CSO

CSO is a type of compression method used specifically on ISO disk images.

The format is used normally to compress raw images of PlayStation Portable UMD games, and is an alternative to DAX compression.

CSO was the first of many compression methods for ISO. Created by Booster, CSO was notably used in the now outdated application homebrew loader DevHook.

CSO has nine levels of compression but using the higher levels of compression will noticeably slowdown gameplay (lag) or introduce lengthy load times.

Less usefully, CSO also supports compressed Sega Mega-CD disc images.