What happens when your computer starts up? Watch Mac Bowley take you through the components that are involved.

You are watching: Where are the instructions stored that the computer needs when it starts up

What happens inside your computer from the moment you press the power button? In this step, you’ll learn about the startup sequence. But first, let’s explore the various components involved. The central processing unit or CPU is the brain of your computer, and it controls everything. The CPU is a large chip which reads instructions and data from the computer’s memory. The CPU performs the instructions and writes the data back into your computer’s random access memory or RAM. RAM temporarily stores data while your computer is running. The size of this storage depends on your computer and is measured in gigabytes. For example, your laptop might have something like 4, 6, or 8 gigabytes of RAM.
RAM is fast, and also “read and write”, which means you can add, change and delete the data stored in RAM. RAM is also volatile. Once you power down, you lose all the data stored in RAM. Read only memory, or ROM, is a chip installed and programmed by the manufacturer. You can’t overwrite what’s stored in ROM without risking damage to your computer. Like RAM, the ROM is also fast, but ROM is non-volatile memory, meaning it doesn’t need power to store data, which is the opposite of RAM. The ROM also stores the basic input output system, or BIOS. The BIOS contains all of the basic code for controlling your computer hardware. This includes things like your keyboard, mouse, monitor, and hard drive.
Once the startup sequence is complete, the BIOS does very little as your computer’s operating system takes control. The hard drive, sometimes to the hard disk, is the main storage device in your computer. The hard drive is like RAM, but it is non-volatile and slower. The hard drive stores your computer’s files and folders like your operating system. It stores data on spinning disks and reads it with a mechanical arm. Some computers might have a solid state drive or SSD. These are faster storage devices with no moving parts, which makes them less likely to break. SSDs aren’t as cheap as hard drives yet, but they are becoming more common. So how do these components work together in the startup sequence?
First, your computer CPU starts and fetches instructions from the BIOS stored in your ROM. The BIOS start the monitor and keyboard. It also performs some basic checks to make sure your computer is working properly. For example, it will look for the RAM. The BIOS then starts the boot sequence. It looks for the operating system stored on your hard drive and loads it into the RAM. The BIOS then transfers control to the operating system, and with that, your computer has now completed the startup sequence. Once you know the sequence of events, you can create some really fun lesson plans for your students. Check out the activity and step below and share your own ideas in the comments.
In this step, you’ll learn how the components of a computer work together from the moment you press the “on” button.

The startup sequence

From the moment you press the power button, a whirlwind of tasks happen inside your computer.
Let’s have a look at each of the components and systems that work together to start up your computer.


The CPU, or central processing unit, is a large chip inside the computer. This is the brains of the computer: it controls everything. It works by reading instructions and data from the random access memory (RAM), performing an instruction, and then writing the data back to RAM. Some of the instructions may involve other components like the hard drive, but the CPU is in control.
BIOS stands for Basic Input Output System. The BIOS is stored in the ROM. It contains all the basic code for controlling your computer hardware (such as keyboards, mice, monitors and hard drives). After the startup sequence is complete, and control has gone to the operating system, the BIOS does very little.
When you start up your computer, you may see a black screen displaying “Press F2 for Setup”. This is the BIOS. By pressing F2, you enter a setup screen where you can change where the BIOS loads the operating system from.
The operating system is normally stored on the hard drive, but you can load an operating system from a USB drive or a CD instead.

Startup sequence

So, how are these components used in the startup sequence?The CPU starts and fetches instructions into RAM from the BIOS, which is stored in the ROM.The BIOS starts the monitor and keyboard, and does some basic checks to make sure the computer is working properly. For example, it will look for the RAM.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.
Hopefully, this step has helped you understand what happens underneath the bonnet of a computer when you turn it on. Once you know this sequence, you can create some really fun lesson plans based on it…

An example lesson

Place them at different tables around the room with some paper. There should be some distance between the students so that everyone can clearly see what’s happening.
Get each team to discuss the key features of their component and write them down on a large piece of paper. They could use their notes, or this could be a test of what they’ve learnt so far.
Run through a simple scenario. For example, “I’m writing a document in Word and my computer crashes. What does RAM do?”
Get the students to physically throw a piece of paper that was on the RAM table into the bin. It’s lost without power. Throw ROM’s data in the bin, and ask the students: is this correct?
Run through the more complicated startup sequence. Have a piece of card with the word CONTROL on it to indicate who has control. Each team has to say what they’re doing as they’re doing it. For example:
Go through several iterations of the startup sequence until the students understand it without looking at their notes. If you can, get them to perform the startup sequence for another class.
Physically modelling the startup sequence is a great learning opportunity for students. Once they can visually see what is happening with the components during startup, they will remember the sequence more easily.
For more detailed descriptions of what’s happening inside some of these components, check out our How Computers Work course.

Our purpose is to transform access to education.

We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.

We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.

See more: What Does A 15 Degree Angle Look Like, Protractor, 15 Degrees

Learn more about how y2kcenter.org is transforming access to education

Sign up to our newsletter and we'll send fresh new courses and special offers direct to your inbox.