• Thursday January 1, 2009 - 12:44:25 am GMT
• registered : 1143348
• online : 19449
• questions/day : 6144
• Answer rate : 76.12%

Introduction to BIOS

BIOS ("Basic Input/Output System" is an essential component in computers, which is used for controlling hardware. It is a small software program, part of which is loaded in ROM (read-only memory, which cannot be modified), and part of which is in EEPROM (electrically erasable programmable read-only memory).

The POST

When a computer system is turned on or reset, the BIOS does an inventory of the hardware found on the computer and carries out a test (called POST for "Power-On Self Test") in order to verify that all of it is functioning properly.

• Testing the processor (CPU)
• Checking the BIOS
• Checking CMOS configuration
• Initialising the timer (the internal clock)
• Initialising the DMA controller
• Checking RAM and cache memory
• Installing all BIOS functions
• Checking all configurations (such as the keyboard, disk drives, and hard drives)

If the POST discovers an error, it will attempt to continue booting the computer. However, if the error is serious, the BIOS will stop loading the system and:

• display a message on the screen, if possible (as the display device might not yet have been initialised, or might be defective);
• emit a sequence of beeps, which refer to the source of the error;
• send a code (called the POST code) to the computer's serial port, which may be retrieved using special diagnostic hardware.

If everything is correct, the BIOS will usually play a short beep to report that there are no errors.

For an Award BIOS, only video-related errors will trigger beeps. Other errors are sent as POST codes and are displayed onscreen.

For example, a long beep, followed by two short beeps, indicates an error in a video device (graphics card). In such a case, you will have to try to place the video card in its slot correctly, or replace it altogether. Any other beep indicates a memory-based error.

Here is the list of POST codes, and the meaning of beep sequences for each of the three main BIOS manufacturers:

• Phoenix - Phoenix BIOS POST code
• AMIBIOS - AMIBIOS POST code
• Award - BIOS Award POST code

BIOS setup

Most BIOSes have a setup program for modifying basic system configurations. This kind of information is stored in self-powered memory (using a battery) so that the data remains saved even when the computer is off (RAM is reinitialised each time the system boots).

Each machine has several BIOSes:

• The motherboard BIOS
• The BIOS which controls the keyboard
• The video card BIOS
• and possibly
  • the BIOS for SCSI controllers, used for booting from the SCSI device, which then communicate with the DOS without requiring an additional driver
  • (The network card BIOS for booting from the network)

When the computer is turned on, the BIOS displays a copyright message on the screen, then carries out diagnostic and initialisation tests. After these tests are complete, the BIOS displays a message prompting the user to press one or more keys in order to enter BIOS setup.

Depending on what brand of BIOS it is, it may be the F2 key, the F10 key, the DEL key, or one of the following key sequences:

• Ctrl+Alt+S
• Ctrl+Alt+Esc
• Ctrl+Alt+Ins

On Award BIOSes, the following message is displayed during POST:

TO ENTER SETUP BEFORE BOOT PRESS CTRL-ALT-ESC OR DEL KEY

Reinitialising the BIOS

As BIOS setup is used to edit hardware settings, changing them might cause the system to become unstable, and it might not even restart. When this happens, the changes to the BIOS must be cancelled, and the default settings must be restored.

If the computer boots up and you can access the BIOS, it will usually allow you to return to the default settings. In PhoenixBIOS, press F9 to return the configuration to the defaults set by the manufacturer. In AwardBIOS, press F9 to restore the previous settings, F6 to restore Award BIOS's default settings, and F7 to restore the defaults set by the motherboard's manufacturer.

If you cannot access the BIOS using standard procedures, most motherboards include a jumper for resetting the default values. Simply change the jumper's position, then leave it there for about ten seconds.

It is strongly recommended to shut off the computer's power before making these changes. Whenever doing so, refer to the manual that came with your motherboard.