.GB

Introduction

A .gb file is a copy of the ROM in a cartridge for Nintendo’s Game Boy, for playing in an emulator or writing to a flash cartridge. Games made for the Game Boy Color are usually named .gbc. Unlike most ROM image formats, there is no header added by the dumping tool: the file is the ROM exactly, and the information an emulator needs is in the cartridge header that every game already contains.

Structure

The first 256 bytes hold the restart and interrupt vectors of the console’s processor. The cartridge header follows at 0x100. It starts with the entry point, usually a jump past the header, and the Nintendo logo, which the console’s boot ROM checks before it runs the game. Then come the title, a flag for Game Boy Color support (0x80 for games that also run on the original, 0xC0 for Color-only games), a flag for Super Game Boy features, the cartridge type, which names the memory bank controller and any battery or real-time clock, and the ROM and RAM sizes. The header checksum at 0x14D covers bytes 0x134 to 0x14C, and the boot ROM refuses to start a game whose checksum is wrong.

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