8KiB for game saves on a CD1 board.
Battery powered RAM (made non-volatile) is used as backup RAM in the MVS to store cab related information (bookkeeping) and score tables (sometimes called "SRAM").
The CD systems use it to store game saves instead of using memory cards.
Except V-Liner, no game cartridges store player data.
BIOS calls are used to handle the specific formats, although nothing prevents read/writes being done the game's 68k program itself.
See Battery replacement.