How does boot up process work?

Answer 1

There are many steps that take place before your computer boots up. This is a long explanation, so be prepared.

BIOS:

This is the first thing that happens. It stands for Basic Input Output System. It is actually a hard-coded piece of software (or was; you can update your BIOS, so it is now just a memory chip that is kept operating). This means that it is a combination of hardware and software. The BIOS keeps track of settings that you have in a memory chip. It has to keep track of where to boot from, what graphics adapter to use, and can also keep track of what time to automatically boot up, and even a password to let you change settings. (you can access the BIOS by pressing a button before you see windows start, usually F2. You have to do this while the only text on the screen is black and white and all pixelated. Other systems will display a pleasing picture so you don't have to see the ugly text. I don't recommend pressing F2 unless you know what you are doing. I repeat DON'T.)

First, the BIOS checks the first storage device in the boot order. This is usually a CD drive, which is why you can boot from your operating system installation disk. If there is something there, it tries to boot. If not, it goes to the next entry. If it reaches the end of the list, and the computer hasn't booted, DOS style words come up and say it couldn't boot.

Let's say the computer is booting from the hard drive. The BIOS checks the first sector of the drive (yeah, the hard drive isn't only a disk, but it also is organized into tiny pieces that are easy to work with). That sector of the hard drive is the Master Boot Record, MBR. It holds information about how to boot up the rest of the way.

MBR and boot program:

The BIOS follows where the MBR tells it to go. The MBR will point to a boot program. This program starts to load what it needs to operate. After a little bit, the program has loaded enough to take over from the BIOS.

The rest of the boot:

So now, Windows (or Macintosh, or Linux, Unix, or whatever) has woken up enough to load itself. At this point, it needs to load how to log you on, system services, everything that supports the end result of you browsing the internet or opening a document. A minute or so later, at the logon screen, the computer has basically finished booting up. It has a few last-minute things to do while you log on. Speaking of that, it still isn't done. If you have a Windows computer, the startup folder executes. You can put shortcuts to your favorite programs in there, like Microsoft Word, a movie player, or whatever, and every time you log on, it will open as if you opened it yourself. But beware! Everything in that folder will open every time, and that takes processing power and time. Use the startup folder sparingly.

Overview:

BIOS looks for the Master Boot Record, which points it to the boot program. The BIOS starts loading the operating system, until it can load itself. The boot-up ends when you have logged on and startup programs finish loading.

Quite lengthly. Hope this is what you were looking for.

Answer 2

Go to the link below for a good explanation on how your computer boots up. http://www.pccomputernotes.com/operating_systems/osintro02.htm