>    - ISR modifies the
>      program counter on return to userspace by decrementing or
>      incrementing it by a fixed offset, or to a nearest multiple of
>      16/whatever.
> 


... and similar tricks:

The ISR needs to know if the value already had been modified (i.e
stored) or not. So it needs to check if the PC was before or after the
store instruction and have the user space function either redo the
critical section or just proceed.

-Michael
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