ROM Formats Explained: CHD, CSO, and Compression

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Cartridge games are simple. One game, one file. Disc-based games are where it gets confusing, with formats like BIN, CUE, ISO, CHD, CSO, and PBP all floating around. Pick the wrong one and your game will not load, or it will eat far more SD card space than it needs. This guide explains the common formats in plain English, so you know what to use and why.

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Cartridge Games Are Easy

Cartridge-based systems use a single file with a system-specific extension.

SNES games end in .sfc or .smc.
Genesis games end in .md or .bin.
GBA games end in .gba.
N64 games end in .z64, .n64, or .v64.

These are already small and need no special handling. You can zip them to save a little space, and many cores read zipped cartridge games directly. Disc games are where formats matter.

Disc Games and Why Format Matters

CD and DVD-based systems like the PS1, Saturn, Sega CD, Dreamcast, PSP, and PS2 store games as disc images. A raw disc image is large, so the community uses compressed formats to shrink them without losing anything. The trick is using a format your emulator actually reads.

The Common Disc Formats

Here is what each one means.

BIN and CUE. The classic raw format. The .bin holds the actual data and the .cue is a small text file that describes the disc layout. You need both, and the CUE must point to the right BIN. A lone BIN with no CUE often will not load. This format is uncompressed, so it takes the most space.
ISO. A simpler single-file disc image, common for PSP and some other systems. Uncompressed.
CHD. The modern favorite. CHD compresses a disc image into one tidy file with no quality loss, and you can convert it back anytime. It saves a lot of space and is widely supported. For most PS1, Saturn, Sega CD, and Dreamcast games, CHD is the format to use.
CSO. A compressed format mainly for PSP games. It shrinks ISOs to save space. Support is good but slightly less universal than ISO, and heavy compression can cost a little performance.
PBP. A PSP-friendly container that can hold PS1 games, sometimes bundling multi-disc games into one file. Handy for PS1 on PSP-style setups.

What You Should Actually Use

Keep it simple with these rules of thumb.

PS1, Saturn, Sega CD, Dreamcast: convert to CHD. It saves space, keeps quality, and is well supported.
PSP: keep games as ISO if you have space, or CSO if you need to save room. Watch for a small performance cost with heavy CSO.
If a game will not load: check that you have all the parts. A BIN needs its CUE. A multi-disc game needs every disc file plus, sometimes, an M3U playlist file.

What Is an M3U File?

Multi-disc games like some RPGs need disc swapping. An .m3u file is a tiny text playlist that lists each disc in order. Point your emulator at the M3U and it treats the discs as one game, swapping automatically when prompted. It keeps your game list tidy and disc swaps painless.

How to Convert Formats

Converting is straightforward with the right tool.

To make CHD files, use a CHD conversion tool to turn a BIN/CUE or ISO into a CHD. The process is quick and reversible.
To convert CSO, use a PSP compression tool to turn an ISO into a CSO or back.
Always keep the format your emulator supports best for each system.

A Note on Space and Quality

The big win with CHD and CSO is space. A library of PS1 games in raw BIN/CUE can be huge, and CHD can cut that dramatically with no loss of quality. On a handheld where SD card space is precious, converting your disc games to CHD is one of the best things you can do. Our best microSD cards guide helps if you need more room.

Quick Reference

Cartridge games: single file, zip them if you like
PS1, Saturn, Sega CD, Dreamcast: use CHD
PSP: ISO for performance, CSO for space
BIN always needs its CUE
Multi-disc games: use an M3U playlist