BlogJanuary 4, 2025

How ALU (Arithmetic Logic Unit) Works

Vaibhav Jha

This article is part of the Vaibhav Jha technical writing series on backend systems, software architecture, and core computer science topics explained with practical developer context.

Overview

The Arithmetic Logic Unit (ALU) is a core component of the CPU responsible for performing all arithmetic and logical operations. Every calculation—from simple addition to complex decision-making—ultimately passes through the ALU.

What Does the ALU Do

The ALU performs two main types of operations:

Arithmetic Operations

Addition
Subtraction
Multiplication (sometimes delegated)
Division (often handled separately)

Logical Operations

AND
OR
NOT
XOR
Comparisons (greater than, less than, equal to)

How ALU Works (Basic Flow)

CPU sends instructions to the ALU
Operands are fetched from registers
ALU processes the operation
Result is stored back in registers

This happens in a single clock cycle for simple operations.

Internal Structure of ALU

The ALU is built using digital circuits:

1. Adders

Half adder
Full adder Used for binary addition.

2. Logic Gates

AND, OR, NOT, XOR Used for logical operations.

3. Multiplexers

Select which operation result to output based on control signals.

Example: Binary Addition

Adding two numbers:

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