add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/axrom.zig

@@ -0,0 +1,64 @@
+const types = @import("types.zig");
+const Mirroring = types.Mirroring;
+
+const Axrom = @This();
+
+prg_bank_select: u8 = 0,
+mirroring_select: u1 = 0,
+prg_bank_count: usize,
+
+pub fn init(
+    prg_size: usize,
+) Axrom {
+    return .{
+        .prg_bank_count = prg_size / 0x8000,
+    };
+}
+
+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 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 {
+    return false;
+}