CISO File

What are CISO files and how to open them

Are you having problems opening a CISO file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CISO files.

What is a CISO file?

A .CISO file is a CISO Compressed ISO CD Image file.

The CISO Compressed ISO CD Image is a file format for storing optical disc images in a compressed form so that they take less space on a storage device. It was created for the PlayStation Portable to hold full UMD game disc images more compactly, yet allow games to be played without fully decompressing the image to a separate ISO file. These images are commonly used by PSP firmware with homebrew support and by PSP emulators.

The format is produced by compression utilities (notably the original ciso tool by Booster or its modern variants like maxcso). It works by splitting a standard ISO CD/DVD image into equal-size blocks (sectors) and compressing each block separately using the DEFLATE algorithm. If a particular block does not compress smaller than the raw data, it is stored uncompressed. When the compressed image is used (or loaded by firmware/emulator), only the necessary blocks are decompressed on demand during gameplay, saving RAM and I/O operations. This block-based compression also enables use of higher compression levels (1 through 9) where higher numbers trade speed for smaller size.

Support for compressed ISO images of this type usually requires a tool or system that recognizes the format's header and block index table. PSP firmware versions with custom or homebrew support and most modern PSP emulators can mount or read the compressed image directly, presenting it as though it were a normal uncompressed ISO. To revert to the full-size ISO, the same tools that created the compressed image can decompress it.

A file of this format always begins with the ASCII signature "CISO" (bytes 43 49 53 4F at offset 0). The header is 24 bytes long and includes fields such as header size, uncompressed total size, block size (commonly 2048 bytes, matching ISO-9660 sector size), and a version identifier. The header is followed by an index table, then a sequence of compressed or raw data blocks. Any block exceeding the raw size threshold is flagged in the index for raw storage.

How to open CISO files

Important: Different programs may use files with the CISO file extension for different purposes, so unless you are sure which format your CISO file is, you may need to try a few different programs.

We have not yet verified any programs for Windows that work with this specific file format. If you know one, please use the 'Suggest a program' link below. Thanks!

Last updated: August 31, 2026

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