Linux on Arm Cortex-M

A testbed for running and experimenting with Linux on Arm Cortex-M microcontrollers. Cortex-M cores have no MMU, so the kernel runs in nommu mode and userspace binaries use the FDPIC ELF format with shared uClibc-ng. The entire system -- cross-toolchain, C library, userspace, kernel, and boot wrapper -- is built from source by a single shell script and boots under QEMU.

Use this project to study how Linux behaves without an MMU, to prototype kernel patches for Cortex-M, or to experiment with minimal embedded Linux configurations on a microcontroller-class CPU.

Prerequisites

A Linux host is required. The build downloads and compiles an entire cross-toolchain, C library, BusyBox, and kernel from source, so expect significant build time and several gigabytes of disk space.

Install the host packages needed for a cross-toolchain build (Debian/Ubuntu):

$ sudo apt-get install build-essential bison flex texinfo \
    libncurses-dev wget git qemu-system-arm

Quick start

$ ./build.sh
$ qemu-system-arm -M mps2-an386 -cpu cortex-m4 -nographic \
    -kernel bootwrapper/linux.axf

Exit QEMU with Ctrl-a x.

What gets built

The build runs in a fixed order, downloading sources automatically:

Step	Component	Version	Purpose
1	binutils	2.46.0	Assembler, linker (with Thumb-only patch)
2	GCC	15.2.0	C-only cross-compiler, first-pass
3	Linux headers	7.0	Kernel headers for C library build
4	uClibc-ng	1.0.57	Shared FDPIC C library and dynamic loader for nommu targets
5	BusyBox	1.37.0	Minimal userspace (shell, coreutils)
6	Linux kernel	7.0	Kernel with embedded initramfs and FDPIC loader
7	Boot wrapper	--	Reset vectors for QEMU Cortex-M emulation

Individual stages can be re-run selectively:

$ ./build.sh busybox finalize_rootfs linux bootwrapper

Run ./build.sh clean to remove build artifacts while keeping downloaded sources.

Target platform

• CPU: Arm Cortex-M4 (ARMv7E-M, Thumb-2 only, no Arm-mode instructions)
• Machine: MPS2-AN386 (QEMU emulation of the ARM MPS2 FPGA board)
• No MMU -- the kernel uses CONFIG_MMU=n with a bootwrapper-loaded image
• Binaries are FDPIC ELF, loaded by CONFIG_BINFMT_ELF_FDPIC

QEMU's MPS2-AN386 model does not provide the normal ARM Linux boot path, so this project uses a boot wrapper (from the cortex-m-linux branch of ARM-software/bootwrapper) that supplies the Cortex-M vector table and jumps to the kernel at 0x21000000.

FDPIC and XIP

Userspace now defaults to shared FDPIC ELF binaries via the arm-uclinuxfdpiceabi toolchain. This removes the old elf2flt conversion path and lets nommu processes share text from shared libraries.

Measured against the historical FLAT build on main, the trade-off is mixed:

• the FDPIC BusyBox executable is smaller (112,972 bytes vs 172,316 bytes FLAT)
• the shipped rootfs is larger (340,906 bytes vs 172,654 bytes FLAT) because ld-uClibc.so and libuClibc.so are extra payload in a single-binary system

In other words, shared FDPIC is the default here for dynamic-linking capability and future multi-binary sharing, not because it is the smallest possible image for the current BusyBox-only userspace.

Application XIP is still not enabled in this repository. The current boot flow embeds the root filesystem into an initramfs and boots it through QEMU -kernel, so userland is loaded into RAM rather than executed from a flash-backed XIP filesystem. To pursue Application XIP with FDPIC, the project would need a flash-mapped romfs/cramfs layout on MTD instead of the current initramfs design. FDPIC is a prerequisite for shared-code XIP, but it is not sufficient on its own.

Experiments you can try

• Adjust kernel configs to measure the size floor for a bootable Cortex-M Linux system.
• Add or remove BusyBox applets and observe the FDPIC binary and shared-library size impact.
• Write small C programs, cross-compile them with the toolchain, and drop them into the initramfs to see how nommu affects process behavior (traditional fork is unavailable; programs use vfork or, where the C library provides it, posix_spawn).
• Test newer kernel versions for Cortex-M support regressions.

License

Build scripts are released under the MIT license. See LICENSE for details. Each upstream component carries its own license.