Valve’s SteamOS expansion goes beyond Proton with FEX and a second emulation layer

Valve built much of the Steam Deck’s success on Proton, the translation layer that lets Windows games run on the Linux-based SteamOS. But with the SteamOS-powered Steam Frame, the company is adding two additional compatibility layers to its platform architecture, signaling a broader push to make Windows-era software work across heterogeneous hardware.

The most notable of the new layers is FEX, Valve’s version of the open source fex-emu project, which translates x86 processor instructions so they can run on Arm-based chips. What began in 2018 as a modest cross-platform emulation effort has matured into what the project describes as an advanced binary recompiler supporting all modern extensions of the x86-64 instruction set.

According to Steam’s developer documentation, the emulator offloads calls to graphics libraries such as OpenGL and Vulkan to cut overhead, and relies on code caching to reduce in-game stuttering — a critical concern for real-time gaming performance.

Unlike Proton, which grew out of the long-established Wine compatibility tool, FEX followed a different path. Valve engineer Pierre-Loup Griffais told The Verge that the company has been funding lead developer Ryan Houdek since the project existed only as a prototype. Even then, Valve anticipated that close to a decade of work would be required before the emulator would be reliable enough for players to depend on with their game libraries.

Why it matters

The move underscores a wider industry shift. Arm Holdings plc, whose CPU designs dominate mobile and increasingly target PCs, has seen its shares trade at $243.98, up 1.11% on the day, valuing the semiconductor IP company at roughly $256 billion. As game and application ecosystems probe the limits of x86’s dominance, translation layers like FEX lower the barrier for Windows software to reach Arm hardware — a trend Microsoft Corporation has also leaned into with its Windows-on-Arm initiatives, though Microsoft shares slipped 0.32% today to $491.65 amid a broader software-sector pullback.

For Valve, the stakes are practical: the Steam Frame needs to run a large existing game library on Arm silicon, and Proton alone cannot solve the processor instruction mismatch. By combining Proton’s Windows API translation with FEX’s instruction emulation, Valve is assembling a stack capable of running x86 Windows titles on an Arm-based device with minimal developer involvement.

The second compatibility layer in Valve’s new Steam architecture has received less attention so far, but its presence alongside FEX suggests the company is building flexibility for multiple emulation and translation scenarios as SteamOS expands beyond the handheld Deck.

Source: original release

What to watch

• Developer documentation updates from Valve detailing FEX performance characteristics and supported titles on SteamOS.
• Steam hardware announcements clarifying the second compatibility layer’s role and target workloads.
• Arm-based PC momentum, including Windows-on-Arm software compatibility progress from major platform vendors.