kahlonel / Rt0s

rt0s is a preemptive, multithreaded, realtime OS built for ARM Cortex-M0

Usable Features

• Create threads at runtime using rt0s_thread
• Threads can yield using rt0s_yield
• Threads can quit using rt0s_kill
• Increase semaphore count using rt0s_sem_signal
• Threads can wait on a semaphore using rt0s_sem_wait

See rt0s.h file for the function signatures.

Design Notes

• Kernel fits under 1k bytes, 1k assembly lines
• 100 bytes allocated for kernel stack
• Kernel RAM usage = 20*(Max threads) + 4*(Max semaphores) + 3 bytes
• Max number of threads and semaphores are known at compile-time
• Thread stack size is known at compile-time (same for all threads)
• Static memory usage / no dynamic memory allocation
• Round-robin pre-emptive scheduling
• All threads have same priority
• Thread switch latency = 192 + 50.4*(Number of threads created) cycles
  • For demo board STM32F0308-DISCO with 8MHz crystal, and for N=10, the thread latency is 696 cycles / 87 microseconds
• Interrupts are disabled during thread switching

Attempt at a scheduling overview diagram

              +----------+                             +----------+
        +-----+ Thread N +-----------+                 |Thread N+1|
        |     +-------+--+           |                 +-----+----+
        |             |              |                       ^
        |             |              |                       |
        v             v              v                       |
+-------+------+   +--+--+     +-----+-----+           +-----+----+
|Wait Semaphore|   |Yield|     |SysTick IRQ|           |PendSV IRQ|
+-------+------+   +---+-+     +-------+---+           +-----+----+
        |              |               |                     ^
        |    +-------+ |               |                     |
        +--->+SVC IRQ+<+               |                     |
             +---+---+                 v                     |
                 |               +-----+-------+             |
                 +-------------->+Schedule Next+-------------+
                                 +-------------+

Demo code

Code for STM32F0308-DISCO board has been added to boards/ folder. The demo code uses semaphores and yield function to debounce a button.

You can add your board into the boards/ folder. Create a folder with your board's name and add these files into it:

• linker.ld: with correct addresses/sizes for flash and RAM.
• cfg.s: SysTick clock frequency.
• main.c: actual application code.

Then change the BOARD value in Makefile to your board's name, and run make. Flash rt0s.elf or rt0s.hex.