???
The only difference I've saw is when
x0-x1=FFFF and y0-y1=FFFF
the status bit C remains zero in mine's and 1 in yours. In that case we have
to add clrc?
No problem. I've changed and still works fine.
Perhaps that explains some sporadic negative "spikes" in some histograms.
I will try the two forms just to see what happens.

Dennis.
You have to see Andrews solution.hats :)

Scott Dattalo wrote:
> Shouldn't the 6-instruction 16-bit add be written:
>
>       movf    x0,w
>       addwf   y0,f
>       movf    x1,w
>       skpnc
>        incfsz x1,w
>         addwf y1,f
>
>   and then the rotate...
>
>       rrf     y1,f
>       rrf     y0,f
>
> Scott

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