Disc Image Formats — ISO, BIN/CUE, CSO, ZSO and CHD, and When to Use Each
Work with retro game files for any length of time and you will meet image files with a bewildering assortment of extensions. Some are plain copies, some are compressed, and some come as a set of several files. This article sorts out the differences and the criteria for choosing between them.
At a glance
| Format | Compressed | Mainly used for | Characteristics |
|---|---|---|---|
| ISO | No | Data-only discs | The simplest possible copy of the original |
| BIN/CUE | No | CDs with audio tracks | The .cue describes the track layout |
| CSO | Yes | PSP | Smallest file size |
| ZSO | Yes | PSP | Larger than CSO but loads faster |
| CHD | Yes | Emulators generally | Bundles every track into one file |
Why ISO and BIN/CUE are separate things
ISO is a straight copy of a disc that contains nothing but data. One file, simple structure, done.
BIN/CUE is what you use when the CD holds audio tracks alongside the data. Older games very often put their background music on the disc as CD audio. Such a disc has multiple tracks, which is awkward to express as a single file.
.bin— the actual data (sometimes one per track).cue— a text file recording how many tracks there are, what format each is, and where each begins
Open a .cue in a text editor and you will see something like this:
FILE "game.bin" BINARY
TRACK 01 MODE2/2352
INDEX 01 00:00:00
TRACK 02 AUDIO
INDEX 00 03:12:00The important part: a .cue is documentation, not game data. When you apply a patch or run a conversion, the real target is the .bin. Conversely, throw the .cue away and keep only the .bin, and a game with audio tracks may end up with no music.
Compression for the PSP — CSO and ZSO
PSP memory sticks are not generous with space, so storing disc images compressed is common practice.
CSO compresses with deflate. The compression ratio is good and files shrink substantially, but the data has to be decompressed every time the game reads it, which weighs on loading times.
ZSO compresses with LZ4. The ratio is lower than CSO so files are somewhat bigger, but decompression is far faster, which makes loading noticeably more comfortable.
To put it plainly:
- Memory stick space is tight → CSO
- Loading speed matters → ZSO
- Space is plentiful and you want the original untouched → ISO
The PSP format converter moves between all three. Converting a compressed image back to ISO produces a byte-for-byte identical file, so you can go back and forth freely as needs change.
Compression for emulators — CHD
CHD is a compressed container that came out of the MAME project and is now supported broadly across emulators.
The biggest nuisance with BIN/CUE is that the files scatter. A game with twenty tracks can leave you with twenty .bin files. CHD folds all of that — track layout included — into a single file, and compresses it along the way.
- File management gets simpler
- Size goes down
- Track information lives inside the file, so no separate
.cueis needed
You can convert with the BIN/ISO → CHD tool. Compression levels run from 0 to 9, and the default of 6 is the balance point. Level 0 only bundles without compressing, so the size does not drop.
Upload several discs at once and you get one CHD per disc.
What order should I work in?
If you intend to apply a translation patch and then run the game on a PSP or an emulator, the order matters.
- Get the original — as BIN/CUE or ISO
- Apply the patch — to the
.bin, with the ROM patch tool - Convert the format — to suit the destination: CSO/ZSO (PSP), CHD (emulators), EBOOT.PBP (PS1 games on a PSP)
Do not try to patch after compressing. A patch is written against byte positions in the original, and a compressed image has a completely different structure. Always patch before you compress.
Confirming a file is sound
Drop a file into the file inspector and it identifies the format automatically and shows you what is inside.
- ISO/BIN — sector size, disc ID, track layout
- CSO/ZSO — header information, block size, compression ratio
- CUE — the track list and the files it references
- Unrecognised formats — a hex dump of the opening bytes
When a file you downloaded turns out to be a different format than its name suggests, or is simply damaged, this tells you where the problem actually is before you go any further.
Summary
- The
.cueis the documentation; the.binis the data - For the PSP: CSO for size, ZSO for speed
- For emulators, CHD is the easiest to live with
- Keep to the order: patch, then convert
Detailed instructions for the individual tools are in How to Apply a Translation Patch and Running PS1 Games on a PSP.