The term CHD started out as an abbreviation for Compressed Hard Drive.
Many of the newer games have too much data to store in ROM, so they had to add a hard drive to the game to store it all. The MAME Developers took a copy of the data, compressed it, and called it a CHD.
Since then, they've added CD-based games, and the CD image is also stored as a CHD. So, CHD got a new name... Compressed Hunks of Data.
What does that mean to you, as a player? Not much, really; just think of them as huge ROM files. The game doesn't play without them!
Last updated: 1 March 2004