f401-rtos

FreeRTOS on the STM32F401 with a custom linker script and bare-metal startup, no STM32Cube, no vendor HAL, just CMSIS headers and the FreeRTOS kernel.

What's in here

Path	Purpose
src/boot.S	Vector table + reset handler (branches to start)
src/start.cpp	.data/.bss init, PLL clock setup (84 MHz), calls main()
map.ld	Linker script, FLASH 512 K @ 0x08000000, RAM 96 K @ 0x20000000
FreeRTOS-Kernel/	FreeRTOS kernel submodule (CM4F port, heap_4)
driver/	stm32f401-cpp-hal submodule, see driver/README.md

The build uses C++20 with -fno-exceptions -fno-rtti, no runtime overhead from those features.

Build

# clone with submodules
git clone --recurse-submodules <repo-url>

# build
make

# flash (requires openocd running: make gdbserver)
make flash

# debug
make gdbserver   # terminal 1
make debug       # terminal 2

Toolchain: arm-none-eabi-g++, OpenOCD + ST-Link

Project structure

src/
  boot.S       # vector table, reset_handler, weak IRQ aliases
  start.cpp    # memory init, HSI PLL → 84 MHz, entry point
  main.cpp     # FreeRTOS tasks (application code)
  syscalls.cpp # minimal newlib syscall stubs (_write → UART)
map.ld         # custom linker script
driver/        # stm32f401-cpp-hal submodule (GPIO, UART)
FreeRTOS-Kernel/
include/       # FreeRTOSConfig.h and project headers