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Albert,

>Now I'm curious. In my experience, a subroutine call involved saving the
>current address (usually on a stack), branching to the subroutine, and
>branching back to the saved address ( plus 1 instruction) at the end of the
>subroutine. If RPG doesn't do it this way, how does it do it?

(Disclaimer:  I don't work for IBM; this is just my observations on
what RPG does.)

It does save the address, but not with push/pop operations on a stack
the way most languages do.  It in essence can save one address per
subroutine.  It just stores the address, and when (or if) you reach
the ENDSR it branches to the address stored.  Since nothing was put on
a stack, if you branch out of the subroutine (via GOTO or *PSSR) prior
to reaching the ENDSR, nothing is left unbalanced.

Since there is only one return addresss storable per subroutine, RPG
cannot let you perform recursion (except now with subprocedures).
This helps explain this admonition from the RPG reference manual:

"An EXSR or CASxx specification within a subroutine cannot directly
call itself.  Indirect calls to itself through another subroutine
should not be performed, because unpredictable results will occur."

Also:

"A GOTO within a subroutine in the main procedure can be issued to a
TAG within the same subroutine, detail calculations or total
calculations."

Having the *PSSR branch back to *DETC (or another spot in the cycle)
is somewhat like performing a GOTO from with the subroutine -- it just
never reaches the ENDSR to branch back to the calling address.  The
next time the subroutine is called, its caller's address overlays the
previous return address, but the stack remains balanced.

Or at least, that is my understanding of what RPG is doing...

Doug

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