What is Adder?

In the world of computers, an adder may refer to a specific component in the processor that adds two numbers according to the instructions given.

Technically, it refers to the electronic circuit that adds the amplitudes of the input signals.

Understanding Adder

What is Adder

The digital logic circuit in the CPU of a computer that is used for adding two binary numbers is called an adder.

Typically, these form the fundamental component of the Arithmetic Logic Unit or ALU.

This combination of logic circuits adds two binary numbers but does not have any memory circuits.

It performs an operation that is logically given to it by a Boolean expression.

The final result generated by the adder will typically depend on the given input at any given time.

When one’s or two’s complements are used, it may be insignificant to amend an adder into an adder-subtractor, but the basic adder will need more logic if there are other signed number demonstrations.

The circuits in the adder follow a simple working principle. If it is a full adder, it will use two half adder circuits with the power connections connected by a switch to the two inputs.

However, the output is displayed for each logic gate.

Typically, when the switch is ON or +5 volts, it indicates a logic high, and when it is OFF or 0 volts, it indicates a logic low.

What is Adder Used for?

The primary function of this digital logic circuit is also to perform ALU operations, calculate addresses or act as an increment or decrement operator in the processor.

Typically, an adder is used mainly to add numbers in a computer when the processor is given such instructions. However, depending on the type of the adder, it may also be used for several other applications such as:

It is also used to perform other related activities such as calculating table indices.

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What Are the Basic Types of Adder?

There are basically two major types of adders, namely the half adder and the full adder. You will also get multi-bit adders with a couple of other variants.

In addition to the above major variants, you will also find a few other specific types of adders as mentioned hereunder.

Multi-Bit adders:

These circuits are made by using full adders either serially or in parallel and accordingly referred to as follows:

Quantum adders:

The quantum adders are specially designed circuits. These are typically built by using only the Toffoli gate and CNOT quantum logic gates that offer both quantum full adders and quantum half adders.

Since both Toffoli and CNOT are classical logic gates, these circuits can be used for both classical and reversible computation.

And, numbers can also be added due to the low circuit complexity of the Quantum Fourier Transform.

Analog adders:

Finally, there are the analog adders that are just like the binary adders. This effectively combines two input currents by adding them together.

It can be used to add non-binary signals having a base greater than 2 as well in order to calculate a sum within the limitations of the hardware.

The analog adders are known as the summing amplifiers as well and the technique followed is very effective to reduce the number of transistors to be used in the addition circuit.

Which Adder is Best?

You will get the best performance from the parallel prefix adders used in the VLSI or Very large Scale Integrations applications.

However, you will also get a reasonably high performance from the carry look ahead adder if it is used for two 8-bit number addition operations.

On the other hand, if you want to add three or four 8-bit numbers, you will be better off using a carry save adder in which the last stage is built by a ripple carry adder.

Typically, the exclusive adders with unique design that offers low area and delay in order will provide the best performance and will be able to meet the requirements of the current industry.

It is expected that in the future it will be extended even further by simulating and designing the adders with a greater number of bits such as 16 bits, 32 bits and 64 bits.

Advantages of Adder

Disadvantages of Adder

Why Use Adder?

The need for an adder is for its significant role in helping the Arithmetic Logic Unit for adding binary numbers, though the control unit of the CPU decides which particular operation should be performed by the ALU.

Apart from the basic use, a full adder also serves several other useful purposes such as:

As for the half adders, on the other hand, the need of them is more profound in the following areas:

What is Parallel Adder?

Parallel adder is basically a stream of multiple full adders that makes binary addition much faster. The number of full adders in it may vary according to the number of bits that are to be added in the binary digits. Therefore, a parallel adder is actually an n-bit adder which is a cascade of n full adders.

This particular circuit is used mainly to find the sum of two n-bit binary numbers that are more than a bit in length with the use of its combination of several logic circuits.

The bits are added simultaneously by the parallel adders for each clock pulse.

There are many different types of parallel adders available such as:

The different types of adders offer different usage benefits such as:

However, there are a few downsides to it. One of the most significant drawbacks of it is that the inputs received by the adder at the two ports are instantaneous but the carry ports do not get them until the previous adder completes its operations.

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This results in an operational delay which grows even further when more adders are used.

This is also called the carry propagation delay which is related to the traveling of carry bits.

Half Adder Vs Full Adder


Adders are a very important component of the CPU organization that helps in adding two binary numbers as well as in other ALU operations.

There are different types of adders available but the major ones are half adders and full adders with each having their distinctive features, offering different benefits.