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

This area is 48 bytes in size. The data name types are explained in detail below.

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

Two alphanumeric ASCII bytes identifying your company. Ignored by emulators; for ROM hackers and homebrewers, just insert whatever.

Game Code

Four alphanumeric ASCII bytes identifying your game. Ignored by emulators; for ROM hackers and homebrewers, just insert whatever.

Exception: If the game code starts with Z and ends with J, it's a BS-X flash cartridge.

Fixed Value

The fixed value bytes at $00FFB6-$00FFBC should be #$00. The fixed value byte at $00FFDA should be #$33.

Expansion RAM Size

Should be #$00 for most roms. For the exception, see RAM Size.

See: page 1-2-16 of Book I

Special Version

Should be #$00 normally. Can be set for promotional events.

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).

Cartridge Configuration

Map Mode

Common values: #$20 - 2.68MHz LoROM, #$21 - 2.68MHz HiROM, #$23 - SA-1; #$25 - 2.68MHz ExHiROM; #$30 - 3.58MHz LoROM, #$31 - 3.58MHz HiROM; #$35 - 3.58MHz ExHiROM

Cartridge Type

Common values: #$00 - ROM only; #$01 - ROM and RAM; #$02 - ROM, RAM and battery; #$33 - ROM and SA-1; #$34 - ROM, SA-1 and RAM; #$35 - ROM, SA-1, RAM and battery

Sub-Number

ROM Size

2^(this value) would be the size of the ROM in kilobytes. For example, for 512KB, this should be #$09.

RAM Size

2^(this value) would be the size of the SRAM (if present) in kilobytes. Maximum supported value is #$07.

Exception: If you're using Super FX aka GSU-1, move this value to the Expansion RAM Size field, and put #$00 in this byte.

Destination Code

Where the game is intended to be sold. Common values: #$00 - Japan; #$01 - USA; #$02 - Europe (enables 50fps PAL mode)

See https://archive.org/details/SNESDevManual/book1/page/n37

Mask ROM Version

Should be #$00, or increased every time you release a new 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.

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.

Certain enhancement chips like SA-1 and Super FX can override the CPU vectors on special occasions.

Address	Length	Mode	Vector
$00:FFE0	4 bytes	Native	unused
$00:FFE4	2 bytes	Native	COP
$00:FFE6	2 bytes	Native	BRK
$00:FFE8	2 bytes	Native	ABORT
$00:FFEA	2 bytes	Native	NMI
$00:FFEC	2 bytes	Native	unused (would be RESET, but the SNES always boots in Emulation mode)
$00:FFEE	2 bytes	Native	IRQ
$00:FFF0	4 bytes	Emulation	unused
$00:FFF4	2 bytes	Emulation	COP
$00:FFF6	2 bytes	Emulation	unused (would be BRK, but BRK and IRQ share the same vector in Emulation mode)
$00:FFF8	2 bytes	Emulation	ABORT
$00:FFFA	2 bytes	Emulation	NMI
$00:FFFC	2 bytes	Emulation	RESET
$00:FFFE	2 bytes	Emulation	IRQ/BRK

References

page 1-2-14, "Addendum 1" of official Super Nintendo development manual: page 1-2-14 of Book I