from
The Free On-line Dictionary of Computing (8 July 2008)
twos complement
<data> 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)