add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/mmc4.zig

@@ -0,0 +1,117 @@
+const types = @import("types.zig");
+const Mirroring = types.Mirroring;
+
+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 },
+
+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 cpuMapRead(
+    self: *const Mmc4,
+    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: *Mmc4,
+    address: u16,
+    value: u8,
+) void {
+    if (address < 0xa000) return;
+
+    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,
+        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 {
+    return self.mirroring_mode;
+}
+
+pub fn irqAsserted(
+    _: *const Mmc4,
+) bool {
+    return false;
+}