Genesis Programming/68K Memory map/

Standard

Start address	End address	Description
$000000	$3FFFFF	Cartridge ROM/RAM
$400000	$7FFFFF	Reserved (used by the Sega CD and 32x)
$800000	$9FFFFF	Reserved (used by the 32x?)
$A00000	$A0FFFF	Z80 addressing space
$A10000	$A10001	Version register (read-only word-long)
$A10002	$A10003	Controller 1 data
$A10004	$A10005	Controller 2 data
$A10006	$A10007	Expansion port data
$A10008	$A10009	Controller 1 control
$A1000A	$A1000B	Controller 2 control
$A1000C	$A1000D	Expansion port control
$A1000E	$A1000F	Controller 1 serial transmit
$A10010	$A10011	Controller 1 serial receive
$A10012	$A10013	Controller 1 serial control
$A10014	$A10015	Controller 2 serial transmit
$A10016	$A10017	Controller 2 serial receive
$A10018	$A10019	Controller 2 serial control
$A1001A	$A1001B	Expansion port serial transmit
$A1001C	$A1001D	Expansion port serial receive
$A1001E	$A1001F	Expansion port serial control
$A10020	$A10FFF	Reserved
$A11000		Memory mode register
$A11002	$A110FF	Reserved
$A11100	$A11101	Z80 bus request
$A11102	$A111FF	Reserved
$A11200	$A11201	Z80 reset
$A11202	$A13FFF	Reserved
$A14000	$A14003	TMSS register
$A14004	$BFFFFF	Reserved
$C00000	$C00001	VDP data
$C00002	$C00003	VDP data (mirror)
$C00004	$C00005	VDP control
$C00006	$C00007	VDP control (mirror)
$C00008	$C00009	VDP HV counter
$C0000A	$C00010	Reserved
$C00011		PSG output
$C00012	$FEFFFF	Reserved
$FF0000	$FFFFFF	68000 RAM

MegaCD Changes

The following shows changes to the 68K Memory Map with a MegaCD/SegaCD attached:

Sega CD Model 2 attached to a Model 2 Sega Genesis.

Start address	End address	Description
$000000	$01FFFF	MegaCD BIOS ROM
$020000	$03FFFF	MegaCD "Program RAM" Bank Access
$200000	$23FFFF	MegaCD "WORD RAM"
$A12000	$A120XX	MegaCD "Gate Array"
$FFFD00	$FFFDFF	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".