The process of starting a computer is given a special name. It's called booting up. As you can see from the process described below, when you start a computer it literally picks itself up by its bootstraps, thus this otherwise strange term for the process. When you turn on the power the computer first checks its memory and some other components (the POST, or Power On Self-Test). After this, the left (or top) disk drive will begin to spin and the boot process will commence. Follow the steps below:

1. ROM BIOS loads Track 0, Sector 0 of the disk. This sector contains a short program that can read the rest of the input/output system. (Note: every disk, bootable or not, contains a boot sector program. On data disks it's the program that displays the message that tells you the disk is not bootable. Because of this a computer virus can attach itself to any disk, not just disks with the operating system on them.)

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2. The boot loader read in step 1 loads the input/output system (files IBMBIO.COM and IBMDOS.COM [or IO.SYS and MSDOS.SYS]).

3. IBMBIO.COM initializes the hardware of the computer, runs the file CONFIG.SYS (if found on the disk) and then moves IBMDOS.COM into its proper location in RAM. (Again, IO.SYS and MSDOS.SYS respectively for MS-DOS.)

4. IBMDOS.COM loads COMMAND.COM and turns control over to it. COMMAND.COM runs the file AUTOEXEC.BAT (if found) and then command is given to you. (MSDOS.SYS for MS-DOS)

[Note: The process is somewhat more complex when Windows boots but the basics continue to be more or less as described here.]

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