Game Save (Sega Mega CD)
The game saves are stored in the Work RAM (MB8464A, IC16). This Work RAM is enabled by the CS2 pin, which is controlled by the Battery Backup IC (MB3790, IC6) wrongly labelled IC9 in the schematic.
The battery backup IC supplies the power from either the main VCC2 rail, or the coin cell battery when there is no VCC2 (so console turned off), to keep the Work RAM powered at all time.
At the same time it also disables the Work RAM by pulling CS2 low (via its !RESET pin) when power fails.
The Work RAM is used to store the game saves and the Mega Drive is what performs those actions, along with the 68k process on the Mega CD.
Do not confuse the WORD RAM with the WORK RAM.
The Word RAM is the two Video RAM chips (TC511664BZ) used for game loading and video data.
The Work RAM (MB8464A) is for saving game save data.
The actual Mega Engine IC doesn't have any involvement in the Work RAM.
For the Work RAM to operate, it needs VCC at 5V, and CS2 high and !CS1 low.
CS2 is controlled by MB3790 !RESET pin, which should be high at all times.
!CS1 is controled by the BRAM pin 2 of the Mega Engine IC.
Turn on the console and measure with a multimeter through the side that alll the pins (5V, CS2, !CS1 are the expected voltages.
NOTE: The 240p Test Suite Word RAM Tests is NOT the Work RAM test. The Word RAM is the Video RAM () to the right of the board as 2 long chips, and the Work RAM is use the Mega CD menu instead.
To test the Work RAM memory (game backup), turn on the Mega CD without a disc, press Start a few times until you get to the main menu, then go to Option and you can format the memory.
