The Cartridge Header

Each cartridge contains a header, located at the address range $0100—$014F. The cartridge header provides the following information about the game itself and the hardware it expects to run on:

0100-0103 — Entry point

After displaying the Nintendo logo, the built-in boot ROM jumps to the address $0100, which should then jump to the actual main program in the cartridge. Most commercial games fill this 4-byte area with a nop instruction followed by a jp $0150.

0104-0133 — Nintendo logo

This area contains a bitmap image that is displayed when the Game Boy is powered on. It must match the following (hexadecimal) dump, otherwise the boot ROM won’t allow the game to run:

CE ED 66 66 CC 0D 00 0B 03 73 00 83 00 0C 00 0D
00 08 11 1F 88 89 00 0E DC CC 6E E6 DD DD D9 99
BB BB 67 63 6E 0E EC CC DD DC 99 9F BB B9 33 3E

The way the pixels are encoded is as follows: (more visual aid)

• The bytes $0104—$011B encode the top half of the logo while the bytes $011C–$0133 encode the bottom half.
• For each half, each nibble encodes 4 pixels (the MSB corresponds to the leftmost pixel, the LSB to the rightmost); a pixel is lit if the corresponding bit is set.
• The 4-pixel “groups” are laid out top to bottom, left to right.
• Finally, the monochrome models upscale the entire thing by a factor of 2 (leading to somewhat chunky pixels).

The Game Boy’s boot procedure first displays the logo and then checks that it matches the dump above. If it doesn’t, the boot ROM locks itself up.

The CGB and later models only check the top half of the logo (the first $18 bytes).

0134-0143 — Title

These bytes contain the title of the game in upper case ASCII. If the title is less than 16 characters long, the remaining bytes should be padded with $00s.

Parts of this area actually have a different meaning on later cartridges, reducing the actual title size to 15 ($0134–$0142) or 11 ($0134–$013E) characters; see below.

013F-0142 — Manufacturer code

In older cartridges these bytes were part of the Title (see above). In newer cartridges they contain a 4-character manufacturer code (in uppercase ASCII). The purpose of the manufacturer code is unknown.

0143 — CGB flag

In older cartridges this byte was part of the Title (see above). The CGB and later models interpret this byte to decide whether to enable Color mode (“CGB Mode”) or to fall back to monochrome compatibility mode (“Non-CGB Mode”).

Typical values are:

Values with bit 7 and either bit 2 or 3 set will switch the Game Boy into a special non-CGB-mode called “PGB mode”.

0144-0145 — New licensee code

This area contains a two-character ASCII “licensee code” indicating the game’s publisher. It is only meaningful if the Old licensee is exactly $33 (which is the case for essentially all games made after the SGB was released); otherwise, the old code must be considered.

Sample licensee codes:

0146 — SGB flag

This byte specifies whether the game supports SGB functions. The SGB will ignore any command packets if this byte is set to a value other than $03 (typically $00).

0147 — Cartridge type

This byte indicates what kind of hardware is present on the cartridge — most notably its mapper.

0148 — ROM size

This byte indicates how much ROM is present on the cartridge. In most cases, the ROM size is given by 32 KiB × (1 << <value>):

0149 — RAM size

This byte indicates how much RAM is present on the cartridge, if any.

If the cartridge type does not include “RAM” in its name, this should be set to 0. This includes MBC2, since its 512 × 4 bits of memory are built directly into the mapper.

Various “PD” ROMs (“Public Domain” homebrew ROMs, generally tagged with (PD) in the filename) are known to use the $01 RAM Size tag, but this is believed to have been a mistake with early homebrew tools, and the PD ROMs often don’t use cartridge RAM at all.

014A — Destination code

This byte specifies whether this version of the game is intended to be sold in Japan or elsewhere.

Only two values are defined:

014B — Old licensee code

This byte is used in older (pre-SGB) cartridges to specify the game’s publisher. However, the value $33 indicates that the New licensee codes must be considered instead. (The SGB will ignore any command packets unless this value is $33.)

Here is a list of known Old licensee codes (source).

014C — Mask ROM version number

This byte specifies the version number of the game. It is usually $00.

014D — Header checksum

This byte contains an 8-bit checksum computed from the cartridge header bytes $0134–014C. The boot ROM computes the checksum as follows:

uint8_t checksum = 0;
for (uint16_t address = 0x0134; address <= 0x014C; address++) {
    checksum = checksum - rom[address] - 1;
}

The boot ROM verifies this checksum. If the byte at $014D does not match the lower 8 bits of checksum, the boot ROM will lock up and the program in the cartridge won’t run.

014E-014F — Global checksum

These bytes contain a 16-bit (big-endian) checksum simply computed as the sum of all the bytes of the cartridge ROM (except these two checksum bytes).

This checksum is not verified, except by Pokémon Stadium’s “GB Tower” emulator (presumably to detect Transfer Pak errors).