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BIOS- Basic Input Output System: An Introduction

Starting Your System: How the BIOS loads the operating system

When you turn on your computer, several events occur automatically:

  1. The CPU "wakes up" and sends a message to activate the BIOS.
  2. The BIOS then runs a series of tests, called the POST for Power On Self Test, to make sure the system devices are working correctly. In general, the BIOS:
    • Initialises system hardware and chipset registers
    • Initialises power management
    • Tests RAM (Random Access Memory)
    • Enables the keyboard
    • Tests serial and parallel ports
    • Initialises floppy disk drives and hard disk drive controllers
    • Displays system summary information
  1. During POST, the BIOS compares the system configuration data obtained from POST with the system information stored on a CMOS - Complementary Metal-Oxide Semiconductor - memory chip located on the motherboard. (This CMOS chip, which is updated whenever new system components are added, contains the latest information about system components.)
  2. After the POST tasks are completed, the BIOS looks for the boot program responsible for loading the operating system. Usually, the BIOS looks on the floppy disk drive A: followed by drive C:
  3. After being loaded into memory, the boot program then loads the system configuration information (contained in the registry in a Windows environment) and device drivers.
  4. Finally, the operating system is loaded, and, if this is a Windows environment, the programmes in the Start Up folder are executed.


Using the BIOS Setup program, you can modify BIOS settings and control special features of your computer.

For example,  you can select the sequence of drives the BIOS checks when loading the operating system or turn the Numlock on or off.

The Setup program includes several menus for making changes and turning on or off certain features. Before modifying any of these options, however, make sure you are completely familiar with your system and the results of choosing various options.

It is possible to corrupt your hard disc data or disable your computer completely by choosing the wrong settings.

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