Quite similar to the half adder, a half subtractor subtracts two 1-bit binary numbers to give two outputs, difference and borrow. Similarly, for the carry output of the half adder, we need to add Y(A+B) in an OR configuration. So we add the Y input and the output of the half adder to an EXOR gate. The equation for SUM requires just an additional input EXORed with the half adder output. Where X is the equation for EXOR and X’ is the equation for EXNOR.įrom the two equations of SUM and CARRY, we can design the following combinational logic circuit for a full adder.Ĭompare the equations for half adder and full adder.
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