μcosii源码试读17- OSStart系统开始运行

在main函数中不能init中断，只能创建task 和event（消息队列消息邮箱等），在系统的第一个子程序中才能init中断，开启timetick开始调度任务，中断向量表应该在main函数运行之前的汇编boot函数中就已经确定了。

看一下main中的最后一个函数调用

/*
*********************************************************************************************************
* START MULTITASKING
*
* Description: This function is used to start the multitasking process which lets uC/OS-II manages the
* task that you have created. Before you can call OSStart(), you MUST have called OSInit()
* and you MUST have created at least one task.
*
* Arguments : none
*
* Returns : none
*
* Note : OSStartHighRdy() MUST:
* a) Call OSTaskSwHook() then,
* b) Set OSRunning to OS_TRUE.
* c) Load the context of the task pointed to by OSTCBHighRdy.
* d_ Execute the task.
*********************************************************************************************************
*/

void OSStart (void)
{
    if (OSRunning == OS_FALSE) {
        OS_SchedNew(); /* Find highest priority's task priority number */
        OSPrioCur = OSPrioHighRdy;
        OSTCBHighRdy = OSTCBPrioTbl[OSPrioHighRdy]; /* Point to highest priority task ready to run */
        OSTCBCur = OSTCBHighRdy;
        OSStartHighRdy(); /* Execute target specific code to start task */
    }
}

在task创建函数中我们已经将就绪表中我们创建的函数置为rdy，也就是说，所有初始化创建的任务都是就绪态

那么直接开始调度函数OS_SchedNew，就会将OSPrioHighRdy设置为task0

第二部，将调度程序求出来的最高优先级赋值给OSPrioCur

第三步，将指向当前运行TCB的指针OSTCBCur指向最高优先级的taskTCB

第四步，调用OSStartHighRdy函数

;*********************************************************************************************************
; START MULTITASKING
; void OSStartHighRdy(void)
;
; The stack frame is assumed to look as follows:
;
; Entry Point(Task Name) (High memory)
; LR(R14)
; R12
; R11
; R10
; R9
; R8
; R7
; R6
; R5
; R4
; R3
; R2
; R1
; R0 : argument
; OSTCBHighRdy->OSTCBStkPtr --> CPSR (Low memory)
;
; Note : OSStartHighRdy() MUST:
; a) Call OSTaskSwHook() then,
; b) Set OSRunning to TRUE,
; c) Switch to the highest priority task.
;********************************************************************************************************** */
OSStartHighRdy
;----------------------------------------------------------------------------------
; OSRunning = TRUE;
;----------------------------------------------------------------------------------

MSR CPSR_cxsf,#SVCMODE|NOINT ;Switch to SVC mode with IRQ&FIQ disable

BL OSTaskSwHook ;Call user define Task switch hook

LDR R0, =OSRunning ; OSRunning =TRUE
MOV R1, #1
STRB R1, [R0]

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

;----------------------------------------------------------------------------------
; Prepare to return to proper mode
;----------------------------------------------------------------------------------
LDMFD SP!, {R0}
MSR SPSR_cxsf, R0
LDMFD SP!, {R0-R12, LR, PC}^

OSRunning =TRUE，SP = OSTCBHighRdy->OSTCBStkPtr;弹出堆栈中的上下文

 

很有必要认真学习ARM和x86体系的汇编语言和简单的boot驱动

以便更好的移植各种操作系统