**Tribhuvan University – BIM**

*What is the maximum number of half adder that we can use in *an* bit parallel adder & why?*

An FA has both a carry in and a carryout.

An HA Doesn’t have a carry in. A half adder can be used to add the least significant digit of a binary number or two binary digits alone.

FAs can be chained together to add a multi-bit number;

An n bit parallel adder, there must be N number of full adder circuits.

**Things to know before you answer that question.**

**What is a parallel adder? **

The parallel adder is a combinatorial circuit (not clocked, does not have any memory and feedback) adding every bit position of the operands in the same time.

Thus it is requiring a number of bit-adders(full adders + 1 half adder) equal to the number of bits to be added

**What is Full Adder?**

A full adder is a digital circuit that performs addition. Full adders are implemented with logic gates in hardware. A full adder adds three one-bit binary numbers, two operands and a carry bit.

The adder outputs two numbers, a sum and a carry bit. The term is contrasted with a half adder, which adds two binary digits.

**What is Half Adder?**

A half adder is a type of adder, an electronic circuit that performs the addition of numbers. The half adder is able to add two single binary digits and provide the output plus a carry value.

It has two inputs, called A and B, and two outputs S (sum) and C (carry). The common representation uses a XOR logic gate and an AND logic gate.