In FreeRTOS, the task scheduler acts as the heart of the real-time operating system, managing the lifecycle and execution flow of concurrent tasks. On platforms like the STM32, launching the scheduler involves a sequence of low-level hardware and software configurations.

1. Initiating the Scheduler

Calling vTaskStartScheduler() marks the transition from single-threaded code to a multi-tasking environment. This function orchestrates several critical initialization steps:

2. Scheduler Startup Sequence

• Memory Allocation Mode: The system checks if it should use static or dynamic allocation for idle tasks, determined by configSUPPORT_STATIC_ALLOCATION.
• Software Timer Initialization: If configUSE_TIMERS is enabled, the timer task is created to manage system-wide delays and callbacks.
• Interrupt Safeguard: The scheduler disables all interrupts via portDISABLE_INTERRUPTS() to ensure the initialization process remains atomic and undisturbed by external hardware triggers.
• Global State Preparation: System variables like xTickCount (the system clock tick counter) are initialized to zero, and xNextTaskUnblockTime is set to portMAX_DELAY, indicating no pending blocked tasks.
• Hardware Configuration: The vPortStartScheduler routine is invoked, which handles architecture-specific setup. This includes configuring the SysTick timer to trigger periodic interrupts for task switching.

3. Configuring the Tick Timer

The SysTick interrupt frequency defines the OS tick resolution. On an ARM Cortex-M architecture, the timer is configured to decrement from a precalculated value based on the CPU frequency:

void setup_tick_timer(void) {
   // Stop and reset the SysTick controller
   SysTick->CTRL = 0;
   SysTick->VAL = 0;

   // Load value: (System Clock / Desired Tick Rate) - 1
   SysTick->LOAD = (SYSTEM_CORE_CLOCK / TICK_RATE_HZ) - 1;

   // Enable SysTick, set as internal clock, and enable interrupt
   SysTick->CTRL = (1 << 2) | (1 << 1) | 1;
}
   

4. Triggering the First Task

The final step in the scheduler startup is prvStartFirstTask(). This function is responsible for handing control from the bootloader/main entry point to the first user task. It performs the following:

• Stack Reset: It resets the Main Stack Pointer (MSP) to its initial position to effectively discard the startup stack state.
• Enabling Interrupts: It re-enables global interrupts.
• Context Switch Trigger: It issues the SVC 0 (Supervisor Call) enstruction. This triggers an SVC interrupt, which is the mechanism specifically designed to switch from thread mode to the first task execution context.

5. The Context Switch Handler (vPortSVCHandler)

The vPortSVCHandler is the assembly routine that executes the context restoration for the first task. It retrieves the Task Control Block (TCB), loads the hardware registers from the task's stack into the CPU core, and updates the Process Stack Pointer (PSP):

__asm void vPortSVCHandler(void) {
   PRESERVE8
   
   // Load the TCB address for the current task
   ldr r3, =pxCurrentTCB
   ldr r1, [r3]
   ldr r0, [r1] // Get top of stack
   
   // Restore registers R4-R11 from the stack
   ldmia r0!, {r4-r11}
   
   // Set PSP to the restored stack pointer
   msr psp, r0
   
   // Clear basepri and jump to the task function
   mov r0, #0
   msr basepri, r0
   orr r14, #0xd
   bx r14
}
   

Once the bx r14 instruction executes, the CPU switches into thread mode using the process stack, and the execution jumps to the address of the first task, completing the transition into the FreeRTOS environment.