ones complement

A system used in some computers to represent negative

numbers. To negate a number, each bit of the number is

inverted (zeros are replaced with ones and vice versa). This

has the consequence that there are two reperesentations for

zero, either all zeros or all ones.

...

000...00011 = +3

000...00010 = +2

000...00001 = +1

000...00000 = +0

111...11111 = -0

111...11110 = -1

111...11101 = -2

111...11100 = -3

...

Naive logic for ones complement addition might easily conclude

that -0 + 1 = +0.

The twos complement avoids this by using all ones to

represent -1.