WebAssembly on MCUs: What We Learned Evaluating Embedded Runtimes

Fedor Smirnov - Peeriot

Running Wasm on MCUs offers real gains—safer updates, easier testing, dynamic loading—but only if it fits. We examine how runtimes handle MCU limits and how well they meet embedded needs, and share where we believe the ecosystem could evolve to better support tiny devices.

WebAssembly is widely used in browsers and increasingly in the cloud—yet its potential on deeply constrained embedded devices is still largely unexplored. This talk discusses the engineering challenges we encountered while using WebAssembly on microcontrollers, and how these challenges reflect the capabilities and trade-offs of today’s embedded-Wasm tooling.

Beyond portability, we highlight the intrinsic advantages of running applications as Wasm on microcontrollers: safer and more robust updates that reduce the risk of bricking devices, the ability to dynamically load logic without reflashing firmware, and significantly easier testing by running the exact same module on development machines as on the microcontroller. These properties make Wasm attractive for embedded systems—even independently of heterogeneous deployments in the distributed system context.

We outline the runtime requirements that arise in OS-less, interrupt-driven embedded environments with very tight memory budgets and assess the extent to which current runtimes satisfy these requirements when deployed on microcontroller targets. We evaluate Rust-native interpreters such as wasmtime, wasmi, and tinywasm, as well as an embedded integration of WAMR, examining how well they support low memory usage, instance lifecycle control, peripheral interaction through host functions, and coordination with asynchronous interrupt-driven execution on microcontrollers.

On the compilation side, we discuss how we structure and build our Wasm modules to balance ergonomics with binary size, highlight patterns that worked—and those that didn’t—and describe where the toolchain still struggles under MCU constraints.