ROM file formats
Super NES ROM files are usually found in one of 2 variations of the same format.
The most common filename extension is .SFC, followed by .SMC. Less common extensions include: .FIG, .SWC.
The data contained in the file may be unheadered or headered. The only difference between these two is that the headered version has 512 extra bytes at the start of the file.
The 512 byte header is not the same as the ROM header, which is a part of the on-cartridge ROM. Instead it is 512 bytes of metadata gathered by a Copier device used to dump the ROM. This extra data is generally considered useless, except to the specific copier device that it was originally used with. Modern common practice prefers unheadered ROM files.
ROM file layout
See: Memory map
Ignoring the optional 512-byte header, the files contain the contents of the cartridge ROM in linear order. Depending on how the cartridge is mapped, there are 3 orderings:
- LoROM: 32k banks starting from $800000
- HiROM: 64k banks starting from $C00000
- ExHiROM: 64k banks starting from $C00000, then continuing from $400000 after 4MB.
This means that the ROM header could be at one of 3 locations which maps to memory at $00FFC0:
- LoROM: $007FC0
- HiROM: $00FFC0
- ExHiROM: $40FFC0
ROM sizes are not always a power of two, but should at least be the sum of two powers of two. For example: this allows a cartridge that needs 3MB to have one ROM of 2MB and a second ROM of 1MB.
Unpacking a ROM to fill the memory space should use the same mirroring rules as the ROM header's checksum.
Detecting Headered ROM
Because ROM files are generally expected to include complete 32 or 64 kb banks, a simple way of detecting a header is by checking if the file size modulo 1024 is equal to 512.
Alternatively, if a ROM header is not detected at one of the 3 normal locations, looking for them again at a 512 byte offset may also identify the headered ROM. (Note that the ROM header's checksum calculation would not include the 512 bytes of this file's header.)
