FEX allows you to run x86 applications on ARM64 Linux devices, similar to qemu-user and box64. It offers broad compatibility with both 32-bit and 64-bit binaries, and it can be used alongside Wine/Proton to play Windows games.

This project is in the news because of a Verge interview (archive) where someone at Valve explained that they’ve been quietly backing this project from the beginning:

Pierre-Loup Griffais: In 2016, 2017, there was always an idea we would end up wanting to do that, and that’s when the Fex compatibility layer was started, because we knew there was close to a decade of work needed before it would be robust enough people could rely on it for their libraries. There’s a lot of work that went into that.

It all started with the same assumption that you’re going to get the same experience on Arm, you’re going to have the same set of games, and you’re going to be able to run them without having to worry about what architecture your computer is using. That was really our goal, to try to reduce barriers for users not having to worry about what games run and for developers to get a starting point for those new devices.

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The Fex emulator’s sole purpose is to provide compatibility with x86. So it takes the x86 code, and uses a just-in-time translator to emit Arm code that does the exact same thing. Proton built for Arm support will make sure that whenever it’s setting up code segments, any code segment that’s x86 will properly jump into Fex so it can be run through Fex instead of the native CPU.

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You’ve told me Valve is really funding a lot of this Proton development, this Wine development, and I’ve heard that the same thing is happening with Fex. How long has Valve supported Fex and to what degree?

That’s right, all the core developers have been funded by us since the beginning. We definitely started that project with the idea that it would be something that’s useful for the ecosystem at large, but also something that would be really useful for SteamOS and other applications in the future.

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Valve started Fex?

Proton is also something where you could say we started it, but because it’s a derivative of Wine, it’s less clear-cut. Whereas Fex, we were talking with a few developers that we knew were the right fit for an undertaking like that, a long-term thing that needed a very specific set of experts. We worked hard on trying to convince these guys to start the project, and have been funding them ever since.