This field is used to identify a CSO file. Little-endian is assumed for fields larger than a byte, following the architecture endianness of PlayStation Portable.Īlways CISO, or 0x4F534943 when read as a 32-bit integer. The file format consists of a 24-byte header, an index table, and data blocks. While CSO allows a tweakable block size, the official compressor does not allow changing it.

JSO ('JISO') was another now unpopular competing format that was overall similar to CSO, with the difference of added option for LZO compression and a block size tweakable in the official compressor. ĬSO are known to have received later extensions in the form of CSO 'v2' and ZSO ('ZISO'), using LZ4 compression for faster decompression.ĭAX is a competing format, it uses 8 KiB blocks which aims to increase the data compression ratio, and it also allows uncompressed areas just like CSO. In addition to being used for compressing PlayStation Portable UMD games, the PSP port of PicoDrive supports compressed Sega Mega-CD disc images in the CSO format. 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. 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. JSO was basically unused.ĬSO uses Deflate compression with nine levels of presets, usually with 2 KiB blocks handled individually (they could be uncompressed). However at the time, the popular application homebrew ' DevHook' used CSO.DAX needed another loader, and. Other formats tried to replace it by fixing up certain issues such as lag in games, such as. Booster created the first code it was later optimized for better compression. It was created so that more memory space can be saved. It was the first compression method for ISO. It is used to compress dumped PlayStation Portable UMD games, and is an alternative to the. CSO is a compression method for the ISO image format.

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