Full Adder

A full adder circuit performs an addition of two single bit numbers, together with a Carry-In from a less significant stage, producing a Sum output and a Carry-Out to the next stage.

The full adder may be constructed using a pair of half adders, plus a single logic gate, as shown above. Operation of the full adder is summarised in the following truth table.

Full adder truth table

The full adder is the basic building block of electronic circuits designed to perform binary addition, and an interconnected series of full adder circuits may be used to form a multiple bit adder circuit, as shown below.

In fact, with only a small addition, this circuit is capable of performing both addition and subtraction, by using the 2's complement method!

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