Interrupt local variables overlayed?[OK]

[Dany]

Answered by Janni:

This should not be a problem if respective internal registers (R5,6,7) are saved and restored in ISR. You could check this in *.lst file.
Normally, saving context includes all internal registers used both in ISR and main code. And optimizer may replace local variables with internal registers in ISR, just like in any routine - the only difference is that ISR requires restoring previous values of internal registers.

and Janni was of course right: There is no problem, the values of registers R0..R6 are saved. I should have seen that...

Original post:

Hi mE people,

I saw in a project (not mine) that local variables of the interrupt routine are overlayed with some other variables. I was under the impression that was impossible (meaning: should not be allowed).

Apparently, sometines mP for PIC v.6.4.0. optimiser decides that it can be done, see below address 4,5 and 6.

Capture3-12-2014-16.03.05.jpg (161.61 KiB) Viewed 2829 times

The source code looks like this:

Code: Select all

procedure interrupt;
var
  enc, bbb  : byte;
  tas       : byte;
  
begin
   ...
end;

I can not reproduce this behaviour in testcode, and I can not publish the code that gives the phenomenon.
The PIC used in the problem project is P18F67J60.

Thanks for investigating.

[janni]

Hi Dany,

This should not be a problem if respective internal registers (R5,6,7) are saved and restored in ISR. You could check this in *.lst file.

Normally, saving context includes all internal registers used both in ISR and main code. And optimizer may replace local variables with internal registers in ISR, just like in any routine - the only difference is that ISR requires restoring previous values of internal registers.

[Dany]

Hi Dany,

This should not be a problem if respective internal registers (R5,6,7) are saved and restored in ISR. You could check this in *.lst file.

Normally, saving context includes all internal registers used both in ISR and main code. And optimizer may replace local variables with internal registers in ISR, just like in any routine - the only difference is that ISR requires restoring previous values of internal registers.

Thanks Janni,
You are indeed right, there is no problem, the values of registers R0..R6 are saved. I should have seen that...

I am relieved because I did not know why the project worked while some variables were (I assumed) overwritten during interrupts...