555 ISRs in ThreadX

I am using Multi 4.07 and ThreadX.  I am struggling to implement the
profiling feature in Multi which should help me to determine how long
each function is taking to execute.

I have noticed unexpected behaviour on the srr0 register which I
thought was meant to hold the location of the next instruction
following the return from the ISR (rfi).  ThreadX handles all the
context saving and advises against using the __interrupt keyword or
(#pragma interrupt).  

When I step through my ISR, the srr0 register just acts like the
Program Counter (PC).  It is almost as if the 555 is unaware that I am
inside an ISR!  Why might this be?  

My program successfully returns from the ISR so I suspect that ThreadX
relies on the Link Register (instead of srr0) that it saves onto the
Interrupt Stack Frame at the beginning of the ISR.

Any help would be appreciated.




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