The Game Boy has a 16-bit address bus, which is used to address ROM, RAM, and I/O.
|0000||3FFF||16 KiB ROM bank 00||From cartridge, usually a fixed bank|
|4000||7FFF||16 KiB ROM Bank 01~NN||From cartridge, switchable bank via mapper (if any)|
|8000||9FFF||8 KiB Video RAM (VRAM)||In CGB mode, switchable bank 0/1|
|A000||BFFF||8 KiB External RAM||From cartridge, switchable bank if any|
|C000||CFFF||4 KiB Work RAM (WRAM)|
|D000||DFFF||4 KiB Work RAM (WRAM)||In CGB mode, switchable bank 1~7|
|E000||FDFF||Mirror of C000~DDFF (ECHO RAM)||Nintendo says use of this area is prohibited.|
|FE00||FE9F||Sprite attribute table (OAM)|
|FEA0||FEFF||Not Usable||Nintendo says use of this area is prohibited|
|FF80||FFFE||High RAM (HRAM)|
|FFFF||FFFF||Interrupt Enable register (IE)|
The following addresses are supposed to be used as jump vectors:
- RST instructions: 0000, 0008, 0010, 0018, 0020, 0028, 0030, 0038
- Interrupts: 0040, 0048, 0050, 0058, 0060
However, this memory area (0000-00FF) may be used for any other purpose in case that your program doesn’t use any (or only some) RST instructions or interrupts. RST is a 1-byte instruction that works similarly to the 3-byte CALL instruction, except that the destination address is restricted. Since it is 1-byte sized, it is also slightly faster.
The memory area at 0100-014F contains the cartridge header. This area contains information about the program, its entry point, checksums, information about the used MBC chip, the ROM and RAM sizes, etc. Most of the bytes in this area are required to be specified correctly.
The areas from 0000-7FFF and A000-BFFF address external hardware on the cartridge, which is essentially an expansion board. Typically this is a ROM and SRAM or, more often, a Memory Bank Controller (MBC). The RAM area can be read from and written to normally; writes to the ROM area control the MBC. Some MBCs allow mapping of other hardware into the RAM area in this way.
Cartridge RAM is often battery buffered to hold saved game positions, high score tables, and other information when the Game Boy is turned off. For specific information read the chapter about Memory Bank Controllers.
The range E000-FDFF is mapped to WRAM, but only the lower 13 bits of the address lines are connected, with the upper bits on the upper bank set internally in the memory controller by a bank swap register. This causes the address to effectively wrap around. All reads and writes to this range have the same effect as reads and writes to C000-DDFF.
Nintendo prohibits developers from using this memory range. The behavior is confirmed on all official hardware. Some emulators (such as VisualBoyAdvance <1.8) don’t emulate Echo RAM. In some flash cartridges, echo RAM interferes with SRAM normally at A000-BFFF. Software can check if Echo RAM is properly emulated by writing to RAM (avoid values 00 and FF) and checking if said value is mirrored in Echo RAM and not cart SRAM.
The Game Boy uses the following I/O ranges:
|$FF04||$FF07||DMG||Divider and Timer|
|$FF4F||CGB||VRAM Bank Select|
|$FF50||DMG||Set to non-zero to disable boot ROM|
|$FF68||$FF69||CGB||BG / OBJ Palettes|
|$FF70||CGB||WRAM Bank Select|
Nintendo indicates use of this area is prohibited. This area returns $FF when OAM is blocked, and otherwise the behavior depends on the hardware revision.
On DMG, MGB, SGB, and SGB2, reads during OAM block trigger OAM corruption. Reads otherwise return $00.
On CGB revisions 0-D, this area is a unique RAM area, but is masked with a revision-specific value.
On CGB revision E, AGB, AGS, and GBP, it returns the high nibble of the lower address byte twice, e.g. FFAx returns $AA, FFBx returns $BB, and so forth.