学了两三年单片机直到用了操作系统才发现的一个问题

;********************************************************************************************************

;                                         HANDLE PendSV EXCEPTION

;                                     void OS_CPU_PendSVHandler(void)

;

; Note(s) : 1) PendSV is used to cause a context switch.  This is a recommended method for performing

;              context switches with Cortex-M3.  This is because the Cortex-M3 auto-saves half of the

;              processor context on any exception, and restores same on return from exception.  So only

;              saving of R4-R11 is required and fixing up the stack pointers.  Using the PendSV exception

;              this way means that context saving and restoring is identical whether it is initiated from

;              a thread or occurs due to an interrupt or exception.

;

;           2) Pseudo-code is:

;              a) Get the process SP, if 0 then skip (goto d) the saving part (first context switch);

;              b) Save remaining regs r4-r11 on process stack;

;              c) Save the process SP in its TCB, OSTCBCur->OSTCBStkPtr = SP;

;              d) Call OSTaskSwHook();

;              e) Get current high priority, OSPrioCur = OSPrioHighRdy;

;              f) Get current ready thread TCB, OSTCBCur = OSTCBHighRdy;

;              g) Get new process SP from TCB, SP = OSTCBHighRdy->OSTCBStkPtr;

;              h) Restore R4-R11 from new process stack;

;              i) Perform exception return which will restore remaining context.

;

;           3) On entry into PendSV handler:

;              a) The following have been saved on the process stack (by processor):

;                 xPSR, PC, LR, R12, R0-R3

;              b) Processor mode is switched to Handler mode (from Thread mode)

;              c) Stack is Main stack (switched from Process stack)

;              d) OSTCBCur      points to the OS_TCB of the task to suspend

;                 OSTCBHighRdy  points to the OS_TCB of the task to resume

;

;           4) Since PendSV is set to lowest priority in the system (by OSStartHighRdy() above), we

;              know that it will only be run when no other exception or interrupt is active, and

;              therefore safe to assume that context being switched out was using the process stack (PSP).

;********************************************************************************************************

 

OS_CPU_PendSVHandler

    CPSID   I                                  ; 上下文切换期间防止中断-关中断

    MRS     R0, PSP                            ; 获得PSP

    CBZ     R0, OS_CPU_PendSVHandler_nosave ;PSP为0跳到OS_CPU_PendSVHandler_nosave，即不保存上文，直接进入下文。

                                                    ;问什么呢，因为首次调用，是没有上文的。否则，全部执行

                                                                                                                                                                    

                                                    ;接下来是保存上文

    SUBS    R0, R0, #0x20       ;因为寄存器是32位的，4字节对齐，自动压栈的寄存器有8个，所以偏移为8*0x04=0x20

    STM     R0, {R4-R11}                                ;除去自动压栈的寄存器外，需手动将R4-R11压栈

 

    LDR     R1, =OSTCBCur       ; OSTCBCur->OSTCBStkPtr = SP;保存上文的SP指针 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 ;切换下文

    PUSH    {R14}              ; Save LR exc_return value, LR压栈，下面要调用C函数

    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 ,PSP = 新任务SP

    ORR     LR, LR, #0x04   ; Ensure exception return uses process stack,确保异常返回后使用PSP

    CPSIE   I

    BX      LR      ; Exception return will restore remaining context,

                        ;退出异常，从PSP弹出xPSR，PC，LR，R0-R3，进入新任务运行

    END