下面以FreeRTOS源码中arm cortex-r5处理器的栈处理为例来介绍栈结构操作前后变化。

1 FreeRTOS源码下载地址

https://www.freertos.org/

2 ARM CR5栈结构操作宏和接口

• .macro portSAVE_CONTEXT
• .macro portRESTORE_CONTEXT
• pxPortInitialiseStack

2.1 portSAVE_CONTEXT宏

2.1.1 portSAVE_CONTEXT源码

.macro portSAVE_CONTEXT

    /* Save the LR and SPSR onto the system mode stack before switching to
    system mode to save the remaining system mode registers. */
    SRSDB   sp!, #SYS_MODE
    CPS     #SYS_MODE
    PUSH    {R0-R12, R14}

    /* Push the critical nesting count. */
    LDR     R2, ulCriticalNestingConst
    LDR     R1, [R2]
    PUSH    {R1}

    #if defined( __ARM_FP )
        /* Does the task have a floating point context that needs saving?  If
        ulPortTaskHasFPUContext is 0 then no. */
        LDR     R2, ulPortTaskHasFPUContextConst
        LDR     R3, [R2]
        CMP     R3, #0

        /* Save the floating point context, if any. */
        FMRXNE  R1,  FPSCR
        PUSHNE  {R1}
        VPUSHNE {D0-D15}

        /* Save ulPortTaskHasFPUContext itself. */
        PUSH    {R3}
    #endif /* __ARM_FP */

    /* Save the stack pointer in the TCB. */
    LDR     R0, pxCurrentTCBConst
    LDR     R1, [R0]
    STR     SP, [R1]

    .endm

2.1.2 portSAVE_CONTEXT宏操作栈结构变化示意图

2.2 portRESTORE_CONTEXT宏

2.2.1 portRESTORE_CONTEXT源码

.macro portRESTORE_CONTEXT

    /* Set the SP to point to the stack of the task being restored. */
    LDR     R0, pxCurrentTCBConst
    LDR     R1, [R0]
    LDR     SP, [R1]

    #if defined( __ARM_FP )
        /*
         * Is there a floating point context to restore?  If the restored
         * ulPortTaskHasFPUContext is zero then no.
         */
        LDR     R0, ulPortTaskHasFPUContextConst
        POP     {R1}
        STR     R1, [R0]
        CMP     R1, #0

        /* Restore the floating point context, if any. */
        VPOPNE  {D0-D15}
        POPNE   {R0}
        VMSRNE  FPSCR, R0
    #endif /* __ARM_FP */

    /* Restore the critical section nesting depth. */
    LDR     R0, ulCriticalNestingConst
    POP     {R1}
    STR     R1, [R0]

    /* Ensure the priority mask is correct for the critical nesting depth. */
    LDR     R2, ulICCPMRConst
    LDR     R2, [R2]
    CMP     R1, #0
    MOVEQ   R4, #255
    LDRNE   R4, ulMaxAPIPriorityMaskConst
    LDRNE   R4, [R4]
    STR     R4, [R2]

    /* Restore all system mode registers other than the SP (which is already
    being used). */
    POP     {R0-R12, R14}

    /* Return to the task code, loading CPSR on the way. */
    RFEIA   sp!

    .endm

2.2.2 portRESTORE_CONTEXT宏操作栈结构变化示意图

2.3 pxPortInitialiseStack

2.3.1 pxPortInitialiseStack源码

/*
 * See header file for description.
 */
StackType_t * pxPortInitialiseStack( StackType_t * pxTopOfStack,
                                     TaskFunction_t pxCode,
                                     void * pvParameters )
{
    /*
     * Setup the initial stack of the task.  The stack is set exactly as
     * expected by the portRESTORE_CONTEXT() macro.
     *
     * The fist real value on the stack is the status register, which is set for
     * system mode, with interrupts enabled.  A few NULLs are added first to ensure
     * GDB does not try decoding a non-existent return address.
     */
    *pxTopOfStack = ( StackType_t ) NULL;
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) NULL;
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) NULL;
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) portINITIAL_SPSR;

    if( ( ( uint32_t ) pxCode & portTHUMB_MODE_ADDRESS ) != 0x00UL )
    {
        /* The task will start in THUMB mode. */
        *pxTopOfStack |= portTHUMB_MODE_BIT;
    }

    pxTopOfStack--;

    /* Next the return address, which in this case is the start of the task. */
    *pxTopOfStack = ( StackType_t ) pxCode;
    pxTopOfStack--;

    /* Next all the registers other than the stack pointer. */
    *pxTopOfStack = ( StackType_t ) portTASK_RETURN_ADDRESS; /* R14 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x12121212;              /* R12 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x11111111;              /* R11 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x10101010;              /* R10 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x09090909;              /* R9 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x08080808;              /* R8 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x07070707;              /* R7 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x06060606;              /* R6 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x05050505;              /* R5 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x04040404;              /* R4 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x03030303;              /* R3 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x02020202;              /* R2 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) 0x01010101;              /* R1 */
    pxTopOfStack--;
    *pxTopOfStack = ( StackType_t ) pvParameters;            /* R0 */

    /*
     * The task will start with a critical nesting count of 0 as interrupts are
     * enabled.
     */
    pxTopOfStack--;
    *pxTopOfStack = portNO_CRITICAL_NESTING;

    #if ( configUSE_TASK_FPU_SUPPORT == 1 )
    {
        /*
         * The task will start without a floating point context.
         * A task that uses the floating point hardware must call
         * vPortTaskUsesFPU() before executing any floating point
         * instructions.
         */
        pxTopOfStack--;
        *pxTopOfStack = portNO_FLOATING_POINT_CONTEXT;
    }
    #elif ( configUSE_TASK_FPU_SUPPORT == 2 )
    {
        /*
         * The task will start with a floating point context. Leave enough
         * space for the registers and ensure they are initialized to 0.
         */
        pxTopOfStack -= portFPU_REGISTER_WORDS;
        memset( pxTopOfStack, 0x00, portFPU_REGISTER_WORDS * sizeof( StackType_t ) );

        pxTopOfStack--;
        *pxTopOfStack = pdTRUE;
        ulPortTaskHasFPUContext = pdTRUE;
    }
    #elif ( configUSE_TASK_FPU_SUPPORT != 0 )
    {
        #error Invalid configUSE_TASK_FPU_SUPPORT setting - configUSE_TASK_FPU_SUPPORT must be set to 0, 1, or 2.
    }
    #endif /* configUSE_TASK_FPU_SUPPORT */

    return pxTopOfStack;
}

3.2.2 pxPortInitialiseStack栈结构变化示意图

2.4 pxPortInitialiseStack调用关系

		|- xTaskCreate( pxIdleTaskFunction, ...)
			|- prvCreateTask
				|- pxStack = pvPortMallocStack( ( ( ( size_t ) uxStackDepth ) * sizeof( StackType_t ) ) );
				|- pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) );
				|- prvInitialiseNewTask
					|- vListInitialiseItem( &( pxNewTCB->xStateListItem ) );
					|- vListInitialiseItem( &( pxNewTCB->xEventListItem ) );
					|- pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters );

2.5 portRESTORE_CONTEXT调用关系

|- vTaskStartScheduler
	|- prvCreateIdleTasks()
		|- xTaskCreate( pxIdleTaskFunction, ...)
			|- prvCreateTask
				|- pxStack = pvPortMallocStack( ( ( ( size_t ) uxStackDepth ) * sizeof( StackType_t ) ) );
				|- pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) );
				|- prvInitialiseNewTask
					|- vListInitialiseItem( &( pxNewTCB->xStateListItem ) );
					|- vListInitialiseItem( &( pxNewTCB->xEventListItem ) );
					|- pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters );
					|- *pxCreatedTask = ( TaskHandle_t ) pxNewTCB;
			|- prvAddNewTaskToReadyList
				|- prvInitialiseTaskLists
				|- prvAddTaskToReadyList
					|- listINSERT_END( &( pxReadyTasksLists[ ( pxTCB )->uxPriority ] ), &( ( pxTCB )->xStateListItem ) );
	|- xTimerCreateTimerTask()
		|- xTaskCreateAffinitySet
			|- prvCreateTask
					|- pxStack = pvPortMallocStack( ( ( ( size_t ) uxStackDepth ) * sizeof( StackType_t ) ) );
					|- pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) );
					|- prvInitialiseNewTask
						|- vListInitialiseItem( &( pxNewTCB->xStateListItem ) );
						|- vListInitialiseItem( &( pxNewTCB->xEventListItem ) );
						|- pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters );
						|- *pxCreatedTask = ( TaskHandle_t ) pxNewTCB;
				|- prvAddNewTaskToReadyList
					|- prvInitialiseTaskLists
					|- prvAddTaskToReadyList
						|- listINSERT_END( &( pxReadyTasksLists[ ( pxTCB )->uxPriority ] ), &( ( pxTCB )->xStateListItem ) )
			|- pxNewTCB->uxCoreAffinityMask = uxCoreAffinityMask;
			|- prvAddNewTaskToReadyList( pxNewTCB );
	|- xPortStartScheduler()
		|- vPortRestoreTaskContext
			|- portRESTORE_CONTEXT