Genesis Programming/68K Memory map/

Standard Start addressEnd addressDescription$000000 $3FFFFFCartridge ROM/RAM$400000$7FFFFFReserved (used by the Sega CD and 32x)$800000$9FFFFFReserved (used by the 32x?)$A00000$A0FFFFZ80 addressing space$A10000$A10001Version register (read-only word-long)$A10002$A10003Controller 1 data$A10004$A10005Controller 2 data$A10006$A10007Expansion port data$A10008$A10009Controller 1 control$A1000A$A1000BController 2 control$A1000C$A1000DExpansion port control$A1000E$A1000FController 1 serial transmit$A10010$A10011Controller 1 serial receive$A10012$A10013Controller 1 serial control$A10014$A10015Controller 2 serial transmit$A10016$A10017Controller 2 serial receive$A10018$A10019Controller 2 serial control$A1001A$A1001BExpansion port serial transmit$A1001C$A1001DExpansion port serial receive$A1001E$A1001FExpansion port serial control$A10020$A10FFFReserved$A11000Memory mode register$A11002$A110FFReserved$A11100$A11101Z80 bus request$A11102$A111FFReserved$A11200$A11201Z80 reset$A11202$A13FFFReserved$A14000$A14003TMSS register$A14004$BFFFFFReserved$C00000$C00001VDP data$C00002$C00003VDP data (mirror)$C00004$C00005VDP control$C00006$C00007VDP control (mirror)$C00008$C00009VDP HV counter$C0000A$C00010Reserved$C00011PSG output$C00012$FEFFFFReserved$FF0000$FFFFFF68000 RAM MegaCD Changes The following shows changes to the 68K Memory Map with a MegaCD/SegaCD attached: 320x184px|thumbnail|right|Sega CD Model 2 attached to a Model 2 Sega Genesis. Start addressEnd addressDescription$000000$01FFFFMegaCD BIOS ROM$020000$03FFFFMegaCD "Program RAM" Bank Access$200000$23FFFFMegaCD "WORD RAM"$A12000$A120XXMegaCD "Gate Array"$FFFD00$FFFDFFMegaCD 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".