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CSO is a compression method for the ISO image format. It is used to compress the file sizes for image files taken of the Playstation Portable UMD disks. This format came to be as a rival of the DAX format formally found for compressed UMD images.
The DAX format was once the most popular format to find UMD images in, but with the success of the Devhook homebrew application which favors the CSO standard, CSO has become more popular. CSO images often take longer to load due to the system's need to decompress the image file every time it needs to access a new part of the UMD.
CSO provides nine levels of compression. While the highest levels of compression can introduce slowdown and lengthy load-times in software which relies heavily on disc streaming, even the lower levels are capable of substantial compression. This is partially due to the data layout of a UMD, though more frequently due to the use of Dummy Files as both an anti-piracy tool and a means to more optimally lay the data out physically on the disc.