To perform a binary subtraction you first have to
represent the number to be subtracted in its negative
form. This is known as its two's complement.
The two's complement of a binary number is obtained by:
 Replacing all the 1's with 0's and the 0's with 1's.
This is known as its one's complement.
 Adding 1 to this number by the rules of binary
addition.
Now you have the two's complement. Example:
The decimal subtraction 29  7 = 22 is the same as adding
(29) + (7) = 22

Convert the number to be subtracted to its two's
complement:
00000111 
(decimal 7) 
11111000 
(one's complement) 
+ 00000001 
(add 1) 
11111001 
(two's complement) 
11111001 now represents 7.

Add
29 
00011101 
+ 7 
11111001 
22 
(1)00010110 
Note that the final carry 1 is
ignored.
