• Locked Locked
  • Replies 3 replies
  • Subscribers 119 subscribers
  • Views 10362 views
  • Users 0 members are here
2025 survey

We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.

Options
Related
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

[Cortex-M3] PendSV, svc and priorities

brag braga
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...
Parents
  • Arm Employee Badge
    Note: This was originally posted on 15th June 2012 at http://forums.arm.com

    Since SVC is exception 11 and pendSV is exception 14 , if both of them happen at the same time and if both of them are the same priority, the SVC should take place first.regards. Do you have trace tools? For example, even with low cost debug adapter like ulink2 you can get exception trace, which might be useful for debugging problem like this as it shown the exceptions triggered.
    regards,
    Joseph

Reply
  • Arm Employee Badge
    Note: This was originally posted on 15th June 2012 at http://forums.arm.com

    Since SVC is exception 11 and pendSV is exception 14 , if both of them happen at the same time and if both of them are the same priority, the SVC should take place first.regards. Do you have trace tools? For example, even with low cost debug adapter like ulink2 you can get exception trace, which might be useful for debugging problem like this as it shown the exceptions triggered.
    regards,
    Joseph

Children
No data