Memory Map
The Game Boy has a 16-bit address bus, which is used to address ROM, RAM, and I/O.
| Start | End | Description | Notes |
|---|---|---|---|
| 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 | Echo RAM (mirror of C000–DDFF) | Nintendo says use of this area is prohibited. |
| FE00 | FE9F | Object attribute memory (OAM) | |
| FEA0 | FEFF | Not Usable | Nintendo says use of this area is prohibited. |
| FF00 | FF7F | I/O Registers | |
| FF80 | FFFE | High RAM (HRAM) | |
| FFFF | FFFF | Interrupt Enable register (IE) |
I/O Ranges
The Game Boy uses the following I/O ranges:
| Start | End | First appeared | Purpose |
|---|---|---|---|
| $FF00 | DMG | Joypad input | |
| $FF01 | $FF02 | DMG | Serial transfer |
| $FF04 | $FF07 | DMG | Timer and divider |
| $FF10 | $FF26 | DMG | Audio |
| $FF30 | $FF3F | DMG | Wave pattern |
| $FF40 | $FF4B | DMG | LCD Control, Status, Position, Scrolling, and Palettes |
| $FF4F | CGB | VRAM Bank Select | |
| $FF50 | DMG | Set to non-zero to disable boot ROM | |
| $FF51 | $FF55 | CGB | VRAM DMA |
| $FF68 | $FF6B | CGB | BG / OBJ Palettes |
| $FF70 | CGB | WRAM Bank Select |
VRAM memory map
VRAM is, by itself, normal RAM, and may be used as such; however, the PPU interprets it in specific ways.
Bank 1 does not exist except on CGB, where it can be switched to (only in CGB Mode) using the VBK register.
Each bank first contains 384 tiles, of 16 bytes each. These tiles are commonly thought of as grouped in three “blocks” of 128 tiles each; see this detailed explanation for more details.
The ID of a tile can be obtained from its address using the following equation:
ID = address / 16 mod 256.
This is equivalent to only looking at the address’ middle two hexadecimal digits.
After the tiles, each bank contains two maps, 32×32 (= 1024) bytes each. The two banks are however different here: bank 0 contains tile maps, while bank 1 contains the corresponding attribute maps.
Each entry corresponds to a set of coordinates, linked to its address:
- X = address mod 32
- Y = address / 32 mod 32
In fact, the address of any entry can be thought of as a bitfield:
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 0 | 0 | 1 | 1 | tilemap | Y | X | ||||||||
Here is a visualisation of how VRAM is laid out; hover over elements to see some details.
The diagram is not to scale: each map takes up only half as much memory as a tile "block", despite the maps being visually twice as tall.Jump Vectors in first ROM bank
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.
Cartridge Header in first ROM bank
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.
External Memory and Hardware
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.
Echo RAM
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.
FEA0–FEFF range
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.