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How does cortex-M33, for example, know previous exception priority?

Susumu Endoh

Hello

I want to know about exception priority.

In following situation

  • A has highest priority, described as priority A.
  • B has medium priority, described as priority B.
  • C has lowest priority, described as priority C.

when following sequence is occurred.

  1. CPU runs in thread mode
  2. Exception C occurs. The CPU enter the handler mode. And the execution priority is priority C.
  3. Exception B occurs. The execution priority is priority B.
  4. Exception A occurs. The execution priority is priority A.
  5. When ISR for Exception A reaches end of task so use EXC_RETURN to return exception.
  6. Now execution back to ISR for B and the execution priority might be priority B.

My Question is how does the Cortex-M33 know correct priority number(priority B in above sequence #6)

Regards,

Susumu

  • 0

    This is a combination of the context being saved on the "stack" and the LR containing the proper EXC_RETURN value so that the proper context gets restored.

    USP   and Thread Mode to start.

           Execption C, context saved on USP and LR set proper to restore from there and Thread Mode -   on EXC_RETURN will return to USP and Thread Mode

    MSP and Handler Mode

           Exception B, context saved on MSP and LR set proper to restore from there and Handler Mode + Priority C  on EXC_RETURN will return to MSP Handler Priority C

    MSP and Handler Mode

            Exception A, context saved on MSP and LR set proper to restore from there and Handler Mode + Priority B On EXC_RETURN will return to MSP Handler Priority B

          

  • 0 in reply to Robert McNamara

    Robert,

    Thank you for your reply, But I want to clear more.
    In the Exception B handler on priority B, then Exception A occurrs, priority will be Priority A.
    To restore priority B when back from Exception A. Where is location to save Priority-B?

    I Think the saved context which you mentioned is right place to save current priority(Priority-B in above situation).
    But I Could not find out.

    Please show me more details.

    Thanks,
    Susumu

  • +1 in reply to Susumu Endoh

    The "Current Exception Number" is in the xPSR (Special Purpose Program Status Register)

    When there is a new exception taken, this is one of the registers that is saved on the stack.  On Return, this register is "restored".  The Exception Number restored indicates which priority should also be "restored"

  • 0 in reply to Robert McNamara

    Robert,

    Thank you for the additional explanation. I understood priority determination mechanism.

    Susumu