PROPOSED GAME BOY / GAME BOY COLOR ROM IMAGE FORMAT v0.3

GBX v1.0 has now been finalised!
This proposal is now outdated and is retained here for historical purposes only.
The v1.0 spec can be found here.

🦁 by taizou 🦁 (web / email / twitter)

Comments welcome here!

Last updated: 21st February 2018

GBX = Game Boy Xtended (仮)

1. Background

The current and only standard for Game Boy and Game Boy Color ROM images (.GB/.GBC) is simply a raw binary dump of the cartridge ROM.

Current Game Boy emulation bases cartridge type detection largely on the internal ROM header. In licensed games, this is always populated per Nintendo's specs. But in unlicensed games, it often isn't. Likewise for some licensed special carts, notably multicarts.

Until now, emulators have had to rely on heuristics to work out if they're dealing with something non-standard. This was workable when only a few such cartridges had been dumped and emulated, but now many more have been discovered (numbering in at least the low hundreds), with more unique mappers and copy protection schemes, it's becoming increasingly unwieldy.

Even when a game in actuality uses a standard mapper, when the internal header is incorrect, the only way to make most emulators recognise it is to hack that header, thus destroying the integrity of the original data.

So I'm proposing a fairly simple file format to allow this information to be specified somewhere outside of the ROM data itself.

2. File format

A GBX ROM image file will consist of the following:

1. ROM data (any size)
2. Footer (currently 64 bytes)

Recommended file extension: .gbx

Reasons for using a footer instead of a header (commonly used by other ROM formats):

• Backwards compatibility; most existing emulators ignore extra data at the end of a ROM file so would still run a GBX ROM if they would be capable of running the raw binary
• Doesn't offset addresses, meaning ROM hackers don't have to compensate and existing patches will still work (provided they don't validate checksum or file size)

Basically my aim is to make this fairly unintrusive and low-overhead, since I know it'll be a niche/low-uptake thing at this late stage in Game Boy emulation, especially given that plain binary dumps are satisfactory for 99.5% of licensed releases and the cases for a new format are relatively obscure.

TBC: Maybe a plain binary could be considered a valid '0.0' GBX file, meaning 'this is a regular licensed game and the internal header is valid, defer to it'

3. Footer

The footer defines any cartridge metadata required for emulation.

The values at 30-3F in this version shouldange will always be placed at the end of the footer block in future versions, meaning you can always read the last 4 bytes of a file to determine if it's GBX format, and the preceding 12 to determine if it's a supported version and how big the footer is.

Note the footer is not intended to store metadata about the game itself, such as title, release date or publisher, whether it has Super Game Boy or Game Boy Color support, etc.

Layout

MMMM FFFF RRRR SSSS  MMMM = mapper, FFFF = flags, RRRR = romsize, SSSS = ramsize
1111 2222 3333 4444  mapper-specific variables
5555 6666 7777 8888  mapper-specific variables
SSSS MMMM VVVV GBX!  SSSS = footer size, MMMM = major version, VVVV = minor version, GBX! = signature

Valid example (hex)

4D 42 43 35  01 01 00 00  00 10 00 00  00 00 20 00  MBC5, ram+rumble, 1M ROM, 8K RAM
00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00  empty mapper-specific variables
00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00  empty mapper-specific variables
00 00 00 40  00 00 00 01  00 00 00 00  47 42 58 21  64k footer, minor v1, major v0, signature

4. Mapper identifiers

Note the term "mapper" here is used in the NES emulation sense, e.g., any circuitry onboard the cartridge controlling mapping of ROM & RAM and potentially other components such as a rumble motor or tilt sensor. Game Boy cartridges usually contain a custom ASIC (commonly known as "MBC" after the five standard Nintendo chips) for this purpose.

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