In FORTRAN IV, multiple return labels could be passed to a subroutine. This
was carried into PL/I. In FORTRAN 66, this was not supported. But, FORTRAN IV and 66 both support assigned go to, which can be used to replace this -- WATFIV supported this (local routines that use ASSIGN, with a bit of syntax sugar).
Note that this is a LONG time ago (mid sixties)...
(Microsoft F80 was a FORTRAN 66, slightly subset compiler -- no COMPLEX
that Microsoft sold from the late 70s to early 80s).
In there find pop.mac which is a short 4 byte assembly routine to discard
a stack level. That can be used as follows:
ASSIGN 1 TO I -- STORE ADDRESS OF LABEL 1 TO I
CALL F(I) -- PASS I TO SUBROUTINE F
... -- WE WILL NEVER GET HERE
1 ...
SUBROUTINE F(I)
EXTERNAL $POP
INTEGER $POP
JUNK = $POP(0) -- REMOVE RETURN ADDRESS
GO TO I -- GO TO ASSIGNED LABEL I
END
The ASSIGN command puts the address of label 1 into INTEGER I
GO TO I then transfers to that location. Note that we can pass I into the
SUBROUTINE, use $POP to remove a stack level (the return address), then
jump to the variable. Scheme is nicer, which continuations, of course, but
that hadn't been considered yet...
The ASSIGN was deleted as a feature in FORTRAN 95 -- obsolescent by FORTRAN 90.
I guess "very very old-school".
Now, the FORTRAN 66 standard doesn't mention if something like this is allowed... but, since no stack was involved in early implementations, and
recursion was not allowed, I imagine that it would work more widely than
might otherwise be expected.
