The preservation of video game history is a race against physical decay. Arcade circuit boards, laserdiscs, and hard drives rot, capacitors leak, and the original hardware eventually fails. At the forefront of combating this entropy stands the Multiple Arcade Machine Emulator (MAME). However, for the uninitiated, navigating MAME’s file structure reveals a confusing duality: the small, ubiquitous ROM file and the massive, enigmatic CHD file. Understanding the relationship between these two formats is not merely a technical hurdle; it is essential to grasping how modern emulation replicates the complex, multi-layered hardware of arcade history.
Yet, as arcade technology evolved, ROMs alone became insufficient. The late 1980s and 1990s saw the rise of CD-ROMs, laserdiscs, and later, hard disk drives as storage media within arcade systems. Games like Dance Dance Revolution , Gauntlet Legends , or the Killer Instinct series required vast amounts of streaming data—full-motion video, CD-quality audio tracks, and complex 3D texture maps. This data could not fit on a traditional ROM chip. Instead, manufacturers stored it on spinning media. When MAME emulates such a system, it needs access to a byte-for-byte copy of that storage device. Enter the CHD (Compressed Hunks of Data) file. The CHD format is MAME’s ingenious solution for storing lossless, compressed images of CD-ROMs, hard drives, and laserdiscs. A single CHD can be hundreds of megabytes or even gigabytes in size, representing the mass storage that the arcade hardware would have accessed in real-time. mame roms chd
At its core, a MAME ROM (Read-Only Memory) is a digital snapshot of a game’s program code and basic circuitry logic. In a classic arcade cabinet from the 1980s or early 1990s, the game’s instructions, graphics data, and sound samples were etched onto physical ROM chips soldered onto the printed circuit board (PCB). A ROM set, therefore, is a collection of these chip dumps. These files are typically small—ranging from a few kilobytes to several megabytes—because early game logic was lean, and assets were heavily compressed or procedurally generated. When you load Pac-Man or Street Fighter II in MAME, you are feeding the emulated CPU the exact same binary instructions the original Zilog Z80 or Motorola 68000 processor would have read. Without the ROM, there is no software; the emulator is just a silent, idle simulation of silicon. The preservation of video game history is a
The critical relationship between ROM and CHD is one of logic versus data. The ROM contains the game’s executable code—the BIOS, the main program, the input handlers, and the encryption keys. The CHD contains the assets: the song files, the pre-rendered cutscenes, the level geometry, and the sampled voice lines. In emulation terms, the ROM tells the emulated CPU what to do , while the CHD provides the material to do it with . They are inseparable partners. For example, to run NBA Showtime: NBA on NBC , MAME first loads the ROM set, which initializes the Seattle or Vegas hardware. When the game code executes a command to load a specific arena texture or a commentary clip, the emulator reads the request, locates the corresponding sector on the virtual hard drive (the CHD), and streams the data back into the emulated system’s memory. The late 1980s and 1990s saw the rise
Furthermore, the CHD format itself is a marvel of preservation engineering. Unlike a standard ISO or BIN/CUE image, CHD employs hunk-level compression, delta compression (storing only the differences between successive revisions of a game), and metadata for error detection. This allows MAME to store, for instance, five revisions of a laserdisc game like Dragon’s Lair in the space of one and a half discs. It prioritizes accuracy over convenience, ensuring that every sector, including unformatted or damaged areas, is preserved exactly as it was on the original medium.