FreeRTOS移植要点(2)

port.c port.c 中主要实现了几个函数： pxPortInitialiseStack() xPortStartScheduler() vPortEndScheduler() vPortYield() vPortTickInterrupt() 还定义了个全局变量：uxCriticalNesting uxCriticalNesting 定义全局变量uxCriticalNesting 的代码如下。 /*

port.c

port.c 中主要实现了几个函数：

pxPortInitialiseStack()

xPortStartScheduler()

vPortEndScheduler()

vPortYield()

vPortTickInterrupt()

还定义了个全局变量：uxCriticalNesting

uxCriticalNesting

定义全局变量uxCriticalNesting 的代码如下。

/* Calls to portENTER_CRITICAL() can be nested. When they are nested the 
critical div should not be left (i.e. interrupts should not be re-enabled)
until the nesting depth reaches 0. This variable simply tracks the nesting 
depth. Each task maintains it's own critical nesting depth variable so 
uxCriticalNesting is saved and restored from the task stack during a context
switch. */
volatile unsigned portBASE_TYPE uxCriticalNesting = 0xff;

uxCriticalNesting 的初始值并不重要，因为每个任务的堆栈中存了uxCriticalNesting 各自的初始值0。

pxPortInitialiseStack

第一个介绍的是pxPortInitialiseStack()。这个函数的作用与uC/OS-II 中 OS_STK*OSTaskStkInit (void (*task)(void *pd), void *p_arg, OS_STK *ptos, INT16U opt) 函数的作用是相同的，实现代码也大同小异。

portSTACK_TYPE *pxPortInitialiseStack( portSTACK_TYPE *pxTopOfStack, pdTASK_CODE pxCode, void *pvParameters )
{
	/* 
	Place a few bytes of known values on the bottom of the stack.
	This can be uncommented to provide useful stack markers when debugging.
	*pxTopOfStack = ( portSTACK_TYPE ) 0x11;
	pxTopOfStack--;
	*pxTopOfStack = ( portSTACK_TYPE ) 0x22;
	pxTopOfStack--;
	*pxTopOfStack = ( portSTACK_TYPE ) 0x33;
	pxTopOfStack--;
	*/

	/* Setup the initial stack of the task. The stack is set exactly as 
	expected by the portRESTORE_CONTEXT() macro. In this case the stack as
	expected by the HCS12 RTI instruction. */

	/* The address of the task function is placed in the stack byte at a time. */
	*pxTopOfStack = ( portSTACK_TYPE ) *( ((portSTACK_TYPE *) (&pxCode) ) + 1 );
	pxTopOfStack--;
	*pxTopOfStack = ( portSTACK_TYPE ) *( ((portSTACK_TYPE *) (&pxCode) ) + 0 );
	pxTopOfStack--;

	/* Next are all the registers that form part of the task context. */

	/* Y register */
	*pxTopOfStack = ( portSTACK_TYPE ) 0xff;
	pxTopOfStack--;
	*pxTopOfStack = ( portSTACK_TYPE ) 0xee;
	pxTopOfStack--;

	/* X register */
	*pxTopOfStack = ( portSTACK_TYPE ) 0xdd;
	pxTopOfStack--;
	*pxTopOfStack = ( portSTACK_TYPE ) 0xcc;
	pxTopOfStack--;
 
	/* A register contains parameter high byte. */
	*pxTopOfStack = ( portSTACK_TYPE ) *( ((portSTACK_TYPE *) (&pvParameters) ) + 0 );
	pxTopOfStack--;

	/* B register contains parameter low byte. */
	*pxTopOfStack = ( portSTACK_TYPE ) *( ((portSTACK_TYPE *) (&pvParameters) ) + 1 );
	pxTopOfStack--;

	/* CCR: Note that when the task starts interrupts will be enabled since
	"I" bit of CCR is cleared */
	*pxTopOfStack = ( portSTACK_TYPE ) 0x00;
	pxTopOfStack--;
	
	#ifdef BANKED_MODEL
	/* The page of the task. */
	*pxTopOfStack = ( portSTACK_TYPE ) ( ( int ) pxCode );
	pxTopOfStack--;
	#endif
	
	/* Finally the critical nesting depth is initialised with 0 (not within
	a critical div). */
	*pxTopOfStack = ( portSTACK_TYPE ) 0x00;

	return pxTopOfStack;
}

xPortStartScheduler

xPortStartScheduler()函数对应于uC/OS-II 中的OSStartHighRdy() 函数。FreeRTOS的移植代码中并没有直接在xPortStartScheduler() 函数中实现具体功能，而是将真正的工作放到了xBankedStartScheduler()函数中，xPortStartScheduler()函数只是简单的调用xBankedStartScheduler()函数。之所以这样处理是因为相应的代码需放到64K以内的地址空间中。具体可以参看下面代码中的注释部分。

#pragma CODE_SEG __NEAR_SEG NON_BANKED
/* Simply called by xPortStartScheduler(). xPortStartScheduler() does not
	start the scheduler directly because the header file containing the 
	xPortStartScheduler() prototype is part of the common kernel code, and 
	therefore cannot use the CODE_SEG pragma. */
	static portBASE_TYPE xBankedStartScheduler( void );
#pragma CODE_SEG DEFAULT

portBASE_TYPE xPortStartScheduler( void )
{
	/* xPortStartScheduler() does not start the scheduler directly because 
	the header file containing the xPortStartScheduler() prototype is part 
	of the common kernel code, and therefore cannot use the CODE_SEG pragma. 
	Instead it simply calls the locally defined xBankedStartScheduler() - 
	which does use the CODE_SEG pragma. */

	return xBankedStartScheduler();
}
/*-----------------------------------------------------------*/

#pragma CODE_SEG __NEAR_SEG NON_BANKED

static portBASE_TYPE xBankedStartScheduler( void )
{
	/* Configure the timer that will generate the RTOS tick. Interrupts are
	disabled when this function is called. */
	prvSetupTimerInterrupt();

	/* Restore the context of the first task. */
	portRESTORE_CONTEXT();

	/* Simulate the end of an interrupt to start the scheduler off. */
	__asm( "rti" );

	/* Should not get here! */
	return pdFALSE;
}

上面代码中调用了prvSetupTimerInterrupt() 函数，这在 uC/OS-II中是没有对应代码的。prvSetupTimerInterrupt()函数的功能是设置定时中断的频率。在这里放这个函数从程序逻辑上来看并不是太好。我本人还是倾向于uC/OS-II 作者的做法，应该将prvSetupTimerInterrupt() 函数放到第一个运行的任务的代码中，虽然这里的做法也没错误。

vPortEndScheduler

这个函数在uC/OS-II 没有对应的函数，因为uC/OS-II 不允许退出。这个移植代码中也没有实现什么具体的功能，就是个空函数。

void vPortEndScheduler( void )
{
	/* It is unlikely that the HCS12 port will get stopped. */
}

vPortYield

vPortYield()函数等价于uC/OS-II 中的OSCtxSw()函数。具体代码如下：

/*
 * Manual context switch forced by calling portYIELD(). This is the SWI
 * handler.
 */
void interrupt vPortYield( void )
{
	portSAVE_CONTEXT();
	vTaskSwitchContext();
	portRESTORE_CONTEXT();
}

vPortTickInterrupt()

最后一个函数是vPortTickInterrupt() 这个函数是定时中断处理函数，等价于uC/OS-II 移植代码中的：interrupt VectorNumber_Vrti void OSTickISR (void)

由于 FreeRTOS既支持抢占式多任务，也支持协作式多任务，所以vPortTickInterrupt()函数相对uC/OS-II 移植代码中的OSTickISR()来说要复杂些。

/*
 * RTOS tick interrupt service routine. If the cooperative scheduler is 
 * being used then this simply increments the tick count. If the 
 * preemptive scheduler is being used a context switch can occur.
 */
void interrupt vPortTickInterrupt( void )
{
	#if configUSE_PREEMPTION == 1
	{
	/* A context switch might happen so save the context. */
	portSAVE_CONTEXT();

	/* Increment the tick ... */
	vTaskIncrementTick();

	/* ... then see if the new tick value has necessitated a
	context switch. */
	vTaskSwitchContext();

	TFLG1 = 1;

	/* Restore the context of a task - which may be a different task
	to that interrupted. */
	portRESTORE_CONTEXT();	
	}
	#else
	{
	vTaskIncrementTick();
	TFLG1 = 1;
	}
	#endif
}

至此，所有移植代码就都分析完了。

实时操作系统内核其实都大同小异，掌握了一种再学习其余的很容易就能入门。从入门难度来说，uC/OS-II无疑是入门学习的首选。之所以这么说并不是因为uC/OS-II本身很简单，而是国内介绍uC/OS-II的资料非常多。相比起来，介绍FreeRTOS的资料就少的可怜了。我建议想要学习FreeRTOS的人还是应该先学习uC/OS-II，学懂了uC/OS-II，然后对比着学习FreeRTOS，这样会事半功倍。这也是我学习FreeRTOS的路径。