Guides:MAME - chdman
Compressed Hunks of Data Manager
Introduction
A Compressed Hunks of Data file or CHD for short is a standard format developed by the MAME project storing, not surprisingly compressed hunks of data. The format was originally developed as a format to image later 200x arcade machines that had begun shipping with internal hard drives. Beyond that some hardware systems also began shipping with interchangeable discs (e.g. NAOMI) and a format was needed to capture that.
The CHD format provided a solution for both of the above scenarios. The format itself has undergone multiple iterations to add support for various hdd/disc features. For reference the current chd hashes registered in MAME 0.217 are version 5.
Once imaged the dump is a standard format that all emulator authors can implement.
If you have been around the scene for a while (from the late 90s) you will be aware of how many different disc image formats took hold for a short while mostly due to the fact they supported various features other formats didn't. Some example formats were mdf, nrg, img, iso, bin etc. Emulator authors are required to add support for all these formats or provide a limited number of formats and force users to convert from one format to another during use.
Machine Support
We can take a look at the supported image formats for a couple of systems by querying MAME with -listmedia. Let's look at a couple of consoles to start with that have cdrom drives
Sony PlayStation (USA)
$ ./mame64 psu -listmedia
SYSTEM MEDIA NAME (brief) IMAGE FILE EXTENSIONS SUPPORTED
---------------- --------------------------- -------------------------------
psu memcard1 (memc1) .mc
memcard2 (memc2) .mc
quickload (quik) .cpe .exe .psf .psx
cdrom (cdrm) .chd .cue .toc .nrg .gdi .iso .cdr
Sega Saturn (USA)
$ ./mame64 saturn -listmedia
SYSTEM MEDIA NAME (brief) IMAGE FILE EXTENSIONS SUPPORTED
---------------- --------------------------- -------------------------------
saturn cdrom (cdrm) .chd .cue .toc .nrg .gdi .iso .cdr
cartridge (cart) .bin
You'll notice neither of these systems support mdf/mds format.
The mdf/mds format was developed and use by a very popular tool during the early 2000s called Alcohol 120%.
Another popular tool of the early 2000's CloneCD was the only tool for a very long time to support sub channel data dumping and became popular with groups for dumping pc titles. The format from CloneCD was ccd/img or ccd/img/sub when sub channel data was dumped. Again not supported in either system above although dumps exist.
You will notice the nrg format is in the list of compatible file extensions, this was a proprietary format from the tool Nero Burning ROM, it is interesting that its supported. We need to find out why and update information here.
We can see that chd is supported on both systems, so if we can convert our discs to chd we will remain compatible with MAME. You will remember we mentioned this was used as a form to capture hard disk images originally. That is true but it can also create blank images for use with machines where a hdd may be managed, like the ct486. Lets have a look at the output of -listmedia for this machine.
PC/AT 486 with CS4031 chipset
$ ./mame64 ct496 -listmedia
SYSTEM MEDIA NAME (brief) IMAGE FILE EXTENSIONS SUPPORTED
---------------- --------------------------- -------------------------------
ct486 floppydisk1 (flop1) .dsk .ima .img .ufi .360 .d77 .d88 .1dd .dfi .hfe .imd .ipf .mfi .mfm .td0 .cqm .cqi
floppydisk2 (flop2) .dsk .ima .img .ufi .360 .d77 .d88 .1dd .dfi .hfe .imd .ipf .mfi .mfm .td0 .cqm .cqi
harddisk (hard) .chd .hd .hdv .2mg .hdi
printout (prin) .prn
The harddisk option does have the ability to attache a chd formatted file to it along with a number of other formats but we will focus on chd in this document.