The generic designation FxROM refers to cartridge boards made by Nintendo that use the Nintendo MMC4 mapper.
The following FxROM boards are known to exist:
|Board||PRG ROM||PRG RAM||CHR|
|FJROM||128 / 256 KB||8 KB||16 / 32 / 64 KB ROM|
|FKROM||128 / 256 KB||8 KB||128 KB ROM|
- PRG ROM size: up to 256 KB
- PRG ROM bank size: 16 KB
- PRG RAM: 8 KB plus optional battery
- CHR capacity: up to 128 KB ROM
- CHR bank size: 4 KB
- Nametable mirroring: Controlled by mapper
- Subject to bus conflicts: No
Solder Pad config
- PRG RAM with data retention : 'SL' disconnected, D1, D2, R1, R2, R3 present.
- PRG RAM without data retention : 'SL' connected, D1, D2, R1, R2, R3 not present.
On the FKROM board, battery backup appears to be compulsory unless there is a hidden solder pad below the MM1026 chip.
The FKROM board uses a Mitsumi MM1026 chip dedicated to safe battery backup. It's the only Nintendo board to use a specific chip dedicated to safe battery backup.