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The generic designation FxROM refers to cartridge boards made by Nintendo that use the Nintendo MMC4 mapper.

Board Types

The following FxROM boards are known to exist:

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

Battery backup

  • 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 compulsary 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.

See also