An electronic logic circuit for calculating the difference between two binary numbers, the minuend and the number to be subtracted, the subtrahend (see table). A full subtractor performs this calculation with three inputs: minuend bit, subtrahend bit, and borrow bit. It produces two outputs: the difference and the borrow. Full subtractors thus allow for the inclusion of borrows generated by previous stages of subtraction when forming their output signals, and can be cascaded to form n-bit subtractors. Alternatively the subtract operation can be performed using two half subtractors, which are simpler since they contain only two inputs and produce two outputs.

Neither of these devices is commonly encountered since modulo-two subtraction is more conveniently accomplished using two’s complement arithmetic and binary adders.

Subtractor. Modulo-two subtraction