All SNES gamepaks have an internal header that is used to identifying the game, producer, region and technical aspects of the ROM. It's often referred as Internal ROM Header or SNES Software Specification.
Although it's not required to run a game on real hardware, the SNES ROM Header was used during the Nintendo approval process for validation and it's also used by the SNES emulators to identify and determine the memory layout and ROM type.
The data starts at SNES address $00:FFB0 and ends at $00:FFDF. $00:FFE0 though $00:FFFF contains the SNES vector information and it's actually used by the SNES CPU to determine where to execute when an interrupt occurs.
- 1 ROM Registration Data
- 2 Data Name Types
- 2.1 Maker Code
- 2.2 Game Code
- 2.3 Fixed Value
- 2.4 Expansion RAM Size
- 2.5 Special Version
- 2.6 Game Title Specification
- 2.7 Cartridge Configuration
- 2.8 Destination Code
- 2.9 Mask ROM Version
- 2.10 ROM Verification
- 3 CPU Exception Vectors
ROM Registration Data
|Address||Length||Data Name Type|
|$00:FFB0||2 bytes||Maker Code|
|$00:FFB2||4 bytes||Game Code|
|$00:FFB6||7 bytes||Fixed Value|
|$00:FFBD||1 byte||Expansion RAM Size|
|$00:FFBE||1 byte||Special Version|
|$00:FFBF||1 byte||Cartridge Type (Sub-number)|
|$00:FFC0||21 bytes||Game Title Registration|
|$00:FFD5||1 byte||Map Mode|
|$00:FFD6||1 byte||Cartridge Type|
|$00:FFD7||1 byte||ROM Size|
|$00:FFD8||1 byte||RAM Size|
|$00:FFD9||1 byte||Destination Code|
|$00:FFDA||1 byte||Fixed Value|
|$00:FFDB||1 byte||Mask ROM Version|
|$00:FFDC||2 bytes||Complement Check|
|$00:FFDE||2 bytes||Check Sum|
Data Name Types
Expansion RAM Size
Game Title Specification
The game title is 21 bytes long, encoded with the JIS X 0201 character set (which consists of standard ASCII plus katakana). If the title is shorter than 21 bytes, then the remainder should be padded with spaces (0x20).
2^(this value) would be the size of the ROM in kilobytes.
2^(this value) would be the size of the SRAM (if present) in kilobytes. Maximum supported value is $07.
Mask ROM Version
This is the 16-bit complement (bit-inverse) of the checksum. This is used so that the checksum value cancels itself out when calculating the real checksum.
This is simply the 16-bit sum of all bytes in the ROM. For power-of-2-sized ROMs, no mirroring is used, each byte of ROM is counted exactly once.
For non-power-of-2-sized ROMs (e.g. 2.5MB or 6MB), first the checksum for the largest power-of-2 area smaller than the ROM size (so 4MB for 6MB ROMs, 2MB for 2.5MB ROMs) is computed normally. Then the remaining part is repeated until it's the same size as the first part (so the last 2MB of a 6MB ROM is repeated once so both halves are 4MB, and the last 512KB of a 2.5MB ROM is repeated 4 times so both halves are 2MB). Then its checksum is computed and the checksums of the 2 halves are added to get the final checksum.
CPU Exception Vectors
The vectors from $00:FFE0-$00:FFEF are used when the SNES is running in native mode, while the vectors from $00:FFF0-$00:FFFF are used when the SNES is running in emulation mode. Note that ABORT is technically unused, but listed for completion.
Also note that the vectors are only 16 bits wide, so they can only jump to addresses within bank $00. If you want to jump to a different bank, or you need FastROM addressing, point them to a JML instruction. With WRAM being mapped at $0000-$1FFF on bank $00, it's possible to point them on that range, being an interesting option for run-time code modification.
|$00:FFEC||2 bytes||Native||unused (would be RESET, but the SNES always boots in Emulation mode)|
|$00:FFF6||2 bytes||Emulation||unused (would be BRK, but BRK and IRQ share the same vector in Emulation mode)|