twos complement

    A system used in some computers to represent negative
   numbers in binary.  Each bit of the number is inverted
   (zeros are replaced with ones and vice versa), as for ones
   complement, but then one (000...0001) is added (ignoring
   overflow).  This avoids the two representations for zero found
   in ones complement by using all ones to represent -1.

   	...
   	000...00011 = +3
   	000...00010 = +2
   	000...00001 = +1
   	000...00000 =  0
   	111...11111 = -1
   	111...11110 = -2
   	111...11101 = -3
   	...

   This representation simplifies the logic required for addition
   and subtraction, at the expense of a little extra complexity
   for negation.

   (1994-10-31)