add NES 2.0 cartridge header parsing support

src/system/nes/cartridge.zig

@@ -53,19 +53,33 @@ pub fn init(
     const flags6 = rom[6];
     const flags7 = rom[7];
 
-    // NES 2.0 signature.
-    if ((flags7 & 0x0c) == 0x08)
-        return error.UnsupportedNes2;
+    const is_nes2 = (flags7 & 0x0c) == 0x08;
 
-    const mapper_id: u8 =
-        (flags6 >> 4) |
-        (flags7 & 0xf0);
+    var mapper_id: u16 = (flags6 >> 4) | (flags7 & 0xf0);
+    var prg_size: usize = @as(usize, prg_banks) * 0x4000;
+    var chr_size: usize = @as(usize, chr_banks) * 0x2000;
 
-    const prg_size =
-        @as(usize, prg_banks) * 0x4000;
+    if (is_nes2) {
+        mapper_id |= @as(u16, rom[8] & 0x0f) << 8;
 
-    const chr_size =
-        @as(usize, chr_banks) * 0x2000;
+        const prg_msb = rom[9] & 0x0f;
+        if (prg_msb < 0x0f) {
+            prg_size = ((@as(usize, prg_msb) << 8) | prg_banks) * 0x4000;
+        } else {
+            const exponent = (prg_banks >> 2) & 0x3f;
+            const multiplier = prg_banks & 0x03;
+            prg_size = (@as(usize, 1) << @intCast(exponent)) * (multiplier * 2 + 1);
+        }
+
+        const chr_msb = (rom[9] >> 4) & 0x0f;
+        if (chr_msb < 0x0f) {
+            chr_size = ((@as(usize, chr_msb) << 8) | chr_banks) * 0x2000;
+        } else {
+            const exponent = (chr_banks >> 2) & 0x3f;
+            const multiplier = chr_banks & 0x03;
+            chr_size = (@as(usize, 1) << @intCast(exponent)) * (multiplier * 2 + 1);
+        }
+    }
 
     switch (mapper_id) {
         0, 3 => {
@@ -177,21 +191,21 @@ pub fn init(
             .vrc4 = _mapper.Vrc4.init(
                 prg_size,
                 chr_size,
-                mapper_id,
+                @truncate(mapper_id),
             ),
         },
         22, 23 => .{
             .vrc2 = _mapper.Vrc2.init(
                 prg_size,
                 chr_size,
-                mapper_id,
+                @truncate(mapper_id),
             ),
         },
         24, 26 => .{
             .vrc6 = _mapper.Vrc6.init(
                 prg_size,
                 chr_size,
-                mapper_id,
+                @truncate(mapper_id),
             ),
         },
         34 => .{
@@ -1103,3 +1117,15 @@ test "NES Cartridge - Nintendo MMC5 Mapper 5 PRG, CHR Banking & Multiplier" {
     try std.testing.expectEqual(@as(?u8, 0xd2), mmc5.readRegister(0x5205));
     try std.testing.expectEqual(@as(?u8, 0x00), mmc5.readRegister(0x5206));
 }
+
+test "NES Cartridge - NES 2.0 Header Parsing" {
+    // Header for 128KB PRG, 128KB CHR, Mapper 1 (MMC1), NES 2.0 signature (flags7 0x08)
+    const nes2_header = [_]u8{
+        'N', 'E', 'S', 0x1A, 0x08, 0x10, 0x13, 0x08,
+        0x00, 0x00, 0x70, 0x00, 0x00, 0x00, 0x00, 0x00,
+    } ++ ([_]u8{0} ** (128 * 1024 + 128 * 1024));
+
+    const cart = try Cartridge.init(&nes2_header);
+    try std.testing.expectEqual(@as(usize, 128 * 1024), cart.prg_rom.len);
+    try std.testing.expectEqual(@as(usize, 128 * 1024), cart.chr_rom.len);
+}