RTOS — From-Scratch Preemptive RTOS on ARM Cortex-M3

A preemptive real-time operating system kernel built from scratch in C and ARM Thumb-2 assembly, running on an emulated ARM Cortex-M3 (QEMU mps2-an385) — no RTOS library or HAL as a base.

What's in here

• Context switching — hand-written PendSV_Handler, synthetic exception-frame construction so a never-yet-run task looks identical, to the restore path, to one resuming mid-execution (rtos/pendsv.s, rtos/kernel.c).
• Priority-array scheduler with SysTick-driven time-sliced preemption (rtos/scheduler.c, rtos/systick.c).
• Blocking primitives — delay(), semaphore, mutex — built on a sorted wake-tick list with PRIMASK-protected critical sections (rtos/sync.c).
• Priority inheritance — the classic 3-task priority-inversion scenario, constructed and then fixed, compiled two ways from one source tree via a single flag so the comparison is a controlled experiment (rtos/sync.c, rtos/main.c).
• Real-time layer — periodic tasks, a Rate-Monotonic Scheduling (RMS) schedulability check (Liu & Layland bound, integer-only arithmetic), and a runtime deadline-miss counter that catches violations the static check doesn't (rtos/rtos.c).

Building and testing

cd rtos
make clean && make
cd ..
./tests/m5_test.sh   # the latest milestone's test -- see tests/README.md for why

Requires arm-none-eabi-gcc/gdb and qemu-system-arm (see CI workflow for exact packages).

Docs

• docs/superpowers/specs/ — the approved architecture spec
• docs/superpowers/plans/ — one implementation plan per milestone
• rtos-whiteboard.md — concept notes (real-time fundamentals, schedulability math)
• CLAUDE.md — full project history/decisions log