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Computers may seem smart, but really all they are is a big pile of transistors and other electronic components.  Until you turn on a computer it's more more useful than a pile of old tin cans.  Actually, just turning on a computer (letting electricity flow into it) wouldn't be much of an improvement if it weren't for some instructions stored in the hardware that kick into action as soon as the electricity hits them.  This process is called booting up, and its the first step in getting your computer ready to do the stuff you really want to do.

Booting Up 

Booting up starts when electricity hits the CPU, or Central Processing Unit.  The CPU does the main calculating in a computer.  You know CPUs as Pentiums, Athlons and other brand names that show up in computer ads.  CPUs mainly do calculating, or processing, but they also have some memory in them for storing instructions or data.  


When you throw the switch on your computer, the CPU digs into a bit of its memory that holds the Basic Input/Output System.  People usually call it the BIOS (pronounced bye-'ohs).  The bios comes with the CPU and generally doesn't change much.  (You actually can change it, but few people do).

The BIOS checks for some basic hardware, like the monitor, keyboard, floppy disk drive, CD drive and the hard drive.  If the right hardware is hooked up to the computer, the BIOS has enough instructions built into it to let it work with the CPU.  

Booting up usually happens automatically -- it's over before you ever start using your computer.  But if you ever do use your computer with only the BIOS loaded, you'll notice that it's pretty rudimentary.  The monitor shows just text on a blank background and you move the cursor around the screen with the up/down arrows on the keyboard.  

Loading the Operating System

The BIOS lets the computer work just enough to load the operating sytem.  You know operating systems as Microsoft Windows, Linux, Mac OS, etc.  They're usually stored on your hard drive.  The BIOS gets eveything working together just enough to transfer (or load) the operating system from the hard drive to the memory.  Once that happens, the OS takes over and BIOS goes mostly dormant until you restart the computer.

The OS doesn't have to be on the hard drive.  It can also be on a floppy, a CD or even a USB flash drive.  In fact, the BIOS on your computer is probably set up to check some of these other things before loading the OS from the hard drive.  For instance, it will check the CD ROM drive first then, if there's no CD in it, move on to the hard drive.  This is important.  If something goes wrong with the OS stored on your hard drive, you have to get your computer working to fix it.  If you put an OS on a CD and load it in the drive, your computer will boot up to that OS instead of the one stored on your hard drive.  You've done this if you've ever upgraded to a new version of your OS, or if you've repaired your existing OS.