.MD

Introduction

A .md or .gen file is a copy of the ROM in a cartridge for Sega’s Mega Drive, sold in North America as the Genesis, for playing in an emulator. Like a Game Boy ROM, it is the cartridge’s contents exactly, with no header added by the dumping tool. Many files are also simply named .bin.

An older form, .smd, comes from the Super Magic Drive copier: it adds a 512-byte header and stores the ROM in interleaved 16 KB blocks, so it must be converted before most tools can read it.

Structure

The console’s 68000 processor starts by reading the first 256 bytes, its vector table: the initial stack pointer, the address to start running at, and the handlers for interrupts and errors. The cartridge header follows at 0x100, written as text padded with spaces. It begins with the console name, SEGA MEGA DRIVE or SEGA GENESIS ; later models of the console check for SEGA there before running a game. All numbers are big-endian.

Also used for

• Markdown .md

Links