Simple RTOS for STM32F401RE

Implemented a simple real-time operating system on an ARM Cortex-M4 (STM32F401RE) to learn and gain a foundational understanding of C development in a real-time embedded environment. Main features implemented include cooperative & pre-emptive multitasking, SVC system calls, context switching, and basic kernel services.

Main Features

• Hardware Abstraction & I/O

  • Uses STM32 HAL for GPIO, UART, SysTick, PendSV, SVC

• Stack Management

  • Divides the main stack (MSP) into 0x400-byte PSP regions for up to eight contexts (main + seven threads)
  • Maintains a simple “stack pool” allocator for up to eight stacks (main + seven threads)

• System Calls & Context Initialization

  • Defines custom SVC numbers and a handler (SVC_Handler_Main) for supervisor calls
  • Builds each thread’s initial context on its PSP: xPSR, PC, LR R0–R3 (arguments), R4–R11

• Kernel API

  • osKernelInitialize() — capture initial MSP and reset kernel state
  • osCreateThread(void (*fn)(void*), void *arg) — allocate stack, set up context, register a TCB
  • osCreateThreadWithDeadlines(fn, arg, slice_ms, deadline_ms) — same as above with time-slice and deadline information
  • osKernelStart() — start the first thread via SVC
  • osYield() — cooperative context switch trigger

• Scheduling (Round-Robin)

  • Cooperative: threads call osYield(), triggering an SVC exception; the PendSV handler then switches to the next thread.
  • Preemptive: a 1 ms SysTick interrupt decrements the current thread’s time slice; when it hits zero, PendSV is pended to rotate to the next thread.

• Argument Passing & Exit Handling

  • Uses the ARM ABI (R0) to enable uses to pass arguments when creating threads

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Below is a simplified example usage of the RTOS API

int main(void) {
    HAL_Init();

    osKernelInitialize();

    osCreateThread(thread1, arg1);
    osCreateThreadWithDeadlines(thread2, arg2, 20, 50);

    osKernelStart();

    while (1) 
}

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This is a very bare-bones RTOS implementation to learn about real-time systems and related concepts. There are many features omitted that I hope to implement; for example, thread exit handling, dynamic task management, priority-based scheduling, and inter-thread communication, which would make this a more complete implementation.