GBX v1.0

a Game Boy / Game Boy Color ROM image format

by taizou (web / email / twitter)

Spec v1.0 finalised: 22nd February 2018 / Document revision 2: 28th May 2023

1. Background

As of early 2018, the current and only standard for Game Boy and Game Boy Color ROM images (.GB/.GBC) was simply a raw binary dump of the cartridge ROM.

Game Boy emulators therefore base 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.

GBX is a fairly simple file format which aims to solve this problem by allowing this information to be specified somewhere outside of the ROM data itself.

2. File format

A GBX ROM image file consists of the following:

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

File extension: .gbx

GBX is intended to be fairly unintrusive and low-overhead, since it'll most likely be a niche proposition at this late stage in Game Boy emulation, especially given that plain binary dumps are satisfactory for almost all licensed releases.

Specifically, a footer was chosen instead of a header because:

• Backwards compatibility; most existing emulators ignore extra data after the ROM size specified by the internal header, so would still run a GBX ROM if they would be capable of running the raw binary.
• It 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).

The intention is that this format will be used only where required. In other words, if the game is licensed and the internal header is correct, a plain binary is still perfectly adequate - there'd be little benefit in mass-updating licensed ROM sets to use GBX.

3. Footer

The footer defines any cartridge metadata required for emulation.

The "GBX metadata" section - the values at 30-3F in this version - 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  64 byte footer, minor v1, major v0, signature

4. Mapper identifiers

This is NOT an exhaustive list of mappers, just a list of those known and supported at the time of this document's creation.

An up to date list of mappers will be maintained here for now.

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.

Document revisions

The following revisions have been made to correct errors in this document without affecting the spec:

1. 26th March 2018 - Fixed numbers mistakenly specified as being little-endian instead of big-endian
2. 28th May 2023 - Fixed example footer annotation mistakenly referring to "64k" instead of "64 bytes"

🦁