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A fast ISO to CSO compression program for use with PSP and PS2 emulators, which uses multiple algorithms for best compression ratio.

Basic usage

maxcso myfile.iso

Or, drag the iso file into maxcso.exe on Windows.


Get the latest release from the GitHub releases. Use maxcso.exe on modern systems, and use maxcso32.exe on Windows XP and other 32-bit versions of Windows.


  • Can use as many CPU cores as you want.
  • Can use zlib, 7-zip's deflate, and Zopfli.
  • Processes multiple files in one command.
  • Can take a CSO or DAX file as a source.
  • Able to output at larger block sizes.
  • Support for experimental CSO v2 and ZSO formats using lz4 (faster decompression.)
  • Tuning of deflate or lz4 compression threshold.
  • Decompression of all supported inputs (including DAX and CSO v2.)


maxcso always uses compression level 9. Decompression speed is about the same regardless of level, and disk access is faster with smaller files.

Using 7-zip's deflate and Zopfli improves compression ratios, but don't expect a lot. Usual results are between 0.5% to 1.0% smaller.

Larger block sizes than the default will help compression, in the range of 2-3%. However, the files may not be compatible with some software. For example, PPSSPP versions released after 2014-10-26 will support larger block sizes.

Avoid DAX where CSOs using larger block sizes are supported, since DAX is less efficient.

LZ4 support is mostly for experimentation.


Compared to other tools like ciso and CisoPlus, maxcso can run much faster and achieve the same compression. Use --fast to get the fastest compression, which matches level 9 in other tools.

Additionally, if you have better than a dual core processor, maxcso will use all of your cores, and perform even better.

In usage, CSOs typically perform well in all known emulators. Some versions of PSP firmware with support for CSOs have bugs in their CSO support, but this doesn't affect emulators.

Full program usage

Usage: maxcso [--args] input.iso [-o output.cso]

Multiple files may be specified.  Inputs can be iso or cso files.

   --threads=N      Specify N threads for I/O and compression
   --quiet          Suppress status output
   --crc            Log CRC32 checksums, ignore output files and methods
   --measure        Measure compressed size without saving output
   --fast           Use only basic zlib or lz4 for fastest result
   --decompress     Write out to raw ISO, decompressing as needed
   --block=N        Specify a block size (default depends on iso size)
                    Many readers only support the 2048 size
   --format=VER     Specify cso version (options: cso1, cso2, zso, dax)
                    These are experimental, default is cso1
   --use-zlib       Enable trials with zlib for deflate compression
   --use-zopfli     Enable trials with Zopfli for deflate compression
   --use-7zdeflate  Enable trials with 7-zip's deflate compression
   --use-lz4        Enable trials with lz4hc for lz4 compression
   --use-lz4brute   Enable bruteforce trials with lz4hc for lz4 compression
   --use-libdeflate Enable trials with libdeflate compression
   --only-METHOD    Only allow a certain compression method (zlib, etc. above)
   --no-METHOD      Disable a certain compression method (zlib, etc. above)
                    The default is to use zlib and 7zdeflate only
   --lz4-cost=N     Allow lz4 to increase block size by N% at most (cso2 only)
   --orig-cost=N    Allow uncompressed to increase block size by N% at most
   --output-path=X  Output to path X/, use basename for default outputs

Because Zopfli is significantly slower than the other methods, and uses a lot more memory, it is disabled by default. Add --use-zopfli for maximum compression.

The cost arguments enable you to allow each block to be N% bigger by using lz4 or no compression. This makes the file read faster (less cpu power), but take more space.


maxcso has been tested on Windows, macOS, and Linux so far. The code was written to be portable. If you'd like to port it to another platform, pull requests are accepted. It may just compile without any changes.


To build on Windows, simply open cli/maxcso.sln and build. Visual Studio 2017 or higher is required.

Mac OS X

Aside from gcc/g++ or clang (from Xcode or brew), you will also need:

brew install lz4
brew install libuv
brew install libdeflate

And then just compile using make.

Linux / Unix

Aside from gcc/g++ or clang, you will also need liblz4-dev, libdeflate-dev, and libuv1-dev - or similar.


Community provided packages are available on some platforms under "maxcso". Please confirm version and security before using. Thanks should go to their respective maintainers.

Credits and licensing

The larger portion of code here is from others' wonderful work in decompression and I/O libraries. Licensing is as follows:

  • maxcso is licensed under ISC.
  • 7-zip and p7zip are licensed under LGPL.
  • Zopfli is licensed under Apache 2.0.
  • libuv and libdeflate are licensed under MIT.
  • zlib is licensed under zlib.
  • lz4 is licensed under BSD.

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