Arm Development Studio forum [Cortex-M3] PendSV, svc and priorities

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[Cortex-M3] PendSV, svc and priorities

Note: This was originally posted on 15th June 2012 at http://forums.arm.com

Is it possible a case when PendSV will execute earlier than SVcall when both have the same priority? The cpu is Cortex-M3 STM32F1xx

I had an issue when PendSV and SVC have the same priority levels (lowest) with this code:

ePendSV_handler:
 mrs r0,PSP
 stmdb r0!,{r4-r11,lr}

; ... load ... load new PSP valu here to R0

 ldmia r0!,{r4-r11,lr}
 msr PSP,r0
 bx lr


 void eSVCall(void){
 unsigned int a0,a1,a2,a3,svc,*psp;
 
 psp=(U32*)__get_PSP();
 a0=psp[0];a1=psp[1];a2=psp[2];a3=psp[3]; //get registers from user stack
 svc=psp[4]; // service call ID
 
 a0=service(svc,a0,a1,a2,a3);
 
 psp[0]=a0; //overwrite R0(return value) in user stack
}

 SVC usage in thread level code:
 mov R12, #1 ; service call ID 
 mov R0, #123 ; arg
 svc #0
 ; now R0 holds return value

PENDSVSET can be set asynchronously from timer interrupt or from service() function. Sometimes I had a situations when ePendSV was called before eSVCall and overwrited PSP register and then eSVCall fails because new PSP is not tend to the thread which called svc instruction. When I incrased priority of SVcall exception the code above began working correct.

Can some exception execute and terminate executiong svc instruction when svc instruction is fetched but not executed yet?

sorry for bad english...