ROM header

Nintendo required SNES developers to include a header in the game's data that describes what hardware the game cartridge contains. Emulators and flashcarts rely on this header to know how to emulate the game. Therefore, homebrew games also need to provide a header. lorom-template contains an example of how to set one up.

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.

Things that increase an emulator's confidence include the checksum and checksum compliment adding up to $FFFF, the memory map value being correct for the header location, and the ROM and RAM size being reasonable values. If the first instruction in the reset routine is  then that increases confidence even further. At least one flash cart actually checks to see if the checksum is correct, so it's recommended to set a correct checksum.

$FFD5
Address $00FFD5 indicates the ROM speed and map mode.

001smmmm |++++- Map mode +- Speed: 0=Slow, 1=Fast

Available modes include:
 * 0: LoROM
 * 1: HiROM
 * 5: ExHiROM

$FFD6
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:
 * $00 - SPC7110
 * $01 - ST010/ST011
 * $02 - ST018
 * $03 - CX4

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.