The CSO format (short for Compressed ISO) was developed to address the PSP’s hardware constraints. A standard PSP game, when ripped from a UMD, exists as an ISO file—an exact, uncompressed replica of the disc. While perfect in quality, these files were often too large for the Memory Stick Duo cards of the era, which were expensive and offered limited capacity. The CSO format emerged as a method of compressing these ISOs, stripping away padding data and compressing the game’s assets. This allowed players to fit more games into a "full archive" on a single memory card, transforming the PSP from a device that carried one game at a time into a library that could hold dozens.
That said, the emulation community often focuses on — many games are out of print, unavailable digitally, and face disc rot. For abandoned titles, archives serve a historical role. Always check your local laws. cso psp archive full