The Sega Saturn 4MB RAM Cartridge. It's just used for increasing the video RAM. It's required for some later games to play correctly, or enhance any where it's optional. There is also a 1MB RAM cart as well.
TOPBOTTOMTOP CLEANSCHEMATICIC1 - SEGA 315-6122 (ASIC?)
IC2 - HY5118164B JY-60
IC3 - HY5118164B JY-60
CE1 - 16v, 47uF
C1, C2, C3 - Decoupling, could be anything from 0.01uF to 1uF each