> Does anyone know how to determine whithin a MPASM macro whether a
parameter
> has been specified?

Hello, Dwayne!

Suggestion: use simple arythmetic to make compiler think it
performs a smart job :)

test_parm       MACRO             p1
;
; Parameter p1 is parsed here as 0 if the value
; is not supplied to the list.
;
        if ( p1+0 ) == 0                ; parser with 0 default value
        messg "Parm 1 default"  ; so this is the case of your
        retlw   0x0                     ; 1st example
        else
        messg "Parm 1 defined"
        retlw   p1
        endif
        ENDM

I guess something more complex can parse zeros (eg. logical operation "!")
There is also a #v() "text substitution" operation which may be of some
use.
                             sincerely yours, Andrew.