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Format Guide

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Claus Valca edited this page Aug 18, 2026 · 1 revision

📼 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.

CHD — The King 👑

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.

CSO / ZSO — The Mobile Pick 📱

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.

ECM — The Old School One 🕹️

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.

XISO — The Xbox One 🎮

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.

Which format should I use?

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

Size comparison (rough estimates)

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

→ Your First Conversion → Conversion Matrix

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