How does computer start

Have you ever think what happens as soon as you press the power button of your computer(PC/Laptop)?

Why does it take so long to show your desktop?

So in this article, we are going to see how does computer start

The startup sequence

As soon as you press the power button, a lot of processing takes place inside your computer.
Let’s discuss each component and systems that work together to start up your computer.


When a computer is powered on, electricity flows through the components, which causes the central processing unit (CPU) to begin executing instructions.

Power-on Self-Test (POST)

The first set of instructions that the CPU retrieves is known as the power-on self-test (POST). During POST, the CPU performs a series of checks on the computer’s hardware to ensure that everything is functioning properly.

This process includes checking the memory, verifying the integrity of the BIOS or UEFI firmware, and initializing the various hardware components.

Initializing the BIOS/UEFI

The BIOS or UEFI firmware is responsible for configuring the hardware components of the computer, such as the memory, storage, and peripherals.

The firmware also provides a basic interface for interacting with the hardware, such as setting the date and time, configuring the network settings, and so on.

Loading the Bootloader

Once the BIOS or UEFI has finished its tasks, it passes control to the bootloader. The bootloader is a program that is responsible for loading the operating system.

The bootloader is stored on a hard drive or other non-volatile storage devices, and it is executed by the CPU as soon as the BIOS or UEFI hands over control.

Starting the Operating System

The bootloader loads the operating system from the hard drive or other non-volatile storage devices into memory and then transfers control to the operating system.

The operating system then takes over and begins to initialize its own components and load any necessary drivers and services. Once the operating system has fully initialized, the user is presented with a login prompt or the desktop, depending on the operating system.


The CPU, or Central Processing Unit, is a large chip inside the computer called the brain of the computer which controls everything.

It takes data from RAM, performs instructions and writes the data back to RAM.
Some of the instructions may also involve other components like the HDD.

How does computer start



RAM (Random Access Memory) stores data temporarily.

RAM is both readable and writable.

It is volatile. i.e., when the computer is power off, all the data stored in RAM is lost.

It is fast.



ROM (Read-Only Memory) is a chip which usually programmed by the manufacturer, and cannot be changed or overwritten.

ROM is read-only because its data cannot be changed or overwritten.

ROM is non-volatile memory i.e., it does not need the power to keep the data inside it.

Hard drive


The hard disk drive is the main/secondary storage device in your computer. The operating system is also stored on the hard drive.


BIOS stands for Basic Input Output System.

The BIOS is stored in the ROM which contains all the instructions for controlling your hardware (like keyboard, mouse, monitors and hard disk drives).

BIOS does very little after the startup sequence is complete, and control has gone to the operating system.

When you start up your computer by pressing the power button, the black screen displaying “Press Esc, F2 or Del for Setup” is the very first screen from where you can enter the BIOS.

By pressing the given button, you enter a setup screen where you can change the different settings.

The operating system is usually stored on the hard disk drive but you can load an operating system from a USB drive or a CD instead.

For booting from the other devices, you have to select the boot device priority from BIOS.

The CPU starts and fetches instructions into RAM from the BIOS, which is stored in the ROM.

The BIOS starts the hardware peripherals and performs some basic checks to make sure the computer is working properly.

The BIOS then starts the boot sequence. It will look for the operating system.

If you don’t change any of the settings, the BIOS will fetch the operating system from the hard drive and load it into the RAM.

The BIOS then transfers control to the operating system.  

I hope this information will help you. Please write below if you have any suggestions.

Leave a Comment