# CS201 Lab: Design Adders & Subtractors

## Purpose

Arithmetic circuits play a central role in the operation of a microprocessor.
This week we will look at how basic arithmetic circuits are built.

## Half-Adder

A **Half-adder** is a combinational circuit that performs the addition of 2 bits.
Inputs: 2 input bits to be added
Outputs: sum bit (least significant)
carry bit (most significant)
**For example:**
1 + 1 = 1 0 c = 1; s = 0
1 + 0 = 0 1 c = 0; s = 1

## Full-Adder

A **Full-adder** is a combinational circuit that forms the arithmetic sum of 3 input bits.
Inputs: 2 input bits to be added (x, y)
1 carry bit from previous lower significant position (z)
Outputs: sum bit (least significant)
carry bit (most significant)
**For example:**
1 + 1 + 1 = 1 1 c = 1; s = 1
1 + 0 + 1 = 1 0 c = 1; s = 0
Note: it takes 2 half-adders to make a full adder.

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Copyright: Department of Computer Science, University of Regina.
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