Convert DAX to CSO

A PSP emulator or custom firmware may support .cso images but not the older .dax format. Converting the image preserves the disc data while moving it into a container supported by PPSSPP and most current PSP software.

What the DAX format is

DAX is a compressed image format for PlayStation Portable UMD discs. It stores an ISO disc image in compressed blocks and was used by older PSP custom-firmware loaders and homebrew utilities.

DAX files commonly come from PSP ISO Compressor and older DAX-capable PSP tools. Modern emulators and firmware generally support standard ISO and CSO images more consistently than DAX.

What the CSO format is

CSO, also called CISO, is a compressed ISO format widely used for PSP game images. It divides the disc image into blocks, compresses the blocks, and retains an index for random access.

CSO normally uses less storage than an uncompressed ISO while preserving the original disc-image data. Excessive compression can increase loading delays or cause stuttering on some real PSP models; moderate compression is a safer default.

How to convert the file

PSP ISO Compressor 1.4 is the most practical desktop option for this job. It is an older Windows utility, so run it locally and obtain it from a reputable archive or software source. Its menus vary slightly between releases.

1. Copy the original game.dax to a backup location. Keep enough free space for an extracted ISO and the resulting CSO.
2. Open PSP ISO Compressor and choose its DAX conversion or decompression function, usually under Conversion or a source/output format selector.
3. Select game.dax as the source and create game.iso. If the program offers a direct DAX-to-CSO mode, select CSO as the output instead.
4. If conversion produced an ISO, run the utility again, choose ISO-to-CSO, select game.iso, and save the result as game.cso.
5. Start with a moderate compression setting and do not overwrite the DAX or intermediate ISO until testing is complete.
6. Open the CSO in PPSSPP, or copy it to the PSP memory card's /ISO/ directory, then test booting, loading, and in-game data access.

A two-stage conversion is a format change rather than a media-quality conversion: DAX is decompressed to ISO, then the same disc data is compressed into CSO. If PSP ISO Compressor cannot complete the second stage, maxcso can create a CSO from the extracted ISO on Windows, macOS, or Linux; it should not be treated as a general DAX decoder.

Online converters and alternatives

Online conversion is generally impractical for PSP disc images. Files are often hundreds of megabytes or larger, browser services may impose upload and processing limits, and uploading a copyrighted or private disc image creates avoidable security and legal risks. No widely established online service is a better choice than local PSP-specific software for DAX input.

Compatibility and quality limits

The conversion is normally lossless. It changes compression and container structure, not the audio, video, or game data. Two CSO files may differ byte-for-byte because compression levels, block layouts, and index structures differ.

Keep the DAX until the CSO has been verified. A damaged source can yield an image that opens but fails during loading. If PPSSPP reports read errors, recreate the file from the original DAX, try a lower compression level, and test the extracted ISO before compressing it again.

CSO is usually more compatible than DAX, but some hardware performs better with an ISO, especially when the CSO uses aggressive compression. If gameplay stutters or loading becomes unusually slow, recreate the image with moderate compression or use the ISO instead. Convert only disc images you are legally entitled to possess and use.