ucOS-II基于ARM920T的OSCtxSw实现分析

首先必须了解，在将ucos-II移植到ARM920T时，使用了如下结构的任务栈：

程序清单：基于ARM920T的OSCtxSw实现分析

注意：ARM需手动保存pc和psw

OSCtxSw

; Special optimised code below:

；1、根据任务栈结构，分别压栈，保存旧任务的现场

stmfd sp!,{lr}; push pc (lr should be pushed in place of PC)

stmfd sp!,{r0-r12,lr}; push lr & register file

mrs r4,cpsr

stmfd sp!,{r4}; push current psr

mrs r4,spsr

stmfd sp!,{r4}; push current spsr

; 2、OSPrioCur = OSPrioHighRdy

ldr r4,=OSPrioCur

ldr r5,=OSPrioHighRdy

ldrb r6,[r5]

strb r6,[r4]

; 3、Get current task TCB address:r5=OSTCBCur

ldr r4,=OSTCBCur; r4 = &OSTCBCur

ldr r5,[r4]

；4、OSTCBCur->OSTCBStkPtr = SP;

str sp,[r5]; store sp in preempted tasks's TCB

bl OSTaskSwHook; call Task Switch Hook

; 5、Get highest priority task TCB address:r6=OSTCBHighRdy

ldr r6,=OSTCBHighRdy

ldr r6,[r6]

；6、SP = OSTCBHighRdy ->OSTCBStkPtr;

ldr sp,[r6]; get new task's stack pointer

; 7、OSTCBCur = OSTCBHighRdy

str r6,[r4]; set new current task TCB address

；8、根据任务栈结构，分别出栈，恢复新任务的现场

ldmfd sp!,{r4}; pop new task's spsr

msr SPSR_cxsf,r4

ldmfd sp!,{r4}; pop new task's psr

msr CPSR_cxsf,r4

ldmfd sp!,{r0-r12,lr,pc}; pop new task's r0-r12,lr & pc