add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/mapper76.zig

@@ -0,0 +1,91 @@
+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;
+}