add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/mapper65.zig

@@ -0,0 +1,118 @@
+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;
+}