Have you ever wondered what happens when you press the power button of your PC/Laptop? Why does it take so long to show your desktop screen?
The startup sequence
From the moment you press the power button, a hurricane of tasks happens inside your computer.
Let’s have a look at each of the components and systems that work together to start up your computer.
CPU
The CPU, or Central Processing Unit, is a large chip inside the computer. This is the brain of the computer: it controls everything.
It works by reading instructions and data from 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.
RAM
RAM (Random Access Memory) temporarily stores data while your computer is running.
- RAM is both readable and writable. You can add, change and delete data stored in RAM.
- It is volatile. When the computer is switched off, all the data stored in RAM is lost.
- It is fast.
ROM stands for Read-Only Memory. It is a chip containing data installed by the manufacturer that is not typically replaced or upgraded. It stores the BIOS.
ROM- ROM is read-only. It is usually programmed by the computer manufacturer, and cannot be changed or overwritten. (There is a process for overwriting the ROM, called “flashing” it, but it’s difficult and can lead to a completely broken and unrecoverable computer.)
- ROM is non-volatile memory, which means it does not need the power to keep the data inside it.
- It is fast.
Hard drive
The hard drive (sometimes called the hard disk) is the main storage device in your computer. Like RAM, it can be added to and changed, and like ROM it is non-volatile, but it is slow. If you have files and folders on your computer, are stored on the hard drive. The operating system is also stored on the hard drive.BIOS
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.
An example lesson
Give students or teams of students a component each:- CPU
- BIOS
- ROM
- RAM
- Operating system
I am the BIOS. I’m just going to go over here and check that I have some RAM.Give a high five to the RAM teamOK, RAM is there. Now I’m going to look for the operating system.Go over and give a thumbs up to the OS teamI’m going to pick the OS up and put it in the RAM.
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 easily remember the sequence.The OS team follows BIOS over to the RAM table
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