A clean PSP archive structure:

Contains the "CISO" magic number, the total uncompressed size, and the index size.

Many users maintain a "complete PSP archive" as a personal backup. While a full PSP ISO Redump set exceeds 1.2 TB, a well-compressed CSO set can shrink to under 600 GB—far more manageable.

A CSO file is structured into several key components that allow the PSP to read data on the fly:

Many archives also host fan-translated titles that never left Japan, giving western players access to "new" retro experiences. How to Use a CSO Archive

: A "PSP archive" generally refers to a collection of PSP game backups (ISOs or CSOs), save data, or firmware updates. A CSO is the compressed format of a PSP ISO.

✅ Smaller file sizes (more games per card). ✅ No loading times from physical discs. ✅ Easy transfer to emulators (PPSSPP) on mobile/PC.