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PSP: ISO vs. CSO — Which Format Should You Use?

Both ISO (uncompressed) and CSO (compressed) work on a hacked PSP, but CSO is generally preferred for most users.

Comparison

Feature	ISO	CSO
File Size	Full UMD size (up to 1.8 GB)	30–70% smaller (compression varies)
Compression	None	Lossless (zlib-based)
Load Times	Fastest (direct read)	Slightly slower (decompression overhead)
Battery Life	Better (less CPU work)	Slightly worse (CPU decompresses)
Storage Efficiency	Poor	Excellent
Compatibility	100%	~99% (rare edge-case bugs)

Recommendation

Use CSO for: Most games, especially large RPGs (FF Tactics, Persona 3, Monster Hunter). Saves massive space on Memory Stick Pro Duo / microSD adapter.
Use ISO for: Games with known CSO issues (streaming audio/video hiccups), or if you have abundant storage and want absolute fastest loads.

Conversion Tools

PSP ISO Compressor (Windows) — batch convert ISO → CSO / CSO → ISO.
UMDGen — advanced: rip, rebuild, optimize, dummy-file removal.
cso / maxcso (command line) — Linux/macOS.

Prerequisite: Custom Firmware (CFW) like 6.61 PRO-C2 / ME / Infinity, or HEN. Official firmware (OFW) cannot run ISOs/CSOs.

Bottom Line

Default to CSO. The load-time difference is negligible on real hardware (often <1 sec), while storage savings are huge. Keep ISOs only for the few titles that misbehave as CSO.

PSP Games