A CSO file, which stands for "Compressed ISO," is a compressed disk image format primarily associated with the PlayStation Portable (PSP) handheld console. It serves as a space-saving alternative to the standard ISO file format, which is a sector-by-sector copy of an optical disc. Developed to address the limited storage capacity of PSP's Memory Stick Duo cards, CSO files allow users to store more game backups on their devices. The compression typically employs the zlib algorithm, applied in blocks, which enables on-the-fly decompression by PSP emulators or custom firmware running on the actual console. While CSO files significantly reduce file sizes, the trade-off often includes increased game loading times and a potential, albeit minor, performance impact during gameplay due to the continuous decompression process. Despite these drawbacks, CSO remains a popular format for distributing and playing PSP game ROMs, especially within the emulation community. It is widely supported by various PSP emulators across different platforms, allowing users to enjoy their favorite PSP titles on computers and mobile devices.