The Cartridge Header
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.
0104-0133 - Nintendo Logo
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 13h MBC3+RAM+BATTERY 01h MBC1 15h MBC4 02h MBC1+RAM 16h MBC4+RAM 03h MBC1+RAM+BATTERY 17h MBC4+RAM+BATTERY 05h MBC2 19h MBC5 06h MBC2+BATTERY 1Ah MBC5+RAM 08h ROM+RAM 1Bh MBC5+RAM+BATTERY 09h ROM+RAM+BATTERY 1Ch MBC5+RUMBLE 0Bh MMM01 1Dh MBC5+RUMBLE+RAM 0Ch MMM01+RAM 1Eh MBC5+RUMBLE+RAM+BATTERY 0Dh MMM01+RAM+BATTERY FCh POCKET CAMERA 0Fh MBC3+TIMER+BATTERY FDh BANDAI TAMA5 10h MBC3+TIMER+RAM+BATTERY FEh HuC3 11h MBC3 FFh HuC1+RAM+BATTERY 12h MBC3+RAM
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)
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.