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The half-subtractor is a combinational circuit which is used to perform subtraction of two bits. It has two inputs, X (minuend) and Y (subtrahend) and two outputs D (difference) and B (borrow).
As in the case of the addition using logic gates, a full subtractor is made by combining two half-subtractors and an additional OR-gate. A full subtractor has the borrow in capability (denoted as BORIN in the diagram below) and so allows cascading which results in the possibility of multi-bit subtraction. The circuit diagram for a full subtractor is given below.
Subtractor - Video:
Subtractor - Presentation: