There were various math libraries available then and I'd used one for
solving simultaneous equations.  I have the source code for the routine I've
called (and perhaps others called SIMEQ).

Subroutine SIMEQ frequently does this:

CALL OVERFL (IBIG)

Now, I don't think there's such a library function on my DVF system and I'd
just as soon go ahead and continue to use this SIMEQ subroutine with a
suitable replacement to these OVERFL calls - or to just eliminate them
altogether.

Does anyone know what I'm dealing with here specifically?  I believe one of
the main math libaries was called Mathpack.

The OVERFL subroutine tests for exponent overfow.
Form: CALL OVERFL ( i ) where i is an integer variable or array element.
OVERFL determines whether or not an exponent overflow has occurred since
last call to OVERFL/OVUNFL (or the start of the program, if  OVERFL/OVUNFL
has not been previously called).
..
If an overflow condition has occurred, the value returned in i is 1.
..
If non overflow condition has occurred, the value returned in i 2.
After the value of i has been set, the overflow indicator is cleared.

Hope this helps,
Kurt