Free online CSO to ISO converter

Some PSP emulators, mounting tools, disc-image utilities, and archival workflows require an ISO instead of a compressed CSO file. Converting it restores the uncompressed image, but the ISO needs more storage space.

What the CSO format is

CSO, also called CISO, is a compressed ISO disc-image format developed for PlayStation Portable software. It stores the source image in compressed blocks, reducing file size when the original data compresses well.

PSP emulators, PSP hardware, and utilities such as maxcso and ciso commonly support CSO files. A CSO is useful for reducing storage requirements, but some general-purpose disc-image programs and devices recognize only ISO.

What the ISO format is

ISO is a sector-based image of an optical disc or other disc layout. It can preserve the source image's files, directory structure, boot data, and sector contents.

ISO has broader software support than CSO. Operating systems can often mount it directly, while disc-image utilities can inspect or burn it and emulators or devices that lack CSO support can usually open it. An ISO is generally an uncompressed image, so it is normally larger than the CSO from which it was created.

How to convert CSO to ISO

Keep the original CSO until the ISO has been verified. Decompression requires enough free space for the complete output image.

Using maxcso

1. Install maxcso from its official project distribution for your operating system.
2. Open a terminal or command prompt in the directory containing the CSO file.
3. Run maxcso --decompress game.cso -o game.iso.
4. Replace the input and output names with the actual paths, then wait for the command to finish.
5. Check that the ISO exists and has an expected size before deleting or replacing the CSO.

Quote paths containing spaces, for example maxcso --decompress "My Game.cso" -o "My Game.iso". The --decompress option reconstructs the ISO image rather than compressing it again.

Using 101convert

You can convert the file for free on 101convert.com without installing software. Upload the CSO, choose ISO as the output format, start the conversion, and download the resulting file when processing finishes. For sensitive or irreplaceable images, use a local converter instead of uploading the file.

Quality and compatibility limitations

CSO decompression is normally lossless: it restores the data represented by the compressed image and does not intentionally alter the disc contents. It cannot repair a damaged or incomplete CSO, remove encryption, bypass copy protection, or make software compatible with a device that has other requirements.

The converted ISO may be substantially larger than the CSO. A PSP emulator that already supports CSO does not need a conversion; convert only when a specific program or device requires ISO, or when mounting, inspection, or uncompressed-image archiving is needed.

When a known checksum is available, compare it with the converted file. Otherwise, verify that the ISO mounts, opens in the intended emulator or utility, and contains the expected image data.