CSO is used to compress dumped PlayStation Portable UMD games, and is an alternative to the DAX compression method. It is also sometimes called CISO.

CSO files are compressed ISO image files. They are often used as a medium to play psp games. CSO files may include up to nine levels of compression. While the highest levels of compression can introduce slowdown and lengthy load-time in software which relies heavily on disc streaming, even the lower levels are capable of substantial compression.

In addition to being used for compressing PSP UMD games, the PSP port of PicoDrive supports compressed Sega Mega-CD disc images in the CSO format.

The easiest way to open a CSO file is to double-click the file and let the PC decide which default application should open it. If it does not work out, your PC may lack an application which can open or edit CSO files.

You can try the following programs to open CSO files on your computer.

PSP ISO Compressor

CisoPlus

UMDGen

You can also try to use Notepad or another text editor to open the CSO file. Many files are text-only files meaning no matter the file extension, a text editor may be able to properly display the file's contents.

Warning: Take great care when opening executable file formats received via email or downloaded from websites you're not familiar with.

CSO stands for "Compressed ISO." It is a format used by the game console Portable Sony Playstation, or the PSP. This format is simply a compressed CD image. In order to use this format on your computer, you'll have to decompress the format into "regular" ISO, so that it can be used by a program that can run the ISO format.

Instrctions

Step 1

Visit a website that has a PSP Compressor available for download.

Step 2

Download the program and open it.

Step 3

Click on the button with the three dots on it ("...") next to the box labelled "Input File(s)." Find your CSO file and double-click on it.

Step 4

Click on the circle on the left next to the text that reads "Uncompress CSO-ISO."

Step 5

Then click "Compress" at the bottom of the program to begin the conversion.

If you want to play games on PSP, you should first learn to install CSO files on PSP. The following instrctions can help you to install CSO files on PSP.

Instrctions

Step 1---Install Custom Firmware on PSP

First of all you need to install custom firmware on your PSP. Download Custom Firmware.

Step 2---Download PSP Games (CSO File)

Once you’ve installed the custom firmware on your PSP, download your desired PSP Game (ISO/CSO). Google is your best friend. If the downloaded PSP game is a compressed .zip or.rar file then make sure to extract iso/cso using WinRAR.

Step 3---Copy CSO on PSP

• Connect your PSP to your computer via data cable. On your PSP, go to Settings > USB Connection.

• Your PSP memory stick will appear as a removable device on your computer.

• Open the memory stick and go to ISO folder. If there’s no ISO named folder then create one and make sure you name it in Caps (ISO not iso).

• Now copy the downloaded PSP Game ISO / CSO file into ISO folder.

• Disconnect your PSP from your computer.

Step 4---Configurations

• Turn your PSP completely OFF by holding down the power button for 7 seconds.

• Hold down the R button and turn your PSP ON to boot into Recovery Mode.

• Go to Configuration and make the following changes:

      o Game folder hombrew (**currently: 5.xx Kernel**)
  
      o UMD Mode (**Currently: M33 driver -NO UMD-**)
  

• Go back to Main Menu then Advance > Advance Configuration.

      o Enable Plain modules in UMD/ISO
  
      o Disable “Execute BOOT.BIN in UMD/ISO”
  

• Go back to main menu and select Exit. Your PSP will Reboot and turn back ON.

Step 5---Play the game on PSP

From the PSP home Menu, go to:

Game > Memory Stick

and play your desired PSP game (ISO/CSO) on Custom firmware.