pymcu-arm

ARM (Cortex-M) support for the PyMCU compiler: LLVM-IR codegen backend + LLVM toolchain driver.

Unlike the AVR/PIC/RISC-V backends, this backend does not emit assembly directly. It lowers PyMCU's architecture-agnostic IR (.mir) to LLVM IR text (.ll) and hands it to LLVM — so register allocation, instruction selection, the AAPCS calling convention, and all optimization passes are done by LLVM.

Currently supported targets:

The codegen is chip-agnostic: only -mtriple/-mcpu change per target.

Pipeline

pymcuc --emit-ir        →  firmware.mir    (target-agnostic IR, 32-bit pointers)
pymcuc-arm (this pkg)   →  firmware.ll     (LLVM IR text)
LLVM toolchain          →  opt → llc → ld.lld → llvm-objcopy
                            + generic boot2 (crc32) + crt0  →  firmware.bin
UF2 packer              →  firmware.bin    →  firmware.uf2  (BOOTSEL image)

The flat flash image (firmware.bin, boot2 at offset 0) is what the RP2040Sharp emulator's PicoSimulation.LoadFlash(...) consumes in the integration tests. firmware.uf2 is the same image in UF2 form (family 0xe48bff56 on RP2040, 0xe48bff59 on RP2350) — it is what pymcu flash uploads with picotool or by drag-and-drop onto the RPI-RP2 volume.

Installation

pip install pymcu-arm

LLVM tools are resolved (in order) from the pymcu-arm-toolchain wheel cache, common system install paths (/opt/homebrew/opt/llvm/bin, /usr/lib/llvm/bin), or PATH. A system LLVM (brew install llvm lld / apt install llvm lld) also works with no extra package.

Layout

src/python/pymcu/backend/rp2040/    backend plugin — wraps pymcuc-arm
src/python/pymcu/toolchain/rp2040/  LLVM toolchain driver (opt → firmware.bin)
src/csharp/lib/                     Rp2040BackendProvider + Targets/RP2040/Rp2040LlvmCodeGen
src/csharp/cli/                     pymcuc-arm runner CLI
src/python/pymcu/programmer/rp2040/ UF2/picotool flash programmer plugin
src/runtime/                        boot2 (crc32), crt0 / crt0_m33, picobin block, rp2040.ld / rp2350.ld

Status

Alpha. Single-core (core0). GPIO, UART0, SPI, I2C, ADC, PWM, DMA, PIO. Exceptions (portable T-flag model), f32 (RP2040 via the bootrom fast-float library, RP2350 via the M33 FPU in softfp mode), operand-form inline asm. No GC yet.