Patching Snes Roms
Updated:
International Patching System (IPS) is a file format for applying patches to snes roms.
Patches are smaller to distribute. Also, there are no concerns regarding copyright law when distributing a patch, as opposed to the distributing a post-patched rom.
File Format
The file format is very simplistic diff patch. It’s similar to git patches but binary.
| Section | Size | Description |
|---|---|---|
| Offset | 3 bytes | Position in rom to start patching |
| Size | 2 bytes | Amount of data to be patched |
| Data | Amount of bytes specified by size | Bit dump of data to overwrite |
As you can see, this is very simple file format. There’s no checksum, compression, or rom validation. It will literally overwrite those blocks of the rom.
The only way to validate the rom is complete playthrough (100% rom coverage).
Tools
Lunar IPS is a simple tool to apply a patch to a ROM. It’s how you apply rom hacks.
Rom Expansion
Some rom hacks add more data and the rom needs to be resized. The rom data section is enlarged, while keeping headers and footers intact.
Coilsnakes is a tool for editing Earthbound and it features the ability to expand a rom.