add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/mapper93.zig

@@ -0,0 +1,73 @@
+const types = @import("types.zig");
+const Mirroring = types.Mirroring;
+
+const Mapper93 = @This();
+
+prg_bank: u8 = 0,
+mirroring_mode: Mirroring,
+
+prg_bank_count: usize,
+
+pub fn init(
+    prg_size: usize,
+    _mirroring: Mirroring,
+) Mapper93 {
+    return .{
+        .prg_bank_count = prg_size / 0x4000,
+        .mirroring_mode = _mirroring,
+    };
+}
+
+pub fn cpuMapRead(
+    self: *const Mapper93,
+    address: u16,
+) ?usize {
+    if (address < 0x8000) return null;
+
+    if (address < 0xC000) {
+        const offset = @as(usize, address - 0x8000);
+        const bank = @as(usize, self.prg_bank) % self.prg_bank_count;
+        return bank * 0x4000 + offset;
+    } else {
+        const offset = @as(usize, address - 0xC000);
+        const fixed_bank = if (self.prg_bank_count > 0) self.prg_bank_count - 1 else 0;
+        return fixed_bank * 0x4000 + offset;
+    }
+}
+
+pub fn cpuWrite(
+    self: *Mapper93,
+    address: u16,
+    value: u8,
+) void {
+    if (address >= 0x8000) {
+        self.prg_bank = (value >> 4) & 0x07;
+        self.mirroring_mode = if ((value & 1) != 0) .horizontal else .vertical;
+    }
+}
+
+pub fn ppuMapRead(
+    _: *const Mapper93,
+    address: u16,
+) ?usize {
+    if (address >= 0x2000) return null;
+    return address;
+}
+
+pub fn ppuMapWrite(
+    _: *const Mapper93,
+    address: u16,
+) ?usize {
+    if (address >= 0x2000) return null;
+    return address;
+}
+
+pub fn mirroring(
+    self: *const Mapper93,
+) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqAsserted(_: *const Mapper93) bool {
+    return false;
+}