Memory Map; Description Of Registers - Nintendo GAME BOY Programming Manual

5.3 Memory Map

Bank 3
Maximum of 256
Bank 2
Kbits (Banks 0-3)
Set by register RAMB
Bank 1
Accessible only when
RAMG is 0Ah
Bank 0

5.4 Description of Registers

! Register for Specifying External Expansion Memory (RAMG)
Specifies whether external expansion RAM is accessible. Access to this RAM is enabled by
writing 0Ah to the RAMG register (any single address in 0000h-1FFFh). Writing any other
value to this register disables reading to and writing from RAM.
! Lower ROM Bank Register (ROMB0)
Specifies the lower-order 8 bits of a 9-bit ROM bank.
The ROM bank can be changed by writing the desired ROM bank number to the ROMB0
register (any single address in 2000h-2FFFh).
CPU Address
FFFFh
RAM
E000h
C000h
Expansion
Working RAM
A000h
8000h
Empty
(no image)
6000h
5000h
RAMB
4000h
ROMB1
3000h
ROMB0
2000h
RAMG
0000h
Write
* During a write, data is written to the bank control registers at
CPU addresses 0000h-7FFFh. During a read, the contents of
ROM are read from these addresses.
Chapter 8: Game Boy Memory Controllers (MBC)
Unit
Registers
Internal
Working
RAM
External
Display
RAM
Bank Switching
Area
Bank 00h - Bank
1FFh
(Default bank 0x01)
Program Residence
Area
Fixed at Bank 0
Read
229
ROM
Highest bank, 0x1FF
: :
: :
Maximum 64
: :
Mbits
Set by registers
ROMB0 and
ROMB1
: :
: :
Bank 1
Bank 0