RE: Missing parameter

Have you tried using %PARMS on the external procedure?
Heres a quick example...
P prExtProc B EXPORT
D PI N
* Optional Display only
D vbOptional N OPTIONS(*NOPASS) VALUE

* Default Display only Parameter
D pbDisplayOnly S N

/FREE
// If parameter passed, use it
pbDisplayOnly = %PARMS > *ZEROS AND vbOptional;
// Rest of stuff...
RETURN pbDisplayOnly;
/END-FREE
P prExtProc E

That's the approach I generally use, but with a twist for readability (and
to simplify/clarify the default settings)

Using your example, I would set pbDisplayOnly to a default value (in this
case, it would be INZ(*Off)).

Then, you'd see:
If %Parms >= 1 ;
pbDisplayOnly = vbOptional ; // Override default with specified
setting
EndIF ;

If pbDisplayOnly ;
// et cetera

In this manner, the default is clearly stated, and the override is simple to
understand. Plus, adding a second (or third, or more) optional parameter
becomes a relatively simple maintenance chore.

Regards,

Dennis Lovelady
http://www.linkedin.com/in/dennislovelady
--
"Life is what happens while you are making other plans."
-- John Lennon