ucos上下文切换

刘宇shop · 发表于 2016-12-12 11:01:51

有两个问题请教一下大神！！！

-->1
在ucos中的上下文切换时发生在pendSV异常中，代码见下：

PendSV_Handler
CPSID I                                                    ; Prevent interruption during context switch
MRS    R0, PSP                                           ; PSP is process stack pointer
CBZ    R0,  PendSV_Handler_nosave                ;OS_CPU_PendSVHandler_nosave                   Skip register save the first time

SUBS R0, R0, #0x20                                  ; Save remaining regs r4-11 on process stack
STM    R0, {R4-R11}

LDR    R1, =OSTCBCur                               ; OSTCBCur->OSTCBStkPtr = SP;
LDR    R1, [R1]
STR    R0, [R1]                                           ; R0 is SP of process being switched out

                                                                  ; At this point, entire context of process has been saved
;OS_CPU_PendSVHandler_nosave
PendSV_Handler_nosave
; PUSH {R14}                                              ; Save LR exc_return value
; LDR    R0, =OSTaskSwHook                         ;OSTaskSwHook()
; BLX    R0
; POP    {R14}

LDR    R0, =OSPrioCur                                     ; OSPrioCur = OSPrioHighRdy;
LDR    R1, =OSPrioHighRdy
LDRB R2, [R1]
STRB R2, [R0]

LDR    R0, =OSTCBCur                                  ; OSTCBCur  = OSTCBHighRdy;
LDR    R1, =OSTCBHighRdy
LDR    R2, [R1]
STR    R2, [R0]

LDR    R0, [R2]                                              ; R0 is new process SP; SP = OSTCBHighRdy->OSTCBStkPtr;
LDM    R0, {R4-R11}                                     ; Restore r4-11 from new process stack
ADDS R0, R0, #0x20
MSR    PSP, R0                                              ; Load PSP with new process SP
ORR    LR, LR, #0x04                                     ; Ensure exception return uses process stack
CPSIE I
BX    LR                                                    ; Exception return will restore remaining context

; END

问题：假如在任务A的某处发生了pendSV异常，则LR此时存储的是该处的地址，而在pendSV中对任务栈等信息处理了，但是没有更改LR的值，那么在pendSV处理结束的时刻我们看到有条
BX    LR 指令，那岂不是又回到了任务A的这个地方呀，怎么能切换到别的任务上去啊？？？？？？

---->问题2：
参见附件:
图示中上下文切换为什么会回到任务B呀？不是IRQ一直悬起了，上下文切换完后，立刻应该进入到IRQ而不是进入到任务B啊。