The BIOS does a few jobs every time the computer is turned on.
- The BIOS runs a POST (Power On Self Test) program to check hardware. It usually tests the hard disk drive, keyboard, memory, mouse and video monitor. If POST finds problems then it describes the problem in its own funny little way, by beeping.
- The BIOS checks whether the person who turned on the computer wants to fiddle with the configuration, such as changing the boot order. If the person presses a certain key the BIOS runs a shows the settings and waits for any changes.
- The BIOS looks for the OS (Operating System). If the boot-order settings tell it to look on a floppy disk, then a CD-ROM, then a hard disk, then you will hear these devices clunking and whirring for a couple of seconds and see their LEDs flash. A few things may go wrong here.
- If a non-bootable floppy disk was left in the floppy drive then an error message appears, saying something along the lines of "Non-system disk or disk error. Replace and press any key when ready".
- If the hard disk is knackered then an error message appears such as "invalid system disk", "missing operating system" or "corrupt MBR".
- The BIOS starts the first OS program it finds. The OS handles the rest of the start-up.
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The main reason for tinkering with BIOS settings is to change the boot order. The BIOS looks for an OS program to hand over to. It can look on a variety of devices, such as hard disk, CD-Rom and floppy disk.