Not just Proton: Getting to know Valve's new SteamOS compatibility layers

Since the release of the Steam Deck, Valve fans have gotten very familiar with the idea of the Proton compatibility layer that allows games written for Windows to run capably on the Linux-based SteamOS (though Valve’s efforts in this realm date back further than that). To get more games running on the SteamOS-powered Steam Frame, though, Valve is relying on two additional compatibility layers that are new to the Steam architecture. The first, FEX, is Valve’s port of fex-emu, an open source project that emulates x86 processor instructions on an Arm chipset. Since 2018, what started as a small cross-platform emulation project has grown to include “an advanced binary recompiler that supports all modern extensions of the x86(-64) instruction set,” according to the Fex website. The emulator can also make calls to graphics libraries like OpenGL and Vulkan to reduce overhead and uses “code caching to minimize stuttering as much as possible,” according to Steam’s own developer documentation. While Proton was based on the pre-existing Wine conversion tool, Valve engineer Pierre-Loup Griffais told The Verge last year that the company has been funding Fex lead developer Ryan Houdek since the project was just a prototype. Back then, Valve was already anticipating that “close to a decade of work [would be] needed before [Fex-emu] would be robust enough people could rely on it for their libraries.” “I want to thank the people from Valve for being here from the start and allowing me to kickstart this project,” Houdek wrote last November while marking the seventh anniversary of the project. “They trusted me with the responsibility of designing and frameworking the project in a way that it can work long-term; not only for their use cases but also keeping it an open project that anyone can adapt for their own use cases.” For Griffais, the goal is for gamers to be able to consider Arm-based hardware without having to say, “Oh, this device is good, and the specs are good, the value is good, but I have to leave a portion of my catalog behind if I’m even considering it.” The second new compatibility layer hitting Steam is Lepton, an expansion of the pre-existing Waydroid project focused on getting standard Android APKs to run on SteamOS through a software container. This layer is especially necessary for the Steam Frame, since many of the standalone VR titles players will be running on the device were originally designed for the Android-based Quest headsets.