8KiB for game saves on a CD1 board.
Battery powered RAM (made non-volatile) used to store cab related information (bookkeeping) and score tables in MVS systems, called backup RAM in this case.
The CD systems use it to store game saves instead of using memory cards.
BIOS calls are used to handle the specific save format, although read/writes can be freely done by the game's 68k program.
NiMh/NiCd, CR2032, charging resistor removal...