Full subtractor performs subtraction of two bits, one is minuend and other is subtrahend. In full subtractor ‘1’ is borrowed by the previous adjacent lower minuend bit. Hence there are three bits are considered at the input of a full subtractor. There are two outputs, that are DIFFERENCE output D and BORROW output Bo. The BORROW output indicates`that the minuend bit requires borrow ‘1’ from the next minuend bit. Figure shows the truth table of a full
subtractor. The K-maps for the two outputs are shown in figure. If we compare DIFFERENCE output D and BORROW output Bo with full adder`it can be seen that the DIFFERENCE output D is the same as that for the SUM output. Further, the BORROW output Bo is similar to CARRY-OUT. In the case of a half-subtractor, A input is complemented similar things are carried out in full subtractor.
Figure below shows the implementation of full subtractor using logic gates.