add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/mapper87.zig

@@ -0,0 +1,70 @@
+const types = @import("types.zig");
+const Mirroring = types.Mirroring;
+
+const Mapper87 = @This();
+
+chr_bank: u8 = 0,
+prg_bank_count: usize,
+chr_bank_count: usize,
+mirroring_mode: Mirroring,
+
+pub fn init(
+    prg_size: usize,
+    chr_size: usize,
+    _mirroring: Mirroring,
+) Mapper87 {
+    return .{
+        .prg_bank_count = prg_size / 0x8000,
+        .chr_bank_count = if (chr_size > 0) chr_size / 0x2000 else 1,
+        .mirroring_mode = _mirroring,
+    };
+}
+
+pub fn cpuMapRead(
+    self: *const Mapper87,
+    address: u16,
+) ?usize {
+    if (address < 0x8000) return null;
+
+    const offset = @as(usize, address - 0x8000);
+    const bank = if (self.prg_bank_count > 0) (self.prg_bank_count - 1) else 0;
+    return bank * 0x8000 + offset;
+}
+
+pub fn cpuWrite(
+    self: *Mapper87,
+    address: u16,
+    value: u8,
+) void {
+    if (address >= 0x6000 and address <= 0x7FFF) {
+        self.chr_bank = ((value & 1) << 1) | ((value >> 1) & 1);
+    }
+}
+
+pub fn ppuMapRead(
+    self: *const Mapper87,
+    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 Mapper87,
+    address: u16,
+) ?usize {
+    if (address >= 0x2000) return null;
+    return address;
+}
+
+pub fn mirroring(
+    self: *const Mapper87,
+) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqAsserted(_: *const Mapper87) bool {
+    return false;
+}