FreeRTOS任务创建、启动调度器、任务切换的过程分析——基于ARM-CotexM3

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;
}

BaseType_t xPortStartScheduler( void )
{
/* 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. */
    prvStartFirstTask();

    /* Should not get here! */
    return 0;
}

__asm void prvStartFirstTask( void )
{
    PRESERVE8

    /* 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
    /* Globally enable interrupts. */
    cpsie i
    cpsie f
    dsb
    isb
    /* Call SVC to start the first task. */
    svc 0
    nop
    nop
}

__asm void vPortSVCHandler( void )
{
    PRESERVE8

    ldr    r3, =pxCurrentTCB    /* Restore the context. */
    ldr r1, [r3]            /* Use pxCurrentTCBConst to get the pxCurrentTCB address. */
    ldr r0, [r1]            /* The first item in pxCurrentTCB is the task top of stack. */
    ldmia r0!, {r4-r11}        /* Pop the registers that are not automatically saved on exception entry and the critical nesting count. */
    msr psp, r0                /* Restore the task stack pointer. */
    isb
    mov r0, #0
    msr    basepri, r0
    orr r14, #0xd
    bx r14
}

__asm void xPortPendSVHandler( void )
{
    extern uxCriticalNesting;
    extern pxCurrentTCB;
    extern vTaskSwitchContext;

    PRESERVE8

    mrs r0, psp
    isb

    ldr    r3, =pxCurrentTCB        /* Get the location of the current TCB. */
    ldr    r2, [r3]

    stmdb r0!, {r4-r11}            /* Save the remaining registers. */
    str r0, [r2]                /* Save the new top of stack into the first member of the TCB. */
    stmdb sp!, {r3, r14}
    mov r0, #configMAX_SYSCALL_INTERRUPT_PRIORITY
    msr basepri, r0
    dsb
    isb
    bl vTaskSwitchContext
    mov r0, #0
    msr basepri, r0
    ldmia sp!, {r3, r14}

    ldr r1, [r3]
    ldr r0, [r1]                /* The first item in pxCurrentTCB is the task top of stack. */
    ldmia r0!, {r4-r11}            /* Pop the registers and the critical nesting count. */
    msr psp, r0
    isb
    bx r14
    nop
}