Convert CSO to DAX

Some legacy PSP custom-firmware loaders and older emulators accept a DAX image but not a compressed CSO file. Converting the image preserves its disc contents while recompressing it in the format required by that software.

What the CSO format is

CSO, also called CISO, is a compressed ISO image format commonly used for Sony PlayStation Portable games and UMD images. It compresses the ISO in blocks while preserving the disc image's filesystem and data structure.

CSO files are encountered with PSP custom firmware, PSP emulators, and UMD-image archives. Utilities such as maxcso, UMDGen, CISO tools, and PSP ISO Compressor can create or read CSO files. The usual extension is .cso.

What the DAX format is

DAX is an older compressed PSP ISO format associated with custom firmware, homebrew loaders, and legacy conversion utilities. It stores a PSP disc image in compressed blocks and normally uses the .dax extension.

DAX is useful only when the destination loader or emulator specifically supports it. Its support is much narrower than ISO or CSO support, and decompression or random access can be less efficient on some hardware. DAX does not provide a universal compression advantage: the final size depends on the source image and compression settings.

How to convert CSO to DAX

PSP ISO Compressor is the most practical desktop choice because it can normally perform both stages in one Windows application. Menu names differ between releases, so select the operations by their format names rather than relying on an identical interface.

1. Keep a backup of the original game.cso and confirm that the destination drive has enough free space for a full, uncompressed ISO.
2. Open PSP ISO Compressor and choose its CSO decompression operation, commonly labeled CSO → ISO.
3. Select game.cso as the input and save the intermediate image as game.iso.
4. Choose the ISO compression operation labeled ISO → DAX, select game.iso, and choose a compression level if the program offers one.
5. Save the result as game.dax. Open it in the target loader or emulator and test gameplay before deleting the ISO or original CSO.

If the installed version exposes a direct CSO → DAX option, it performs the same decompression and recompression internally. DAX Creator is an alternative for the second stage: first convert the CSO to game.iso with a CSO-capable utility, then use DAX Creator's ISO → DAX function. UMDGen can handle PSP image formats in some releases, but its available import and export options vary; verify that the installed version explicitly offers DAX output before relying on it.

maxcso is a suitable offline tool for decompressing CSO to ISO on Windows, macOS, and Linux. Use the decompressed .iso with a DAX-capable utility for the second stage; maxcso should not be treated as a complete modern CSO-to-DAX converter because its normal output workflow targets CSO rather than DAX.

Compatibility and quality limits

This conversion is lossless at the disc-image level. It changes compression, not the game files, graphics, or audio, and it cannot repair a damaged CSO or remove encryption. A corrupt source can produce a DAX file that fails to boot.

Current PSP software, including many PPSSPP installations, generally favors ISO and CSO, while DAX support may be absent, experimental, or slower. Check the exact loader or emulator documentation before converting. Keep an ISO as the archival master and retain the original CSO until the DAX image has been tested.

Online conversion is usually unsuitable for this task. PSP images are large, DAX is a legacy format that many web services do not encode, and uploading a disc image creates unnecessary privacy, copyright, and integrity risks. Use an offline converter obtained from a reputable software archive or project source instead.