The Multiple Arcade Machine Emulator, or MAME, can emulate more than 1,000 arcade units. However, not all arcade units share the same hardware; with some games, additional files, such as CHD files, are required. Knowing what these files are and where to place them is integral to playing these games in MAME.
MAME was first released in 1997 and is still ongoing at the time of publication; new versions of MAME with support for additional games and performance increases for older games are released often. MAME is a free download available from the developer's website. As many arcade machines featured different control setups, many different controllers can be used with MAME, such as joysticks, mice and gamepads. MAME is a command-line program and doesn't function unless all of the necessary files, such as CHD files, are located in the proper folders.
Over time, arcade games evolved from simple one screen games, such as "Pac-Man" or "Galaga," to games that utilized scrolling screens, many playable characters and impressive, cutting-edge graphics. Hard drives were used in a few arcade games to help store the large amounts of data needed to run the game. A CHD file is the compressed image of that hard drive; this single file holds the information of a game's entire hard drive, and is required to play the game through MAME.
Examples of CHD Games
Arcade games that used hard drives, and thus would have their information stored in a CHD file, were frequent in the 90s. These games range from popular fighting games, such as "Killer Instinct" and "Street Fighter III: Third Strike" to racing games such as "San Francisco Rush." Not many arcade machines used hard drives due to the cost of having one in each machine, so the list of games that used them is a little over 100. Primarily, games in the fighting genre, racing genre and sports genre from larger companies, such as Capcom or Midway, took advantage of a hard drive.
How to Add CHD to MAME
To play a CHD game in MAME, place the CHD file in the appropriate folder. Within the "Roms" folder in MAME, create a new folder that uses the same name as your CHD file. For example, if the file name was xxxx.CHD, the new folder would be called xxxx. Place the CHD file inside of the folder; once the game is launched within MAME, it automatically detects the CHD file in the folder and your game will load.