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Cortex-M3 Thread mode stack

Marcus Harnisch
Note: This was originally posted on 25th August 2010 at http://forums.arm.com

Gentlemen

I am wondering if, while processing an arbitrarily nested ISR, there is a reliable way to determine which stack thread mode is using (process or main). The CONTROL register seems not appropriate since the SPSEL bit is reserved in Handler mode. The only other place I can think of is the (stacked) link register of the ISR preempting thread mode. But how do I get to it? We don't know the sizes of stack frames in between this and the current ISR. I must have missed something...

Thanks
Marcus
  • Note: This was originally posted on 25th August 2010 at http://forums.arm.com

    ...the stack pointer selection can be changed by the value of EXC_RETURN when you carry out an exception return to thread.

    That is exactly what I am doing. Similar to the untrusted ISR example in your book. Ideally, I would want the untrusted ISR use the same stack that "normal" thread mode uses. For that I'd need the EXC_RETURN of the ISR that preempted Thread mode in the first place. Since the goal is a short paper, I am clearly over-engineering this to show some features.
    In my current implementation, it must be ensured that the user ISR is the first ISR, i.e. has lowest prio of all, so that I can access its EXC_RETURN directly. In practice, this limitation is not that big of a deal since giving a non-trusted ISR a higher priority than other ISR creates the potential of many other issues.

    Thanks
    Marcus
  • Arm Employee Badge
    Note: This was originally posted on 25th August 2010 at http://forums.arm.com

    I don't think there is a way to do this, even if you can determine which stack pointer was used, the stack pointer selection can be changed by the value of EXC_RETURN when you carry out an exception return to thread.
  • Arm Employee Badge
    Note: This was originally posted on 25th August 2010 at http://forums.arm.com

    Hi Marcus,

    I see. Yes, trying to do that is difficult. 
    But then when you have untrust ISR, it is likely that you want this ISR to use the process stack only.
    The thread could be running with main stack or process stack, it should still work.

    regards,
    joseph