Hi Olin.

>   old  |  new  |  out
>   -----|-------|-----
>     0  |    0  |    0
>     0  |    1  |    1
>     1  |    0  |    0
>     1  |    1  |    0

!a&b=(a^b)&b

        movfw   in              ;b
        xorwf   old_sh,W        ;a^b
        xorwf   old_sh,F        ;b (stored)
        andwf   old_sh,W
        movwf   out

WBR Dmitry.

PS. I wish I could sleep less ;)



Olin Lathrop wrote:
>
> You have 4 possible cases as shown by this truth table:
>
>   old  |  new  |  out
>   -----|-------|-----
>     0  |    0  |    0
>     0  |    1  |    1
>     1  |    0  |    0
>     1  |    1  |    0
>
> One way (there are others) to describe what you want is OUT = (-OLD and
> NEW).  Here is a brute force example with bank selection left as an exercise
> to the reader:
>
>      movf   portb, w      ;get the NEW bits into W
>      movwf  temp          ;temp save them
>      andwf  nold, w       ;AND the NEW bits with -OLD bits
>      movwf  out           ;save OUT bits
>      comf   temp, w       ;get -NEW bits
>      movwf  nold          ;save as -OLD bits for next time
>
> This can probably be optimized (Dimitry? Scott?) but I'm trying to
> illustrate the algorithm and show how it implements the truth table above.

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