SNES ROM Header
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.
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
Maker Code
Game Code
Fixed Value
Expansion RAM Size
Special Version
Game Title Specification
Cartridge Configuration
Map Mode
Cartridge Type
Sub-Number
ROM Size
RAM Size
Destination Code
Mask ROM Version
ROM Verification
Complement Check
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.
Check Sum
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.