A compressed ISO (CSO) file is used with a Sony PSP as a backup copy of many retail games. An ISO file is the type of file that is initially created when a PSP game is backed up to your computer or to the PSP's memory stick. PSP ISO files range in file size and can be as large as 1.8 GB. Compressing the ISO file into a much smaller size creates a CSO file. CSO files are CD images that have been compressed, to make room for more information on the PSP's memory card.
Download "PSP ISO Compressor" from one of the many software download mirrors, such as Softpedia. PSP ISO Compressor is shareware, which means it is free.
Double-click the "PSP ISO Compressor" file that you downloaded and click "Next" on the installer window that comes up.
Click "Install" and the program is installed on your computer. Click the "Launch PSP ISO Compressor" option that comes up when installation reaches 100%. Click "Finish" to open PSP ISO Compressor.
Click the "Compress ISO->CSO" option on the PSP ISO Compressor's window. Click the "Compression Level" option and click the "9 - (Best Compression)" option when the context menu comes up.
Click the "Input File(s)" option on the PSP ISO Compressor window and click the "ISO" or "CSO" file that you want to compress to a much smaller size.
Click the "Browse to output folder after compression" option so the PSP ISO Compressor opens the folder that it saves the new CSO file in.
Click the "Compress" option on the PSP ISO Compressor window and the CSO file is compressed into a much smaller version.