> >        addlw   256-hival
> >        addlw   (hival-loval)+1
> >        btfsc   status,cf
> >        goto    inrange
>
>    Now try  w = 75, hival = 100, loval = 50
>
>            75 + (256 - 100) = 231
>            231 + (100 - 50 +1) = 252
>    which does not generate a carry.
>
>    In both cases the starting value is in range.   Did I miss something?

What's 231+(100-50 +1) ?