What is a ROM image?

For arcade games, a ROM image or file is a copy of all of the data inside a given chip on the arcade motherboard. For most consoles and handhelds, the individual chips are frequently (but not always) merged into a single file. As arcade machines are much more complicated in their design, you’ll typically need the data from a number of different chips on the board. Grouping all of the files from Puckman together will get you a ROM set of Puckman.

An example ROM image would be the file pm1_prg1.6e stored in the Puckman ROM set.

Why ROM and not some other name?

ROM stands for Read-Only Memory. The chips used to store the game data were not rewritable and were permanent (as long as the chip wasn’t damaged or aged to death!)

As such, a copy of the data necessary to reconstitute and replace a dead data chip on a board became known as a “ROM image” or ROMs for short.

Parents, Clones, Splitting, and Merging

As the MAME developers received their third or fourth revision of Pac-Man, with bugfixes and other code changes, they quickly discovered that nearly all of the board and chips were identical to the previously dumped version. In order to save space, MAME was adjusted to use a parent/clone set system.

A given set, usually (but not necessarily) the most recent bugfixed World revision of a game, will be designated as the parent. All sets that use mostly the same chips (e.g. Japanese Puckman and USA/World Pac-Man) will be clones that contain only the changed data compared to the parent set.

This typically comes up as an error message to the user when trying to run a Clone set without having the Parent set handy. Using the above example, trying to play the USA version of Pac-Man without having the PUCKMAN.ZIP parent set will result in an error message that there are missing files.

Now we add the final pieces of the puzzle: non-merged, split, and merged sets.

MAME is extremely versatile about where ROM data is located and is quite intelligent about looking for what it needs. This allows us to do some magic with how we store these ROM sets to save further space.

A non-merged set is one that contains absolutely everything necessary for a given game to run in one ZIP file. This is ordinarily very space-inefficient, but is a good way to go if you want to have very few sets and want everything self-contained and easy to work with. We do not recommend this for most users.

A split set is one where the parent set contains all of the normal data it should, and the clone sets contain only what has changed as compared to the parent set. This saves some space, but isn’t quite as efficient as

A merged set takes the parent set and one or more clone sets and puts them all inside the parent set’s storage. For instance, if we combine the Puckman sets, Midway Pac-Man (USA) sets, and various other related official and bootleg sets all into PUCKMAN.ZIP, the result would be a merged set. A complete merged set with the parent and all clones uses less disk space than a split set.

With those basic principles, there are two other kinds of set that will come up in MAME use from time to time.

First, the BIOS set: Some arcade machines shared a common hardware platform, such as the Neo-Geo arcade hardware. As the main board had data necessary to start up and self-test the hardware before passing it off to the game cartridge, it’s not really appropriate to store that data as part of the game ROM sets. Instead, it is stored as a BIOS image for the system itself (e.g. NEOGEO.ZIP for Neo-Geo games)

Secondly, the device set. Frequently the arcade manufacturers would reuse pieces of their designs multiple times in order to save on costs and time. Some of these smaller circuits would reappear in later boards that had minimal common ground with the previous boards that used the circuit, so you couldn’t just have them share the circuit/ROM data through a normal parent/clone relationship. Instead, these re-used designs and ROM data are categorized as a Device, with the data stored as a Device set. For instance, Namco used the Namco 51xx custom I/O chip to handle the joystick and DIP switches for Galaga and other games, and as such you’ll also need the NAMCO51.ZIP device set as well as any needed for the game.

Troubleshooting your ROM sets and the history of ROMs

A lot of the frustration users feel towards MAME can be directly tied to what may feel like pointless ROM changes that seem to only serve to make life more difficult for end-users. Understanding the source of these changes and why they are necessary will help you to avoid being blindsided by change and to know what you need to do to keep your sets running.

A large chunk of arcade ROMs and sets existed before emulation did. These early sets were created by arcade owners and used to repair broken boards by replacing damaged chips. Unfortunately, these sets eventually proved to be missing critical information. Many of the early dumps missed a new type of chip that contained, for instance, color palette information for the screen. The earliest emulators approximated colors until the authors discovered the existence of these missing chips. This resulted in a need to go back and get the missing data and update the sets to add the new dumps as needed.

It wouldn’t be much longer before it would be discovered that many of the existing sets had bad data for one or more chips. These, too, would need to be re-dumped, and many sets would need complete overhauls.

Occasionally games would be discovered to be completely wrongly documented. Some games thought to be legitimate ended up being bootleg copies from pirate manufacturers. Some games thought to be bootlegs ended up being legit. Some games were completely mistaken as to which region the board was actually from (e.g. World as compared to Japan) and this too would require adjustments and renaming.

Even now, occasional miracle finds occur that change our understanding of these games. As accurate documentation is critical to detailing the history of the arcades, MAME will change sets as needed to keep things as accurate as possible within what the team knows at the time of each release.

This results in very spotty compatibility for ROM sets designated for older versions of MAME. Some games may not have changed much within 20-30 revisions of MAME, and others may have drastically changed multiple times.

If you hit problems with a set not working, there are several things to check– are you trying to run a set meant for an older version of MAME? Do you have any necessary BIOS or Device ROMs? Is this a Clone set that would need to have the Parent as well? MAME will tell you what files are missing as well as where it looked for these files. Use that to determine which set(s) may be missing files.

ROMs and CHDs

ROM chip data tends to be relatively small and gets loaded to system memory outright. Some games also used additional storage mediums such as hard drives, CD-ROMs, DVDs, and Laserdiscs. Those storage mediums are, for multiple technical reasons, not well-suited to being stored the same way as ROM data and won’t fit completely in memory in some cases.

Thus, a new format was created for these in the CHD file. Compressed Hunks of Data, or CHD for short, are designed very specifically around the needs of mass storage media. Some arcade games, consoles, and PCs will require a CHD to run. As CHDs are already compressed, they should NOT be stored in a ZIP or 7Z file as you would for ROM images.