3.4 ADD-SUBTRACT UNIT         

      The eight-bit adder-subtractor was made with the help of eight-bit ripple carry adder as shown in figure four. Adapting the two’s complement numbering scheme and depending on the fact that subtraction is simply an addition of the negative version of operand, the circuit of the full adder can be improved to perform the subtraction operation by using an XOR gate and the Binvert signal issued by the FSM. Overflow detection for this unit was added which simply an XOR gate for the carry of the last two stages.