Given a

binary number x, the number

NOT(x) is called the

*1's complement* of x. The

NOT operator when applied on a bit reverses the '

truth' of the bit i.e.

Not(TRUE)=FALSE and Not(FALSE)=TRUE
NOT(1)=0 and Not(0)=1

We extend the NOT operator to a multi-bit value, so that Not(x) is the value with all its bits reversed.

So, the

*1's complement* of:

1010 1010
would be:

0101 0101
the

binary number inverted.