Having presented all the necessary building components, we are able to realize the half-adder according to the figure below. The truth table shows us that the binary addition is analogous to decimal, only simpler. The sum output will be 1 only when the A and B inputs are different, which we can realize with the XOR gate. The carry output will be 1 only when both inputs are 1, which we can realize with AND gate.
We can immediately see that the half adder is sufficient for binary addition of only one bit. If we want to add 2 bits in a word, we need to build a circuit that is able to do the addition of 3 bits – 2 position bits and one carry bit from the previous position. That circuit is called the full adder and we will present it the next chapter.
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