add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/mapper13.zig

@@ -0,0 +1,69 @@
+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;
+}