it is called Compact Disc File System as per ISO 9660
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PSP roms are in ISO format. They are in the ISO 9660 standard.
However, it can be compressed to CSO, DAX, or JSO, proprietary
formats developed for the PSP
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It stands for International Organisation for Standardization (or
the incorrect form: International Standards organisation) the .ISO
file type refers to ISO 9660, a standard for DVD and CD file
formats, standardized by the ISO, therefore it is .ISO
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DVDs primarily use the UDF file system, although it is possible
to use the ISO 9660 file system as well.