If you already use PCSX2 (v1.7 or newer), you do not need separate software. The emulator includes a built-in ISO-to-ZSO converter.
maxcso --format=zso game.iso
You might have heard of (Compressed ISO), the older standard for PSP emulation. iso to zso converter
Older versions of OPL do not recognize the .ZSO extension. You must use OPL v1.2.0 Beta/Stable or newer, or customized forks like OPL GrimDoomer edition. If you already use PCSX2 (v1
| Format | Compression Algorithm | Typical Compression Ratio | Best Use Case | Decompression Speed | | :--- | :--- | :--- | :--- | :--- | | | None (Raw) | 0% | Archiving, compatibility | N/A (No decompression) | | CSO | DEFLATE | 40–60% | Older PSP emulators, maximum space saving | Slow to Medium | | ZSO | LZ4 | 30–50% | OPL, PS2/PSP emulators, performance‑sensitive devices | Extremely Fast | | CHD | LZMA / FLAC | 50–70% | Modern emulators (MAME, Dolphin), archiving | Slow (Higher CPU usage) | Older versions of OPL do not recognize the
Fast, supports batch processing, cross-platform (Windows, macOS, Linux).
, an algorithm designed by Facebook (now Meta) for speed. Zstd decompresses significantly faster than zlib while often achieving smaller file sizes.