FreeRtos 任务切换深入分析

一、背景知识：

1、任务切换包含三个基本流程：保护现场、更新TCB、恢复现场并跳转

2、freertos的任务切换是在xPortPendSVHandler 中断函数中完成的

3、中断函数在调用之前，硬件已经保存了r0,r1,r2,r3,r12,r14(LR),r15(pc)，恢复现场的时候由硬件自动恢复r0,r1,r2,r3,r12,r14(LR),r15(pc)，即r0～r3在中断服务函数中是可以随便使用的，剩下的r4~r11需要在中断函数中手动保存。

4、ARM中的栈有两个，msp和psp，主程序和中断中用msp，线程中是psp

5、ARM一般是满减栈

二、xPortPendSVHandler源代码：

cpp 复制代码

void xPortPendSVHandler( void )
{
    /* This is a naked function. */

    __asm volatile
        (
         "	mrs r0, psp							\n"
         "	isb									\n"
         "										\n"
         "	ldr	r3, pxCurrentTCBConst			\n" /* Get the location of the current TCB. */
         "	ldr	r2, [r3]						\n"
         "										\n"
         "	tst r14, #0x10						\n" /* Is the task using the FPU context?  If so, push high vfp registers. */
         "	it eq								\n"
         "	vstmdbeq r0!, {s16-s31}				\n"
         "										\n"
         "	stmdb r0!, {r4-r11, r14}			\n" /* Save the core registers. */
         "	str r0, [r2]						\n" /* Save the new top of stack into the first member of the TCB. */
         "										\n"
         "	stmdb sp!, {r0, r3}					\n"
         "	mov r0, %0 							\n"
         "	cpsid i								\n" /* Errata workaround. */
         "	msr basepri, r0						\n"
         "	dsb									\n"
         "	isb									\n"
         "	cpsie i								\n" /* Errata workaround. */
         "	bl vTaskSwitchContext				\n"
         "	mov r0, #0							\n"
         "	msr basepri, r0						\n"
         "	ldmia sp!, {r0, r3}					\n"
         "										\n"
         "	ldr r1, [r3]						\n" /* The first item in pxCurrentTCB is the task top of stack. */
         "	ldr r0, [r1]						\n"
         "										\n"
         "	ldmia r0!, {r4-r11, r14}			\n" /* Pop the core registers. */
         "										\n"
         "	tst r14, #0x10						\n" /* Is the task using the FPU context?  If so, pop the high vfp registers too. */
         "	it eq								\n"
         "	vldmiaeq r0!, {s16-s31}				\n"
         "										\n"
         "	msr psp, r0							\n"
         "	isb									\n"
         "										\n"
#ifdef WORKAROUND_PMU_CM001 /* XMC4000 specific errata workaround. */
#if WORKAROUND_PMU_CM001 == 1
         "			push { r14 }				\n"
         "			pop { pc }					\n"
#endif
#endif
         "										\n"
         "	bx r14								\n"
         "										\n"
         "	.align 4							\n"
         "pxCurrentTCBConst: .word pxCurrentTCB	\n"
         ::"i"(configMAX_SYSCALL_INTERRUPT_PRIORITY)
         );
}