The Cartridge Header

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An internal information area is located at 0100-014F in each cartridge. It contains the following values:

0100-0103 - Entry Point

After displaying the Nintendo Logo, the built-in boot procedure jumps to this address (100h), which should then jump to the actual main program in the cartridge. Usually this 4 byte area contains a NOP instruction, followed by a JP 0150h instruction. But not always.

These bytes define the bitmap of the Nintendo logo that is displayed when the gameboy gets turned on. The hexdump of this bitmap is:

 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 gameboys boot procedure verifies the content of this bitmap (after it has displayed it), and LOCKS ITSELF UP if these bytes are incorrect. A CGB verifies only the first 18h bytes of the bitmap, but others (for example a pocket gameboy) verify all 30h bytes.

0134-0143 - Title

Title of the game in UPPER CASE ASCII. If it is less than 16 characters then the remaining bytes are filled with 00's. When inventing the CGB, Nintendo has reduced the length of this area to 15 characters, and some months later they had the fantastic idea to reduce it to 11 characters only. The new meaning of the ex-title bytes is described below.

013F-0142 - Manufacturer Code

In older cartridges this area has been part of the Title (see above), in newer cartridges this area contains an 4 character uppercase manufacturer code. Purpose and Deeper Meaning unknown.

0143 - CGB Flag

In older cartridges this byte has been part of the Title (see above). In CGB cartridges the upper bit is used to enable CGB functions. This is required, otherwise the CGB switches itself into Non-CGB-Mode. Typical values are:

 80h - Game supports CGB functions, but works on old gameboys also.
 C0h - Game works on CGB only (physically the same as 80h).

Values with Bit 7 set, and either Bit 2 or 3 set, will switch the gameboy into a special non-CGB-mode with uninitialized palettes. Purpose unknown, eventually this has been supposed to be used to colorize monochrome games that include fixed palette data at a special location in ROM.

0144-0145 - New Licensee Code

Specifies a two character ASCII licensee code, indicating the company or publisher of the game. These two bytes are used in newer games only (games that have been released after the SGB has been invented). Older games are using the header entry at 014B instead.

0146 - SGB Flag

Specifies whether the game supports SGB functions, common values are:

 00h = No SGB functions (Normal Gameboy or CGB only game)
 03h = Game supports SGB functions

The SGB disables its SGB functions if this byte is set to another value than 03h.

0147 - Cartridge Type

Specifies which Memory Bank Controller (if any) is used in the cartridge, and if further external hardware exists in the cartridge.

 00h  ROM ONLY                 15h  MBC4
 01h  MBC1                     16h  MBC4+RAM
 02h  MBC1+RAM                 17h  MBC4+RAM+BATTERY
 03h  MBC1+RAM+BATTERY         19h  MBC5
 05h  MBC2                     1Ah  MBC5+RAM
 06h  MBC2+BATTERY             1Bh  MBC5+RAM+BATTERY
 08h  ROM+RAM                  1Ch  MBC5+RUMBLE
 0Bh  MMM01                    1Eh  MBC5+RUMBLE+RAM+BATTERY
 0Ch  MMM01+RAM                20h  MBC6
 11h  MBC3                     FDh  BANDAI TAMA5
 12h  MBC3+RAM                 FEh  HuC3

0148 - ROM Size

Specifies the ROM Size of the cartridge. Typically calculated as "32KB shl N".

 00h -  32KByte (no ROM banking)
 01h -  64KByte (4 banks)
 02h - 128KByte (8 banks)
 03h - 256KByte (16 banks)
 04h - 512KByte (32 banks)
 05h -   1MByte (64 banks)  - only 63 banks used by MBC1
 06h -   2MByte (128 banks) - only 125 banks used by MBC1
 07h -   4MByte (256 banks)
 52h - 1.1MByte (72 banks)
 53h - 1.2MByte (80 banks)
 54h - 1.5MByte (96 banks)

0149 - RAM Size

Specifies the size of the external RAM in the cartridge (if any).

 00h - None
 01h - 2 KBytes
 02h - 8 Kbytes
 03h - 32 KBytes (4 banks of 8KBytes each)
 04h - 128 KBytes (16 banks of 8KBytes each)
 05h - 64 KBytes (8 banks of 8KBytes each)

When using a MBC2 chip 00h must be specified in this entry, even though the MBC2 includes a built-in RAM of 512 x 4 bits.

014A - Destination Code

Specifies if this version of the game is supposed to be sold in japan, or anywhere else. Only two values are defined.

 00h - Japanese
 01h - Non-Japanese

014B - Old Licensee Code

Specifies the games company/publisher code in range 00-FFh. A value of 33h signalizes that the New License Code in header bytes 0144-0145 is used instead. (Super GameBoy functions won't work if <> $33.)

014C - Mask ROM Version number

Specifies the version number of the game. That is usually 00h.

014D - Header Checksum

Contains an 8 bit checksum across the cartridge header bytes 0134-014C. The checksum is calculated as follows:

 x=0:FOR i=0134h TO 014Ch:x=x-MEM[i]-1:NEXT

The lower 8 bits of the result must be the same than the value in this entry. The GAME WON'T WORK if this checksum is incorrect.

014E-014F - Global Checksum

Contains a 16 bit checksum (upper byte first) across the whole cartridge ROM. Produced by adding all bytes of the cartridge (except for the two checksum bytes). The Gameboy doesn't verify this checksum.