[[Image:FO.DOS Cartridge (photo from mic-cpcrulez).jpg|thumb|280px|right|FO-DOS - A Lower ROM expansion used to replace the internal Firmware]]
A further ROM is present inside the CPC. This Firmware ROM is in all CPCs, but is mapped to a different area of memory and also doesn't have a ROM Number. It's also possible to overwrite this ROM externally, however this can't be done with a standard ROMBoard, only with a Lower ROM expansion such as the [[FO-DOS|FO-DOS]] interfaceor the [[LowerROM|LowerROM Board]].