Maths for Computing

Two's Compliment

The two's compliment of a number is defined as that number which when added to the original number will result in a sum of zero.

The binary number 110110110110 has a two's compliment of 001001001010:

                                             110 110 110 110

                                             001 001 001 001

   Ignore the carried 1    ( 1) 000 000 000 000

Subtraction can also be preformed using the two's compliment method.

To subtract A from B, A must be expressed as it's two's compliment and then the value of B is added to it, e.g.

           001 000      A(8) base 10
 

           111 000     Two's compliment of A (i.e. -A)
           101 111      B(47) base 10
     (1) 100 111      B-A ( 39) base 10

Ignore the carry