add nes cartridge subsystem and 41 ines mapper variants

src/system/nes/mapper/nrom.zig

@@ -0,0 +1,71 @@
+const types = @import("types.zig");
+const Mirroring = types.Mirroring;
+
+const Nrom = @This();
+
+prg_16k: bool,
+mirroring_mode: Mirroring,
+
+pub fn init(
+    prg_size: usize,
+    _mirroring: Mirroring,
+) Nrom {
+    return .{
+        .prg_16k = prg_size == 0x4000,
+        .mirroring_mode = _mirroring,
+    };
+}
+
+pub fn cpuMapRead(
+    self: *const Nrom,
+    address: u16,
+) ?usize {
+    if (address < 0x8000)
+        return null;
+
+    const offset =
+        @as(usize, address) - 0x8000;
+
+    return if (self.prg_16k)
+        offset & 0x3fff
+    else
+        offset;
+}
+
+pub fn cpuWrite(
+    _: *Nrom,
+    _: u16,
+    _: u8,
+) void {
+    // NROM has no mapper registers.
+}
+
+pub fn ppuMapRead(
+    _: *const Nrom,
+    address: u16,
+) ?usize {
+    if (address >= 0x2000)
+        return null;
+
+    return address;
+}
+
+pub fn ppuMapWrite(
+    _: *const Nrom,
+    address: u16,
+) ?usize {
+    if (address >= 0x2000)
+        return null;
+
+    return address;
+}
+
+pub fn mirroring(
+    self: *const Nrom,
+) Mirroring {
+    return self.mirroring_mode;
+}
+
+pub fn irqAsserted(_: *const Nrom) bool {
+    return false;
+}