Mnemósine500 is a little tiny trapdoor expansion board for the Amiga 500. It allows to add 512kb of chipram (512kb of fastram/low, depends on the system). Its just plug and play.
There is a jumpers setup on the board which, depend on the settings you can enable or disable the ram module or depending of the motherboard revision, assign 512kb to the chip or the fast ram area of the Amiga 500.
Amiga 500+ rev8, agnus 8375 (adds 512k of chipram).
Amiga 500 rev6, agnus 8372a (adds 512k of fastram) / 8375 (adds 512kb of chipram)