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Build multi-architecture 'fat' binaries for Windows.

Fatpack screenshot

What does it do

Fatpack combines Windows programs for multiple architectures or Windows versions into a single executable that runs on:

  • 32-bit Intel (XP and up)
  • 64-bit Intel (XP and up)
  • 64-bit ARM (10 and up)

To see it in action, download Fatpack from the releases page. It is a fat binary for 32-bit Intel, 64-bit Intel, and ARM (untested).


As an experiment, although it may turn out useful. Having a fat binary beats making non-technical users pick the most appropriate version for their computer, or having to distribute multiple different versions in the first place.

How does it work

Fatpack embeds the executables as resources in a 32-bit Intel loader stub. 32-bit Intel binaries are supported on 64-bit Intel through Windows on Windows 64, and on 64-bit ARM through emulation.

When launched, the loader tries each embedded executable in sequence until one works. Careful ordering of the packed executables assures that the most appropriate verion is run (see Packing order below).

The Fatpack packer tool itself embeds an empty version of the loader. When packing, it extracts it and uses UpdateResource to embed the executables.

How to use

  1. Download a release from the Releases page on GitHub, or download the and compile the source.
  2. Launch Fatpack.
  3. Click the Add button to add your binaries. Then use Move up and Move down to put them in the best order (see Packing order below)
  4. Click Pack, name the fat binary, and click Save.

Packing order

Fatpack's loader stub does not inspect and compare the current system architecture with the target architecture embedded binaries. This has a few reasons:

  • Simplicity of implementation.
  • Forward compatibility. For example, in the future Windows could shield emulated software from the architecture of the host sytem.
  • Flexibility of use. E.g. this method also allows for executables targeting different Windows versions to be combined (which the launcher stub wouldn't know to check).

For good results, put the least compatible binaries first. Some rules of thumb:

  1. ARM before Intel (32-bit Intel binaries would launch emulated on ARM).
  2. 64-bit before 32-bit.
  3. Executables requiring more recent Windows versions before those requiring older Windows versions (e.g. Windows 10-only before XP+).


If you have four versions of your program: a legacy 32-bit Intel version for Windows XP and up, 32-bit and 64-bit Intel versions for Windows 7 and up, and an ARM 64-bit version for Windows 10 and up, use the following order:

  1. ARM 64-bit
  2. Intel 64-bit (Windows 10 and up)
  3. Intel 32-bit (Windows 10 and up)
  4. Intel 32-bit (Windows XP and up)

On Windows on ARM, the launcer tries the first entry and succeeds, launching the correct ARM version. Had the 32-bit Intel version been tried before the ARM version, it would have launched in emulation (at a performance cost).

On Windows 7 on 64-bit Intel, the launcher fails to run the ARM and Windows 10 versions of the program, and launch the 32-bit XP version.

On Windows 10 on 64-bit Intel, the launcher fails to run the ARM version but then succeed running the 64-bit Intel version. Had the 32-bit Intel version come before, it would have been used instead (which would be suboptimal).


This is all a proof of concept. Use at your own risks. Some specific caveats:

  • Local DLL dependencies are problematic because DLLs support only one architecture but may be referenced by many in the packed binary.
  • The embedded binaries are extracted and run from a temporary directory which may confuse the program as wel as overeager virus scanners.
  • Future Windows editions may not support 32-bit Intel binaries at all, like Windows RT did, breaking this approach.

Future work

Some ideas:

  • Have Fatpack copy version information from one of the embedded binaries into the packed binary so it correctly reflects the product name, version, author, and so on.
  • Build the loader as a freestanding, position-independent executable so it runs even on Win32s in Windows 3.11.
  • Embed DOS versions in the DOS loader stub.
  • Embed appx (UWP) packages for Windows 10.

Author, bugs, etc

By Sijmen J. Mulder (

Use the Issues tab on GitHub to report bugs or feel free to email me directly.

Fatpack is licensed under the 2-clause "Simplified" BSD license. See


Build multi-architecture 'fat' binaries for Windows.







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