Convert CSO to JSO

A JSO image is needed when a PlayStation Portable emulator such as JPCSP accepts JSO but the available dump is CSO. The conversion changes the compression container, not the game content or platform, and is normally done offline because PSP images are large and may contain copyrighted data.

What the CSO format is

CSO, also called CISO, is a compressed ISO image made from a PlayStation Portable UMD dump. It divides the ISO into blocks and compresses them to reduce storage use. CSO is commonly used with custom PSP firmware and emulators such as PPSSPP; support is not universal on all PSP firmware or image tools. Files normally use the .cso extension.

What the JSO format is

JSO is another compressed PSP ISO container, associated mainly with the JPCSP emulator. Like CSO, it reduces the image size while retaining the original disc-image data. JSO is not a general replacement for CSO: convert only when the receiving emulator or application explicitly supports .jso.

Convert CSO to JSO with maxcso

maxcso is the most practical current command-line option when its release build includes JSO support. Download a release from the official maxcso project, place the executable in the directory containing the image, or add it to the system PATH. Confirm the available formats before starting:

maxcso --help

For builds that support direct transcoding from compressed input, run:

maxcso --format=jso game.cso -o game.jso

If that build cannot use a CSO as input for JSO output, decompress first and then recompress:

maxcso --decompress game.cso -o game.iso
maxcso --format=jso game.iso -o game.jso

Option names and supported formats can vary between releases, so use the executable's help output rather than assuming that an older command line applies. Keep the original file until the new image has been tested.

Use a Windows graphical converter

Older Windows utilities such as PSP ISO Compressor can handle CSO, ISO, and JSO on versions that include JSO in their format list. Open the program, choose its conversion or compression function, select the .cso source, set the output type to JSO, choose a filename ending in .jso, and start the conversion. Verify that the program lists JSO as an output format; changing the extension alone does not convert the file.

UMDGen is not a dependable CSO-to-JSO converter. It is primarily a PSP ISO and UMD-image editing tool, and its format support depends on the version. Use it only if the installed version explicitly offers JSO output.

Online conversion

There is no widely established online service with dependable CSO-to-JSO support. Generic file-conversion websites commonly accept neither PSP image format nor large uploads, and uploading a game dump can disclose private or copyrighted material. An offline converter is the appropriate choice; do not trust a site that merely renames .cso to .jso.

Compatibility and verification

PPSSPP generally supports CSO directly, so converting to JSO may provide no benefit when PPSSPP is the target. JPCSP is the usual reason to create JSO, but support can vary by emulator version and configuration. Test the converted image in the exact application that requires it.

Recompression changes the file bytes and usually the file size, so CSO and JSO hashes will not match even when conversion is correct. For a stronger content check, decompress both images to ISO and compare the resulting ISO hashes, provided the tools produce equivalent raw images. Otherwise, verify that the target emulator boots the game and that loading, audio, and savedata work normally. Preserve a verified CSO or ISO backup until testing is complete.