How to make a CSO (Compressed Image file)
This tutorial will show you how to make a CSO (Compressed Image file / .cso) If you don’t have an iso image of your PSP UMD game please see this tutorial -> How to back-up a PSP UMD to .iso format
Using UMDGen. For this example I’m going to use Metal of Honor Heroes 2 which I just converted to an ISO from my UMD disk. Open UMD gen, then click open and browse to your psp iso image. Select that, UMDGen will load the UMD up. See below, I have Metal of Honor Heroes 2 loaded. Note the file size is 1169.94Mb in total. Lets see if we can slim that down a few megabytes….
Now in UMDGen lets click the properties tab. You’ll see various UMD info. Ignore all that what we want to do is click Optimize image, this will remove the update folder and any padding on the image. (padding out the data helps UMD’s load faster). In my case here we saved 4.05% on the disk size. Not much I know but with some games this can be over 15%.
Now its time to compress this iso into a cso, now click the save tab and select .cso and name your file and select the save location. I’m going to go with a compression level of 9 here. If it lags, reduce this value. My Medal of Honor iso, cso’ed is now just 357mb! thats a huge saving in size from 1169.94Mb. Now I can place more games on my memory stick than if they were iso files.