1.创建必要的任务：空闲任务和软件定时器任务（后者可选）
2.初始化系统的全局变量
3.初始化systick中断 PendSV中断 SVC中断
4.启动调度器
5.触发SVC中断， 启动第一个任务
/*-----------------------------------------------------------*/
void vTaskStartScheduler( void )
{
BaseType_t xReturn;
/* Add the idle task at the lowest priority. */
#if( configSUPPORT_STATIC_ALLOCATION == 1 )
{
StaticTask_t *pxIdleTaskTCBBuffer = NULL;
StackType_t *pxIdleTaskStackBuffer = NULL;
uint32_t ulIdleTaskStackSize;
/* The Idle task is created using user provided RAM - obtain the
address of the RAM then create the idle task. */
vApplicationGetIdleTaskMemory( &pxIdleTaskTCBBuffer, &pxIdleTaskStackBuffer, &ulIdleTaskStackSize );
xIdleTaskHandle = xTaskCreateStatic( prvIdleTask,
configIDLE_TASK_NAME,
ulIdleTaskStackSize,
( void * ) NULL, /*lint !e961. The cast is not redundant for all compilers. */
portPRIVILEGE_BIT, /* In effect ( tskIDLE_PRIORITY | portPRIVILEGE_BIT ), but tskIDLE_PRIORITY is zero. */
pxIdleTaskStackBuffer,
pxIdleTaskTCBBuffer ); /*lint !e961 MISRA exception, justified as it is not a redundant explicit cast to all supported compilers. */
if( xIdleTaskHandle != NULL )
{
xReturn = pdPASS;
}
else
{
xReturn = pdFAIL;
}
}
#else
{
/* The Idle task is being created using dynamically allocated RAM. */
xReturn = xTaskCreate( prvIdleTask,
configIDLE_TASK_NAME,
configMINIMAL_STACK_SIZE,
( void * ) NULL,
portPRIVILEGE_BIT, /* In effect ( tskIDLE_PRIORITY | portPRIVILEGE_BIT ), but tskIDLE_PRIORITY is zero. */
&xIdleTaskHandle ); /*lint !e961 MISRA exception, justified as it is not a redundant explicit cast to all supported compilers. */
}
#endif /* configSUPPORT_STATIC_ALLOCATION */
#if ( configUSE_TIMERS == 1 )
{
if( xReturn == pdPASS )
{
xReturn = xTimerCreateTimerTask();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif /* configUSE_TIMERS */
if( xReturn == pdPASS )
{
/* freertos_tasks_c_additions_init() should only be called if the user
definable macro FREERTOS_TASKS_C_ADDITIONS_INIT() is defined, as that is
the only macro called by the function. */
#ifdef FREERTOS_TASKS_C_ADDITIONS_INIT
{
freertos_tasks_c_additions_init();
}
#endif
/* Interrupts are turned off here, to ensure a tick does not occur
before or during the call to xPortStartScheduler(). The stacks of
the created tasks contain a status word with interrupts switched on
so interrupts will automatically get re-enabled when the first task
starts to run. */
portDISABLE_INTERRUPTS();
#if ( configUSE_NEWLIB_REENTRANT == 1 )
{
/* Switch Newlib's _impure_ptr variable to point to the _reent
structure specific to the task that will run first. */
_impure_ptr = &( pxCurrentTCB->xNewLib_reent );
}
#endif /* configUSE_NEWLIB_REENTRANT */
xNextTaskUnblockTime = portMAX_DELAY;
xSchedulerRunning = pdTRUE;
xTickCount = ( TickType_t ) configINITIAL_TICK_COUNT;
/* If configGENERATE_RUN_TIME_STATS is defined then the following
macro must be defined to configure the timer/counter used to generate
the run time counter time base. NOTE: If configGENERATE_RUN_TIME_STATS
is set to 0 and the following line fails to build then ensure you do not
have portCONFIGURE_TIMER_FOR_RUN_TIME_STATS() defined in your
FreeRTOSConfig.h file. */
portCONFIGURE_TIMER_FOR_RUN_TIME_STATS();
traceTASK_SWITCHED_IN();
/* Setting up the timer tick is hardware specific and thus in the
portable interface. */
if( xPortStartScheduler() != pdFALSE )
{
/* Should not reach here as if the scheduler is running the
function will not return. */
}
else
{
/* Should only reach here if a task calls xTaskEndScheduler(). */
}
}
else
{
/* This line will only be reached if the kernel could not be started,
because there was not enough FreeRTOS heap to create the idle task
or the timer task. */
configASSERT( xReturn != errCOULD_NOT_ALLOCATE_REQUIRED_MEMORY );
}
/* Prevent compiler warnings if INCLUDE_xTaskGetIdleTaskHandle is set to 0,
meaning xIdleTaskHandle is not used anywhere else. */
( void ) xIdleTaskHandle;
}
/*-----------------------------------------------------------*/
/*
* See header file for description.
*/
BaseType_t xPortStartScheduler( void )
{
/* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
See http://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );
#if( configASSERT_DEFINED == 1 )
{
volatile uint32_t ulOriginalPriority;
volatile uint8_t * const pucFirstUserPriorityRegister = ( volatile uint8_t * const ) ( portNVIC_IP_REGISTERS_OFFSET_16 + portFIRST_USER_INTERRUPT_NUMBER );
volatile uint8_t ucMaxPriorityValue;
/* Determine the maximum priority from which ISR safe FreeRTOS API
functions can be called. ISR safe functions are those that end in
"FromISR". FreeRTOS maintains separate thread and ISR API functions to
ensure interrupt entry is as fast and simple as possible.
Save the interrupt priority value that is about to be clobbered. */
ulOriginalPriority = *pucFirstUserPriorityRegister;
/* Determine the number of priority bits available. First write to all
possible bits. */
*pucFirstUserPriorityRegister = portMAX_8_BIT_VALUE;
/* Read the value back to see how many bits stuck. */
ucMaxPriorityValue = *pucFirstUserPriorityRegister;
/* Use the same mask on the maximum system call priority. */
ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
/* Calculate the maximum acceptable priority group value for the number
of bits read back. */
ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;
while( ( ucMaxPriorityValue & portTOP_BIT_OF_BYTE ) == portTOP_BIT_OF_BYTE )
{
ulMaxPRIGROUPValue--;
ucMaxPriorityValue <<= ( uint8_t ) 0x01;
}
#ifdef __NVIC_PRIO_BITS
{
/* Check the CMSIS configuration that defines the number of
priority bits matches the number of priority bits actually queried
from the hardware. */
configASSERT( ( portMAX_PRIGROUP_BITS - ulMaxPRIGROUPValue ) == __NVIC_PRIO_BITS );
}
#endif
#ifdef configPRIO_BITS
{
/* Check the FreeRTOS configuration that defines the number of
priority bits matches the number of priority bits actually queried
from the hardware. */
configASSERT( ( portMAX_PRIGROUP_BITS - ulMaxPRIGROUPValue ) == configPRIO_BITS );
}
#endif
/* Shift the priority group value back to its position within the AIRCR
register. */
ulMaxPRIGROUPValue <<= portPRIGROUP_SHIFT;
ulMaxPRIGROUPValue &= portPRIORITY_GROUP_MASK;
/* Restore the clobbered interrupt priority register to its original
value. */
*pucFirstUserPriorityRegister = ulOriginalPriority;
}
#endif /* conifgASSERT_DEFINED */
/* Make PendSV and SysTick the lowest priority interrupts. */
portNVIC_SYSPRI2_REG |= portNVIC_PENDSV_PRI;
portNVIC_SYSPRI2_REG |= portNVIC_SYSTICK_PRI;
/* Start the timer that generates the tick ISR. Interrupts are disabled
here already. */
vPortSetupTimerInterrupt();
/* Initialise the critical nesting count ready for the first task. */
uxCriticalNesting = 0;
/* Start the first task. */
vPortStartFirstTask();
/* Should not get here! */
return 0;
}
vPortStartFirstTask
/* Use the NVIC offset register to locate the stack. */
ldr r0, =0xE000ED08
ldr r0, [r0]
ldr r0, [r0]
/* Set the msp back to the start of the stack. */
msr msp, r0
/* Call SVC to start the first task, ensuring interrupts are enabled. */
cpsie i
cpsie f
dsb
isb
svc 0 /* 使用svc中断 启动第一个任务， SVC中断不允许被延迟响应，必须立即响应 */
END
vPortSVCHandler:
/* Get the location of the current TCB. */
ldr r3, =pxCurrentTCB /*找到第一个任务的TCB*/
ldr r1, [r3] /*找到保存第一个任务的栈地址的地址，TCB的第一个变量是栈的地址*/
ldr r0, [r1] /*找到第一个任务栈的地址*/
/* Pop the core registers. */
ldmia r0!, {r4-r11} /*手动出栈*/
msr psp, r0 /*设置中断返回后的栈为第一个任务的栈*/
isb /*指令屏障*/
mov r0, #0
msr basepri, r0 /*关闭中断*/
orr r14, r14, #13 /*《权威手册电子版139页》*/
bx r14 /*当r14为0xfffffffd时，该条指令代表从中断中返回，并且返回使用进程栈出栈、返回后进入线程模式*/
解释一下最后一个函数：vPortSVCHandler:。【看注释】
这里使用SVC中断来启动第一个任务，正常的任务切换使用的是PendSV中断。具体原因可以查看《权威指南》或者百度。
任务创建的时候，会初始化任务栈，保存任务函数的地址在栈中，以及一些环境变量，所以从任务栈出栈，PC可以准确获取到任务函数的地址，bx r14 可以返回到第一个任务执行。
StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t pxCode, void *pvParameters )
{
/* Simulate the stack frame as it would be created by a context switch
interrupt. */
pxTopOfStack--; /* Offset added to account for the way the MCU uses the stack on entry/exit of interrupts. */
*pxTopOfStack = portINITIAL_XPSR; /* xPSR */
pxTopOfStack--;
*pxTopOfStack = ( ( StackType_t ) pxCode ) & portSTART_ADDRESS_MASK; /* PC */
pxTopOfStack--;
*pxTopOfStack = ( StackType_t ) prvTaskExitError; /* LR */
pxTopOfStack -= 5; /* R12, R3, R2 and R1. */
*pxTopOfStack = ( StackType_t ) pvParameters; /* R0 */
pxTopOfStack -= 8; /* R11, R10, R9, R8, R7, R6, R5 and R4. */
return pxTopOfStack;
}
《Mastering the FreeRTOS™ Real Time Kernel》
《cortex M3 权威指南》
《freertos 开发手册》
freertos
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