From NESdev Wiki
Jump to navigationJump to search

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 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.

See also