What Does the CPU Do When an RTOS Has No Ready Tasks?

In a bare‑metal program the CPU is stuck in an infinite while(1) loop, continuously executing the same code. For example:

int main(void)
{
    /* 初始化 */
    while(1)
    {
        /* 循环处理多项事情 */
    }
}

When a real‑time operating system (RTOS) is introduced, the goal is to improve CPU utilization by running multiple independent tasks, each typically containing its own infinite loop:

void Task1(void)
{
    /* 初始化 */
    while(1)
    {
        /* 处理事情1 */
    }
}

void Task2(void)
{
    /* 初始化 */
    while(1)
    {
        /* 处理事情2 */
    }
}

/* … more tasks such as Task3, Task4, … */

If all tasks are blocked—e.g., by vTaskDelay or waiting on a queue—the CPU does not simply stop. The RTOS schedules its built‑in idle task (or idle thread/process). This idle task runs whenever no higher‑priority task is ready.

On a typical desktop OS you can see a high‑CPU‑usage process called “System Idle Process”. In embedded RTOSes the same concept exists. For example, in uC/OS the idle task is named OS_TaskIdle, and in FreeRTOS it is called prvIdleTask.

uC/OS idle task: OS_TaskIdle FreeRTOS idle task: prvIdleTask The idle task often contains only a tight loop, optionally calling a user‑definable hook. A typical uC/OS idle implementation looks like:

void OS_TaskIdle(void *p_arg)
{
#if OS_CRITICAL_METHOD == 3u
    /* Allocate storage for CPU status register */
#endif
    OS_CPU_SR cpu_sr = 0u;
    p_arg = p_arg; /* Prevent compiler warning */
    for (;;)
    {
        OS_ENTER_CRITICAL();
        OSIdleCtr++;
        OS_EXIT_CRITICAL();
        OSTaskIdleHook(); /* User‑definable hook */
    }
}

By default the hook does nothing, so the idle task essentially does nothing useful. Developers can add custom work, such as printing a message:

printf("CPU在偷懒了...
\r");

Therefore, when an RTOS has no ready tasks, the CPU executes the system’s idle task, which by default performs no meaningful work unless the programmer adds code to it.