Famicom FAM 2 Format Info
http://www.citilink.com/~terhune/emu/fami_us.html
FAM files are ROM images of 8-bit Nintendo games, designed for version 2 of Seil's Famicom emulator. There is also a previous version of the emulator using the FAM extension for its ROMs as well, although the two formats are *NOT* compatible.
With that, here is the format of FAM images with commentary below:
: Start of File
0000-000F: Famicom Header 16 bytes
0010- : Null xxx
-810C: PRG Data xxx
810D-811C: PRG Footer 16
811D-821E: Null 258
821F-831E: CHR Header 256
831D- : CHR Data xxx
: End of File
FAM Header: 02 00 0D 81 00 01 00 00-00 FF FF 00 00 00 00 00
PRG Footer: 03 00 12 22 00 00 01 00-00 00 00 00 FF FF FF FF
CHR Header: String of 01 in the above range.
It's possible that the Famicom header ends after FF FF, it just seems logical that it would be 16 bytes long.
What I've found from the file structure is that the problem with ROMs bigger than 64K is not in the file size itself, but in PRGs larger than 32K. The emulator essentially works from the middle, expecting the PRG to end at 810C and the CHR to start at 821F. Whatever the PRG size, null bytes are entered from the end of the Famicom header to the actual start of data. In the case of a 64K PRG the data would have to extend past the expected ending location even if it starts immediately after the Famicom header.
Theoretically then, the CHR segment could be as big as it wanted. Unfortunately, there aren't a whole lot of games that have 32K PRGs and 128K CHRs. Right now this means that the Famicom emulator doesn't have a lot of potential for most games in the current format.
Hopefully I got my hexes right...
- Zofo
