freeRtos调度函数代码解读

读了一下下freeRTOS的代码，结合代码过一下freeRTOS调度器是怎么工作的。

vTaskStartScheduler

创建完用户的各种任务之后，调用函数vTaskStartScheduler便进入了任务调度，任务地阿杜这个函数做了三件事。

1. 创建 idle 任务 , 以下便是这部分代码

可以看出idle task的任务优先级被设置为portPRIVILEGE_BIT, 而portPRIVILEGE_BIT被定义为0x00，也就是最低优先级（freeRTOS中数字越大优先级越高）。换句话说，idle task的回调函数只有在系统“无事可做”的时候会被调用。

// The Idle task is created using user provided RAM
vApplicationGetIdleTaskMemory( &pxIdleTaskTCBBuffer, &pxIdleTaskStackBuffer, &ulIdleTaskStackSize );
// Create the idle task
xIdleTaskHandle = xTaskCreateStatic(	prvIdleTask,
							configIDLE_TASK_NAME,
							ulIdleTaskStackSize,
							( void * ) NULL, 
							portPRIVILEGE_BIT, 
							pxIdleTaskStackBuffer,
							pxIdleTaskTCBBuffer );

2. 创建 timer task

		if( xReturn == pdPASS )
		{
			xReturn = xTimerCreateTimerTask();
		}

3. 启动timer task 和 最高优先级task ，即调用xPortStartScheduler

if( xPortStartScheduler() != pdFALSE )

然后进入xPortStartScheduler,这个函数并不在freeRTOS的三大源码文件中，而是属于port的一部分，以下代码来自MSVC-MingW的实现（此代码可运行于windows上）。

• 启动 tick 中断

  pvHandle = CreateThread( NULL, 0, prvSimulatedPeripheralTimer, NULL, CREATE_SUSPENDED, NULL );
  	if( pvHandle != NULL )
  	{
  		SetThreadPriority( pvHandle, portSIMULATED_TIMER_THREAD_PRIORITY );
  		SetThreadPriorityBoost( pvHandle, TRUE );
  		SetThreadAffinityMask( pvHandle, 0x01 );
  		ResumeThread( pvHandle );
  	}
  

• 启动最高优先级任务

ResumeThread( pxThreadState->pvThread );

• 调用函数ProcessSimulatedInterrupts

至此在此层函数范围内程序不在往下执行，即程序进入函数·ProcessSimulatedInterrupts便不再出来了,ProcessSimulatedInterrupts的主题是一个死循环，完成任务的调度，如以下代码。

static void prvProcessSimulatedInterrupts( void )
{
...
	for(;;)
	{
...
		for( i = 0; i < portMAX_INTERRUPTS; i++ )
		{
        ...
					if( ulIsrHandler[ i ]() != pdFALSE )
...
		}

		if( ulSwitchRequired != pdFALSE )
		{
			void *pvOldCurrentTCB;

			pvOldCurrentTCB = pxCurrentTCB;

			vTaskSwitchContext();

			/* If the task selected to enter the running state is not the task
			that is already in the running state. */
			if( pvOldCurrentTCB != pxCurrentTCB )
			{
				pxThreadState = ( ThreadState_t *) *( ( size_t * ) pvOldCurrentTCB );
				SuspendThread( pxThreadState->pvThread );

				xContext.ContextFlags = CONTEXT_INTEGER;
				( void ) GetThreadContext( pxThreadState->pvThread, &xContext );

				pxThreadState = ( ThreadState_t * ) ( *( size_t *) pxCurrentTCB );

				ResumeThread( pxThreadState->pvThread );
			}
		}

		pxThreadState = ( ThreadState_t * ) ( *( size_t *) pxCurrentTCB );
		SetEvent( pxThreadState->pvYieldEvent );
		ReleaseMutex( pvInterruptEventMutex );
	}
}