This instructable will teach you how to compress an .iso file to a .cso file. Using .cso files saves space on your PSP. This can only be done if you are running custom firmware.
This is only for backups of a PSP game you OWN and not DOWNLOADED.
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Step 1: Obtain a .ISO
To obtain a rip the umd of the game you own and then archive it as a .iso. I suggest using Active ISO File Manager 2. This can be downloaded for free at.
Here is a tutorial that I wrote that will help you better understand how to use Active ISO File Manger 2
I am using a copy of Star Wars Battlefront 2. (That I own)
Step 2: Download PSP ISO Compresser V1.4
PSP ISO Compresser v1.4 can be downloaded for free at
Step 3: Convert
Select 'Compress ISO->CSO' on PSP ISO Compresser v1.4. Now input the location of your .iso and press compress.
Step 4: Now Play!
Just place the .cso into your ISO folder in your PSP. Now go to the game selection on your PSP and play it.