RE: *PSSR subroutine and the call stack

Pete Hall <pbhall@execpc.com> wrote:

>Several years ago, I had a program that did something like this:
>
>mainline calls Sub A
>Sub A calls Sub B
>Sub B calls Sub C
>Sub C calls Sub A
>Sub A returns to mainline (not Sub C) - this was not the desired behavior
>
>From this experience, I surmise that subroutine returns are implemented as
>branches to a program counter address which is set at the time the
>subroutine is first invoked.

This didn't seem right to me so I tested it using the following simple program.

     C                     EXSR A
     C                     MOVE *ON       *INLR
     C           A         BEGSR
     C           *IN30     IFEQ *OFF
     C                     MOVE *ON       *IN30
     C                     EXSR B
     C                     ENDIF
     C                     ENDSR
     C           B         BEGSR
     C                     EXSR C
     C                     ENDSR
     C           C         BEGSR
     C                     EXSR A
     C                     ENDSR

When you run it you find mainline calls A, A calls B, B calls C, C calls A.

A then ends and returns to C (not mainline). C returns to B, B returns to A, A
returns to C and so on ad infinitum.
It seems as though the return address is set when the subroutine is called,
overwriting any previous return address.

The ENDSR is therefore effectively a branch to the instruction following the
last EXSR to have called the corresponding BEGSR. However, because the manual
says the behaviour is unpredictable this could change from release to release.
It simply depends how the compiler writer has implemented the EXSR rules.

Dave Kahn, ABB Steward Ltd.


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