All hail Scott, a true Wizard!

This thing contains several great tricks.

>         movwf   temp    ;shift left by 2
>         addwf   temp,f  ;by adding. puts 'a' into carry
>                         ;and bcde into the upper nibble

Very nice way to isolate the pieces needed.

>         andlw   7       ;grab the lower three bits
>         skpnc           ;if the MSB ('a') was set then

Depends on the carry set two instructions ago.

>          addlw  2       ;a*2 == 2, W = a*2 + fgh.
>         swapf   temp,f  ;put bcde into the lower nibble
>         subwf   temp,w  ;W = bcde - (a*2 +fgh)

The comment doesn't say so, but of course the high nibble of
W is trash right now, but ....

>   ;at this point, if N%9 is zero, then W contains -9,0,9
>   ;so just check these three cases
>
>         skpdc           ;If W is less than 0
>          addlw  9       ; then add 9

...it doesn't matter because we use the digit carry instead of the full
carry!

>         andlw   0x0f    ;If W was/is zero then set Z

...and then (and only then!) get rid of the high order bits.

>         skpz            ;If W is greater than 0
>          addlw  -9      ; then subtract 9

Very, very nice!

[and I would be most surprised if anyone could improve on 12 instructions]

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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