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Format Guide
Not all ROM formats are created equal. Here's everything you need to know about each one — and which to use for your situation.
What is it? CHD (Compressed Hunks of Data) is MAME's compression format. It's lossless, meaning your game data is 100% intact — just smaller.
Why use it?
- Typically 30–50% smaller than the original ISO
- Supported by most modern emulators (PCSX2, Dolphin, PPSSPP, RetroArch)
- Single file — no more BIN + CUE juggling
Best for: PS1, PS2, GameCube, Dreamcast
Can I convert back? Yes. CHD → ISO/BIN is fully supported.
What is it? CSO (Compressed ISO) was made for PSP games. ZSO is a newer variant with better compression.
Why use it?
- Smaller than CHD for PSP games specifically
- Works directly on PPSSPP without extraction
Best for: PSP games on mobile emulators
Can I convert back? Yes. CSO/ZSO → ISO works fine.
What is it? ECM (Error Code Modeler) strips redundant error correction data from PS1 discs to save space.
Why use it?
- Very small file size for PS1 games
- Was popular before CHD became mainstream
Honestly? CHD is better in 2026. ECM is mostly useful if you're sharing files where CHD isn't supported.
Best for: PS1 games (legacy use)
Can I convert back? Yes. ECM → ISO/BIN auto-detected by OpenROM.
What is it? XISO is the native Xbox disc format, stripped of the dead zone padding that Microsoft added to original discs.
Why use it?
- Much smaller than raw Xbox ISOs (sometimes 30–40% smaller)
- Required by most Xbox emulators (Xemu)
Best for: Original Xbox games
Can I convert back? Yes. XISO → ISO supported.
What console is it?
│
├── PS1 ──────────────────→ CHD (or ECM if size is critical)
├── PS2 ──────────────────→ CHD
├── PSP ──────────────────→ CSO (mobile) or CHD (desktop)
├── GameCube / Wii ───────→ CHD
├── Dreamcast ────────────→ CHD
└── Original Xbox ────────→ XISO
| Format | Relative Size | Notes |
|---|---|---|
| Original ISO | 100% | Baseline |
| CHD | ~50–70% | Lossless |
| CSO | ~50–65% | Lossy-ish for PSP |
| ECM | ~60–80% | PS1 only |
| XISO | ~60–70% | Xbox only |