D. F. Welch wrote:
>
> Barry,
>    2's complements are simply the "not" of the value + 1
>
>         For example:
>
>         movlw   0xFF            ; Load -1 into W
>         xorwf   value,f ; Form  1's complement
>         incf    value           ; Form 2's complement

That is one brute force way to do it. A better way is this:

        COMF    value,f
        INCF    value

And if you want to complement W:

        XORLW   0xff
        ADDLW   1

I personally prefer Payson's solution for complementing W (which is
suitable for the 12-bit cores):

        ADDWF   some_register,W         ;new W = old W + x
        SUBWF   some_register,W         ;W = x - (old W + x) = -W

Another solution (that I believe Payson also posted) was to do this:

        SUBWF   known_zero,W            ;W = 0 - W = -W

Where 'know_zero' is a file register that is equal to zero (this is very
handy for other stuff too).


Scott