add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/irem78.zig

@@ -0,0 +1,78 @@
+const types = @import("types.zig");
+const Mirroring = types.Mirroring;
+
+const Irem78 = @This();
+
+prg_bank: u8 = 0,
+chr_bank: u8 = 0,
+mirroring_select: u1 = 0,
+
+prg_bank_count: usize,
+chr_bank_count: usize,
+
+pub fn init(
+    prg_size: usize,
+    chr_size: usize,
+) Irem78 {
+    return .{
+        .prg_bank_count = prg_size / 0x4000,
+        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
+    };
+}
+
+pub fn cpuMapRead(
+    self: *const Irem78,
+    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: *Irem78,
+    address: u16,
+    value: u8,
+) void {
+    if (address < 0x8000) return;
+
+    self.prg_bank = value & 0x07;
+    self.mirroring_select = @truncate((value >> 3) & 1);
+    self.chr_bank = (value >> 4) & 0x0F;
+}
+
+pub fn ppuMapRead(
+    self: *const Irem78,
+    address: u16,
+) ?usize {
+    if (address >= 0x2000) return null;
+
+    const bank = @as(usize, self.chr_bank) % self.chr_bank_count;
+    return bank * 0x2000 + @as(usize, address);
+}
+
+pub fn ppuMapWrite(
+    _: *const Irem78,
+    address: u16,
+) ?usize {
+    if (address >= 0x2000) return null;
+    return address;
+}
+
+pub fn mirroring(
+    self: *const Irem78,
+) Mirroring {
+    return if (self.mirroring_select == 0) .single_screen_lower else .single_screen_upper;
+}
+
+pub fn irqAsserted(_: *const Irem78) bool {
+    return false;
+}