The embedonomicon

The embedonomicon walks you through the process of creating a #![no_std] application from scratch and through the iterative process of building architecture-specific functionality for Cortex-M microcontrollers.


By reading this book you will learn

  • How to build a #![no_std] application. This is much more complex than building a #![no_std] library because the target system may not be running an OS (or you could be aiming to build an OS!) and the program could be the only process running in the target (or the first one). In that case, the program may need to be customized for the target system.

  • Tricks to finely control the memory layout of a Rust program. You'll learn about linkers, linker scripts and about the Rust features that let you control a bit of the ABI of Rust programs.

  • A trick to implement default functionality that can be statically overridden (no runtime cost).

Target audience

This book mainly targets to two audiences:

  • People that wish to bootstrap bare metal support for an architecture that the ecosystem doesn't yet support (e.g. Cortex-R as of Rust 1.28), or for an architecture that Rust just gained support for (e.g. maybe Xtensa some time in the future).

  • People that are curious about the unusual implementation of runtime crates like cortex-m-rt, msp430-rt and riscv-rt.


This book is self contained. The reader doesn't need to be familiar with the Cortex-M architecture, nor is access to a Cortex-M microcontroller needed -- all the examples included in this book can be tested in QEMU. You will, however, need to install the following tools to run and inspect the examples in this book:

  • All the code in this book uses the 2018 edition. If you are not familiar with the 2018 features and idioms check the edition guide.

  • Rust 1.31 or a newer toolchain PLUS ARM Cortex-M compilation support.

  • cargo-binutils. v0.1.4 or newer.

  • cargo-edit.

  • QEMU with support for ARM emulation. The qemu-system-arm program must be installed on your computer.

  • GDB with ARM support.

Example setup

Instructions common to all OSes

$ # Rust toolchain
$ # If you start from scratch, get rustup from
$ rustup default stable

$ # toolchain should be newer than this one
$ rustc -V
rustc 1.31.0 (abe02cefd 2018-12-04)

$ rustup target add thumbv7m-none-eabi

$ # cargo-binutils
$ cargo install cargo-binutils

$ rustup component add llvm-tools-preview


$ # arm-none-eabi-gdb
$ # you may need to run `brew tap Caskroom/tap` first
$ brew cask install gcc-arm-embedded

$ # QEMU
$ brew install qemu

Ubuntu 16.04

$ # arm-none-eabi-gdb
$ sudo apt install gdb-arm-none-eabi

$ # QEMU
$ sudo apt install qemu-system-arm

Ubuntu 18.04 or Debian

$ # gdb-multiarch -- use `gdb-multiarch` when you wish to invoke gdb
$ sudo apt install gdb-multiarch

$ # QEMU
$ sudo apt install qemu-system-arm


Installing a toolchain bundle from ARM (optional step) (tested on Ubuntu 18.04)

  • With the late 2018 switch from GCC's linker to LLD for Cortex-M microcontrollers, gcc-arm-none-eabi is no longer required. But for those wishing to use the toolchain anyway, install from here and follow the steps outlined below:
$ tar xvjf gcc-arm-none-eabi-8-2018-q4-major-linux.tar.bz2
$ mv gcc-arm-none-eabi-<version_downloaded> <your_desired_path> # optional
$ export PATH=${PATH}:<path_to_arm_none_eabi_folder>/bin # add this line to .bashrc to make persistent