freertos任务调度关键函数理解-优快云博客

void xPortPendSVHandler( void )
{
/* This is a naked function. */
__asm volatile
(
//保存当前任务上下文
"   mrs r0, psp                           \n" //读取进程栈指针(PSP)到r0
"   isb                                   \n" //指令同步屏障,确保前面的指令执行完毕
"                                       \n"
"   ldr   r3, pxCurrentTCBConst           \n" /* Get the location of the current TCB. */
"   ldr   r2, [r3]                       \n" //保存r4-r11寄存器到当前任务栈(STMDB指令)
"                                       \n" //只保存/恢复r4-r11是因为ARM架构下中断自动保存r0-r3,r12,lr,pc,xPSR
"   stmdb r0!, {r4-r11}                   \n" /* Save the remaining registers. */
"   str r0, [r2]                       \n" /* Save the new top of stack into the first member of the TCB. */
"                                       \n" //更新TCB中的栈顶指针

"   stmdb sp!, {r3, r14}               \n" //临时保存r3和r14寄存器
"   mov r0, %0                           \n"
"   msr basepri, r0                       \n" //提升中断优先级(basepri)保护临界区
"   bl vTaskSwitchContext               \n" //调用vTaskSwitchContext选择新任务
"   mov r0, #0                           \n"
"   msr basepri, r0                       \n" //恢复中断优先级解除所有中断屏蔽 (BASEPRI=0 时允许所有优先级中断)
"   ldmia sp!, {r3, r14}               \n"
"                                       \n" /* Restore the context, including the critical nesting count. */
"   ldr r1, [r3]                       \n" //从新任务的TCB获取栈指针临界区嵌套计数可能是前面版本的注释,此处不适合.
"   ldr r0, [r1]                       \n" /* The first item in pxCurrentTCB is the task top of stack. */
"   ldmia r0!, {r4-r11}                   \n" // Pop the registers. 恢复r4-r11寄存器(LDMIA指令)
"   msr psp, r0                           \n" //更新PSP为新任务的栈指针
"   isb                                   \n" //指令同步屏障,确保前面的指令执行完毕
"   bx r14                               \n" //通过BX指令返回到新任务的执行点
"                                       \n" //会触发处理器的异常返回机制
//处理器会自动从栈中恢复xPSR、PC、LR、R12、R3-R0（这是异常进入时自动压栈的寄存器）从R0开始...到PC结束
//根据EXC_RETURN值切换回线程模式并使用PSP
"   .align 4                           \n"
"pxCurrentTCBConst: .word pxCurrentTCB   \n"
::"i" ( configMAX_SYSCALL_INTERRUPT_PRIORITY )
);
}