The subtraction operation can be implemented with the help of binary adder circuit, because
We know that 2's complement representation of a number is treated as a negative number of the given number.
We can get the 2's complements of a given number by complementing each bit and adding 1 to it.
The circuit for subtracting A-B consist of an added with inverter placed between each data input B and the corresponding input of the full adder. The input carry 
must be equal to 1 when performing subtraction.
The operation thus performed becomes A , plus the 1's complement of B , plus 1. This is equal to A plus 2's complement of B .
With this principle, a single circuit can be used for both addition and subtraction. The 4 bit adder subtractor circuit is shown in the figure. It has got one mode ( M ) selection input line, which will determine the operation,