tinyRTOS

A minimal preemptive RTOS for ARM Cortex-M, built from scratch to understand schedulers, context switching, TCBs, and interrupts at the register and stack level. A learning project — not a FreeRTOS replacement.

Runs on real hardware: STM32F103C8T6 (Blue Pill), Cortex-M3, bare metal — no HAL, own linker script and startup, hand-written context switch in ARMv7-M assembly, SysTick driving the tick, PendSV doing the switch.

More details on my website: rssndr.com

Layout

linux/    reference implementation on Linux — ucontext context switching,
          SIGALRM/setitimer tick. The scheduler was developed and debugged
          here first, with nothing but plain gdb.
stm32/    the real thing — bare-metal Cortex-M3. Preemptive, PendSV context
          switch in assembly, SysTick 1 ms tick.

The split exists because the interesting parts of an RTOS — the scheduler, run queue, blocking primitives, the tick — are independent of how the context switch is implemented. The scheduler logic was built and verified on Linux, then ported to hardware by replacing only the context-switch and tick mechanisms. swapcontext became hand-written register save/restore; SIGALRM became SysTick.

What's implemented

• Preemptive round-robin scheduler with a 1 ms SysTick tick (stm32) / 10 ms SIGALRM tick (linux)
• TCBs with per-task stacks, READY/BLOCKED states, and tick-based wakeup
• task_delay(ticks) blocking primitive
• Context switch in PendSV: hardware stacks half the registers, the handler saves/restores r4–r11 and swaps PSP
• Manual first-task launch (fake exception frame built by task_stack_init)
• Idle task to keep the run queue non-empty

Not implemented (deliberately): synchronization primitives, priorities, dynamic task creation, stack overflow detection.

Build and run

stm32 (needs arm-none-eabi-gcc and stlink; ST-Link V2 wired via SWD):

cd stm32
make
make flash

The onboard LED blinks at 1 Hz; an LED on PB0 (through a resistor to GND) blinks at 5 Hz. Two tasks, visibly independent.

linux:

cd linux
make
make run

Two tasks print at different rates, paced by wall-clock time. Ctrl-C to stop.

Demo

Two LEDs, two tasks, one scheduler: the onboard LED (PC13) toggles every 500 ms, an external LED (PB0) every 100 ms — independent rhythms under preemptive round-robin, with an idle task absorbing the gaps.