μcosii源码试读14- OS_Sched

 

操作系统的调度方法是其核心，下面是μc的调度函数

/*
*********************************************************************************************************
* SCHEDULER
*
* Description: This function is called by other uC/OS-II services to determine whether a new, high
* priority task has been made ready to run. This function is invoked by TASK level code
* and is not used to reschedule tasks from ISRs (see OSIntExit() for ISR rescheduling).
*
* Arguments : none
*
* Returns : none
*
* Notes : 1) This function is INTERNAL to uC/OS-II and your application should not call it.
* 2) Rescheduling is prevented when the scheduler is locked (see OS_SchedLock())
*********************************************************************************************************
*/

void OS_Sched (void)
{
#if OS_CRITICAL_METHOD == 3 /* Allocate storage for CPU status register */
    OS_CPU_SR cpu_sr = 0;
#endif



    OS_ENTER_CRITICAL();
    if (OSIntNesting == 0) { /* Schedule only if all ISRs done and ... */
        if (OSLockNesting == 0) { /* ... scheduler is not locked */
            OS_SchedNew();
            if (OSPrioHighRdy != OSPrioCur) { /* No Ctx Sw if current task is highest rdy */
                OSTCBHighRdy = OSTCBPrioTbl[OSPrioHighRdy];
#if OS_TASK_PROFILE_EN > 0
                OSTCBHighRdy->OSTCBCtxSwCtr++; /* Inc. # of context switches to this task */
#endif
                OSCtxSwCtr++; /* Increment context switch counter */
                OS_TASK_SW(); /* Perform a context switch */
            }
        }
    }
    OS_EXIT_CRITICAL();
}

首先，关中断，判断是否在IRQ中（是否嵌套大于0），如果不在，可以执行调度

然后，判断调度是否被禁止OSLockNesting == 0

调用OS_SchedNew();函数调度

/*
*********************************************************************************************************
* FIND HIGHEST PRIORITY TASK READY TO RUN
*
* Description: This function is called by other uC/OS-II services to determine the highest priority task
* that is ready to run. The global variable 'OSPrioHighRdy' is changed accordingly.
*
* Arguments : none
*
* Returns : none
*
* Notes : 1) This function is INTERNAL to uC/OS-II and your application should not call it.
* 2) Interrupts are assumed to be disabled when this function is called.
*********************************************************************************************************
*/

static void OS_SchedNew (void)
{
#if OS_LOWEST_PRIO <= 63 /* See if we support up to 64 tasks */
    INT8U y;


    y = OSUnMapTbl[OSRdyGrp];
    OSPrioHighRdy = (INT8U)((y << 3) + OSUnMapTbl[OSRdyTbl[y]]);
#else /* We support up to 256 tasks */
    INT8U y;
    INT16U *ptbl;


    if ((OSRdyGrp & 0xFF) != 0) {
        y = OSUnMapTbl[OSRdyGrp & 0xFF];
    } else {
        y = OSUnMapTbl[(OSRdyGrp >> 8) & 0xFF] + 8;
    }
    ptbl = &OSRdyTbl[y];
    if ((*ptbl & 0xFF) != 0) {
        OSPrioHighRdy = (INT8U)((y << 4) + OSUnMapTbl[(*ptbl & 0xFF)]);
    } else {
        OSPrioHighRdy = (INT8U)((y << 4) + OSUnMapTbl[(*ptbl >> 8) & 0xFF] + 8);
    }
#endif
}

当最低优先级小于63时，即优先级只有0-63，那么执行就绪表最高优先级查找算法OSUnMapTbl[]

当优先级大于63时，4位表示一个表元素，

最后都将最高优先级写入OSPrioHighRdy变量

 

检查OSPrioHighRdy和现在运行的任务的优先级是不是匹配，如果不是当前运行的程序

OSTCBHighRdy = OSTCBPrioTbl[OSPrioHighRdy]，修改要运行的最高优先级TCB

如果使能Ext，OSTCBHighRdy->OSTCBCtxSwCtr++;将切换次数变量自加

OSCtxSwCtr++;系统上下文切换计数器自加

OS_TASK_SW();切换任务

;**********************************************************************************************************
; PERFORM A CONTEXT SWITCH (From task level)
; void OSCtxSw(void)
;
; Note(s): 1) Upon entry:
; OSTCBCur points to the OS_TCB of the task to suspend
; OSTCBHighRdy points to the OS_TCB of the task to resume
;
; 2) The stack frame of the task to suspend looks as follows:
;
; PC (High memory)
; LR(R14)
; R12
; R11
; R10
; R9
; R8
; R7
; R6
; R5
; R4
; R3
; R2
; R1
; R0
; OSTCBCur->OSTCBStkPtr ----> CPSR (Low memory)
;
;
; 3) The stack frame of the task to resume looks as follows:
;
; PC (High memory)
; LR(R14)
; R12
; R11
; R10
; R9
; R8
; R7
; R6
; R5
; R4
; R3
; R2
; R1
; R0
; OSTCBHighRdy->OSTCBStkPtr ----> CPSR (Low memory)
;*********************************************************************************************************/
OSCtxSw

STMFD SP!, {LR} ;PC
STMFD SP!, {R0-R12, LR} ;R0-R12 LR
MRS R0, CPSR ;Push CPSR
STMFD SP!, {R0}

;----------------------------------------------------------------------------------
; OSTCBCur->OSTCBStkPtr = SP
;----------------------------------------------------------------------------------
LDR R0, =OSTCBCur
LDR R0, [R0]
STR SP, [R0]

;----------------------------------------------------------------------------------
; OSTaskSwHook();
;---------------------------------------------------------------------------------
BL OSTaskSwHook

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

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

;----------------------------------------------------------------------------------
; OSTCBHighRdy->OSTCBStkPtr;
;----------------------------------------------------------------------------------
LDR R0, =OSTCBHighRdy
LDR R0, [R0]
LDR SP, [R0]

;----------------------------------------------------------------------------------
;Restore New task context
;----------------------------------------------------------------------------------
LDMFD SP!, {R0} ;POP CPSR
MSR SPSR_cxsf, R0
LDMFD SP!, {R0-R12, LR, PC}^

完成三个任务，

第一，OSTCBCur修改指向将要被挂起任务的TCB，将OSTCBHighRdy指向将要被运行的任务的TCB

。。。 

以前看过的ARM体系结构和指令集都忘记了，必经没有实际使用过，没有深刻的印象，今天对照着指令集从新看过这个汇编函数：

这个函数做了三件事情

第一步，把当前任务对应的R0-R12压栈，使用STMFD指令将PC写入LR(R14)寄存器，使用回写‘！’指令使其写入的地址递增，将寄存器值压入栈的顺序为FD满栈递减，从栈底开始向低地址写入寄存器值

第二步，OSTCBCur = OSTCBHighRdy;OSPrioCur = OSPrioHighRdy; SP=OSTCBHighRdy->OSTCBStkPtr;

具体实现了最高优先级prio和TCB对现在运行的prio和TCB的同步

将SP(R13)指向现在要运行的TCB的栈指针

第三步，对切换后的任务TCB的栈进行弹栈操作，使用LDMFD弹出CPSR到R0 使用MSR将R0写入SPSR，弹出R0-R12 R14 R15