In this article, we will discuss those combinational logic building blocks that can be used to perform addition and subtraction operations on binary numbers. Addition and subtraction are the two most commonly used arithmetic operations, as the other two, namely multiplication and division, are respectively the processes of repeated addition and repeated subtraction

We will begin with the basic building blocks that form the basis of all hardware used to perform the aforesaid arithmetic operations on binary numbers. These include half-adder, full adder, half-subtractor, full subtractor and controlled inverter.

A half-adder is an arithmetic circuit block that can be used to add two bits. Such a circuit thus has two inputs that represent the two bits to be added and two outputs, with one producing the SUM output and the other producing the CARRY.

The Boolean expressions for the SUM and CARRY outputs are given by the equations

A full adder circuit is an arithmetic circuit block that can be used to add three bits to produce a SUM and a CARRY output. Such a building block becomes a necessity when it comes to adding binary numbers with a large number of bits.

The full adder circuit overcomes the limitation of the half-adder, which can be used to add two bits only. Let us recall the procedure for adding larger binary numbers. We begin with the addition of LSBs of the two numbers. We record the sum under the LSB column and take the carry, if any, forward to the next higher column bits.

A half-subtractor is a combinational circuit that can be used to subtract one binary digit from another to produce a DIFFERENCE output and a BORROW output. The BORROW output here specifies whether a ‘1’ has been borrowed to perform the subtraction.

A full subtractor performs subtraction operation on two bits, a minuend and a subtrahend, and also takes into consideration whether a ‘1’ has already been borrowed by the previous adjacent lower minuend bit or not.

As a result, there are three bits to be handled at the input of a full subtractor, namely the two bits to be subtracted and a borrow bit designated as Bin . There are two outputs, namely the DIFFERENCE output D and the BORROW output Bo. The BORROW output bit tells whether the minuend bit needs to borrow a ‘1’ from the next possible higher minuend bit.

A controlled inverter is needed when an adder is to be used as a subtractor. As outlined earlier, subtraction is nothing but addition of the 2’s complement of the subtrahend to the minuend. Thus, the first step towards practical implementation of a subtractor is to determine the 2’s complement of the subtrahend. And for this, one needs firstly to find 1’s complement. A controlled inverter is used to find 1’s complement.

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