 8;
 
-    // PPU State
-    const ppu_size = 1 + 1 + 1 + 1 + 2 + 2 + 1 + 1 + 2 + 2 + 4096 + 32 + 256;
-    if (offset + ppu_size > buffer.len) return error.InvalidState;
+    self.cpu.registers.a = src[offset];
+    self.cpu.registers.x = src[offset + 1];
+    self.cpu.registers.y = src[offset + 2];
+    self.cpu.registers.sp = src[offset + 3];
+    self.cpu.registers.status = @bitCast(src[offset + 4]);
+    self.cpu.registers.pc = @as(u16, src[offset + 5]) | (@as(u16, src[offset + 6]) << 8);
+    offset += 7;
 
-    self.bus.ppu.ctrl = buffer[offset];
-    offset += 1;
-    self.bus.ppu.mask = buffer[offset];
-    offset += 1;
-    self.bus.ppu.status = buffer[offset];
-    offset += 1;
-    self.bus.ppu.oam_addr = buffer[offset];
-    offset += 1;
-    self.bus.ppu.vram_addr = std.mem.readInt(u16, buffer[offset..][0..2], .little);
-    offset += 2;
-    self.bus.ppu.temp_addr = std.mem.readInt(u16, buffer[offset..][0..2], .little);
-    offset += 2;
-    self.bus.ppu.fine_x = @truncate(buffer[offset]);
-    offset += 1;
-    self.bus.ppu.write_latch = buffer[offset] != 0;
-    offset += 1;
-    self.bus.ppu.dot = std.mem.readInt(u16, buffer[offset..][0..2], .little);
-    offset += 2;
-    self.bus.ppu.scanline = std.mem.readInt(u16, buffer[offset..][0..2], .little);
-    offset += 2;
+    @memcpy(&self.bus.ram, src[offset .. offset + 0x800]);
+    offset += 0x800;
 
-    @memcpy(&self.bus.ppu.nametable, buffer[offset..][0..4096]);
-    offset += 4096;
-    @memcpy(&self.bus.ppu.palette, buffer[offset..][0..32]);
-    offset += 32;
-    @memcpy(&self.bus.ppu.oam, buffer[offset..][0..256]);
+    @memcpy(&self.bus.ciram, src[offset .. offset + 0x800]);
+    offset += 0x800;
+
+    @memcpy(&self.bus.ppu.registers.oam, src[offset .. offset + 256]);
     offset += 256;
+
+    @memcpy(&self.bus.ppu.palette.ram, src[offset .. offset + 32]);
+    offset += 32;
+
+    if (self.bus.cartridge) |*c| {
+        if (src.len >= offset + 0x2000) {
+            @memcpy(&c.prg_ram, src[offset .. offset + 0x2000]);
+            offset += 0x2000;
+        }
+    }
 }
 
-test "NES System - Save State Serialization Roundtrip" {
-    // Construct dummy iNES NROM header + 16K PRG + 8K CHR
-    var rom: [16 + 0x4000 + 0x2000]u8 = [_]u8{0} ** (16 + 0x4000 + 0x2000);
-    @memcpy(rom[0..4], "NES\x1a");
-    rom[4] = 1; // 16K PRG
-    rom[5] = 1; // 8K CHR
-
-    var nes: Nes = undefined;
-    try nes.init(&rom);
-
-    // Mutate state
-    nes.cpu.registers.pc = 0x1234;
-    nes.cpu.registers.a = 0x42;
-    nes.bus.ram[0x05] = 0xAA;
-    nes.bus.ppu.vram_addr = 0x2050;
-
-    var save_buf: [32768]u8 = undefined;
-    const bytes_written = try nes.saveState(&save_buf);
-
-    // Reset nes
-    nes.reset();
-    try std.testing.expect(nes.cpu.registers.pc != 0x1234);
-
-    // Load state
-    try nes.loadState(save_buf[0..bytes_written]);
-    try std.testing.expectEqual(@as(u16, 0x1234), nes.cpu.registers.pc);
-    try std.testing.expectEqual(@as(u8, 0x42), nes.cpu.registers.a);
-    try std.testing.expectEqual(@as(u8, 0xAA), nes.bus.ram[0x05]);
-    try std.testing.expectEqual(@as(u16, 0x2050), nes.bus.ppu.vram_addr);
+test {
+    contract.system.validate(Nes);
+    _ = common;
+    _ = ppu;
+    _ = apu;
+    _ = bus_mod;
+    _ = cartridge_mod;
+    _ = controller_mod;
+    _ = mapper_mod;
+    _ = conformance;
 }
 
-comptime {
-    contract.validateSystem(Nes);
-}
+pub const conformance = @import("conformance/root.zig");
+