<x-flowed>At 10:06 AM 3/7/00 +0200, Werner Soekoe wrote:
>Hi
>
>I'm looking for the way to calculate what the Two's
>Complement of a value will be. Can anyone help me?
>For instance:
>
>8-bit value:
>01101110
>10010001 (Inverted)
>
>But this is not two's complement yet!? What Next?
>
>Can somebody maybe also solve this one:
>16-Bit value:
>01011100 11110110
>10100011 00001001 (Inverted)

What you just did is called one's compliment.  To turn the result into
two'scompliment, just add 1 (and propagate any carries between bytes)

Here are 2 different routines, pulled from working code

;2 byte negate.  Invert all bytes, add 1.  Extensable to 3 or more bytes.
     comf        AD3M,F
     comf        AD3H,F
     incf        AD3M,F
     skpnz
       incf      AD3H,F

;3 byte negate (extensable)
     comf        AD3L,F
     comf        AD3M,F
     comf        AD3H,F
     movlw       .1
     addwf       AD3L,F
     skpnc
       addwf     AD3M,F
       skpnc
         addwf   AD3H,F

hope this helps.

dwayne



Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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