The following shows changes to the 68K Memory Map with a MegaCD/SegaCD attached:
MegaCD BIOS ROM
MegaCD "Program RAM" Bank Access
MegaCD "WORD RAM"
MegaCD "Gate Array"
MegaCD Interrupt/Exception vectors
MegaCD BIOS ROM is always only visible to the main 68K, and decompresses a separate BIOS for the MegaCD "SubCPU" into "Program RAM". "WORD RAM" access is restricted to one 68K at a time, which is determined by a setting in the "Gate Array". "Program RAM" is $80000 bytes in size, but is only accessible by the Genesis 68K in banks of size $20000, and only when the MegaCD 68K is in "BUSREQ" (stopped). The MegaCD 68K may be stopped/reset, and the available "Program RAM" bank may be set by use of the "Gate Array".