Nintendo required SNES developers to include a header in the game's data that describes what hardware the game cartridge contains. This is not needed by the SNES hardware, though the game software can access it as ROM data.
However, emulators and flashcarts rely on this header to know how to emulate the game cartridge. Homebrew games should also provide a valid header. Example code: lorom-template
The header is located at the CPU address range $00FFC0-$00FFDF, right before the interrupt vectors, with an optional second header at $00FFB0-$00FFBF. This means that the location of the header within the actual ROM file will change based on the cartridge's memory map mode - with LoROM games placing it at $007Fxx, HiROM games placing it at $00FFxx, and ExHiROM games placing it at $40FFxx. Therefore, if it's correctly filled out, an emulator will have a higher chance of being able to figure out where the header is. See: Header Verification below.
This internal ROM header is to be confused with the additional 512-byte headers used by copier devices. See: ROM file formats
See also: Memory map
|$FFC0||21||Cartridge title (21 bytes uppercase ASCII. Unused bytes should be spaces.)|
|$FFD5||1||ROM speed and memory map mode (LoROM/HiROM/ExHiROM)|
|$FFD6||1||Chipset (Indicates if a cartridge contains extra RAM, a battery, and/or a coprocessor)|
|$FFD7||1||ROM size: 1<<N kilobytes, rounded up (so 8=256KB, 12=4096KB and so on)|
|$FFD8||1||RAM size: 1<<N kilobytes (so 1=2KB, 5=32KB, and so on)|
|$FFD9||1||Country (Implies NTSC/PAL)|
|$FFDB||1||ROM version (0 = first)|
|$FFDE||2||Checksum compliment (Checksum ^ $FFFF)|
Address $00FFD5 indicates the ROM speed and map mode.
001smmmm |++++- Map mode +----- Speed: 0=Slow, 1=Fast
Available modes include:
Address $00FFD6 indicates what extra hardware is in the cartridge, if any.
Possible values include:
- $00 - ROM only
- $01 - ROM + RAM
- $02 - ROM + RAM + battery
- $x3 - ROM + coprocessor
- $x4 - ROM + coprocessor + RAM
- $x5 - ROM + coprocessor + RAM + battery
- $x6 - ROM + coprocessor + battery
- $0x - Coprocessor is DSP (DSP-1, 2, 3 or 4)
- $1x - Coprocessor is GSU (SuperFX)
- $2x - Coprocessor is OBC1
- $3x - Coprocessor is SA-1
- $4x - Coprocessor is S-DD1
- $5x - Coprocessor is S-RTC
- $Ex - Coprocessor is Other (Super Game Boy/Satellaview)
- $Fx - Coprocessor is Custom (specified with $FFBF)
When coprocessor is Custom, $FFBF selects from:
Expanded cartridge header
The expanded header's presence is indicate by putting $33 in $00FFDA, which is the developer ID. Some early games may indicate just $00FFBF by setting $00FFD4 to zero.
|FFB0||2||ASCII maker code|
|FFB2||4||ASCII game code|
|FFB6||6||Reserved, should be zero|
|FFBC||1||Expansion flash size: 1 << N kilobytes|
|FFBD||1||Expansion RAM size: 1 << N kilobytes - for GSU?|
|FFBE||1||Special version (usually zero)|
|FFBF||1||Chipset subtype, used if chipset is $F0-$FF|
The checksum is computed as if the ROM is a power of two in size, but some SNES games use multiple ROM chips together. Because of this, the data size is often the sum of 2 powers of two. (E.g. a 3MB game might use a 2MB ROM and a 1MB ROM together.)
Before calculating the checksum, if the data is not already a power of 2 in size we must duplicate some of the data to fill up to a power of 2 in a way that matches how it will be mirrored in the memory map.
- Find the largest power of 2 less than or equal to the data size.
- If data remains past this point:
- Find the smallest power of 2 greater than or equal to the remainder.
- Pad with 0s to meet this power of 2.*
- Now that the remainder is a power of 2, duplicate it to fill 2x the larger power of 2 from the first part of the ROM.
* This is outside the specification, and emulators are inconsistent about how to pad. It is recommended to ensure your ROM file's remainder reaches a power of 2 boundary.
Because the ROM header will be part of the computed checksum, before computing the checksum we must normally fill the header's checksum and complement values with $0000 and $FFFF. These two values will cancel each other out in the checksum process (as would any valid checksum+complement pair).
- Start with a 16-bit checksum = 0.
- Add every pair of bytes from the prepared data to the checksum, as little-endian 16-bit. (Overflow is discarded.)
- Store the checksum in the ROM header ($FFDC or equivalent).
- Store checksum ^ $FFFF in the ROM header ($FFDE).
The primary way to verify a candidate header is to evaluate the checksum it contains. Some flash-carts appear to use only the checksum to distinguish LoROM from HiROM.
- ROM checksum matches.
- Checksum and compliment sum to $FFFF.
- Map mode matches header location.
- Specified ROM size is not smaller than file size.
- A reset vector < $8000 is invalid because it points outside of ROM.
- The first instruction at a valid reset vector is likely to be: sei, clc, sec, stz, jmp, jml
- The first instruction at a valid reset vector is unlikely to be: brk, cop, stp, wdm, $FF (sbc long)
- ROM and RAM sizes are reasonable.
- Game name field is ASCII characters only.