Latest update on Tuesday 18 October 2011 à 15:14 by aquarelle.
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, hence the term Flashing to indicate the action to change the EEPROM).
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 initialized, 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.
Meaning of beeps in recent Award BIOS systems
# of beeps
Meaning
How to resolve the problem
1 short beep
PC is booting normally
2 short beeps
CMOS problem
Reinitialize the CMOS by removing the BIOS stack and replacing it, or by moving jumper JP4
1 long beep / 1 short beep
Problem with motherboard or RAM
Place RAM modules correctly in slot, test RAM or change it
1 long beep / 2 short beeps
Problem with graphics card
Check that the graphics card is correctly placed in its slot. If need be, test with another video card.
1 long beep / 3 short beeps
Problem with keyboard
Check that the keyboard is correctly plugged in, and that no keys are depressed. If need be, test with another keyboard.
1 long beep / 9 short beeps
BIOS failure
The BIOS is invalid, replace it with a more recent version
3 beeps
Base 64K RAM failure
RAM contains errors. Try reinserting it correctly or replacing it.
4 beeps
Refresh error
RAM is not refreshing correctly. Reset the refresh values in the BIOS or reset the BIOS.
5 beeps
Processor error
Check that the processor is correctly plugged in, and that the fan is working. If need be, change it.
6 beeps
Problem with keyboard
Check that the keyboard is correctly plugged in, and that no keys are depressed. If need be, test with another keyboard.
8 beeps
Problem with graphics card
Check that the graphics card is correctly placed in its slot. If need be, test with another video card.
Long incessant beeps
RAM error
Place RAM modules correctly in slot, test RAM or change it
Short incessant beeps
Power supply error
Check that all power cables are correctly connected to the motherboard, test with another power supply, or change them
Meaning of beeps for an AMI BIOS
# of beeps
Meaning
How to resolve the problem
1
Refresh failure
RAM is not refreshing correctly. Reset the refresh values in the BIOS or reset the BIOS. Place RAM modules correctly in slot, or change them.
2
Parity Error
Place RAM modules correctly in slot, or change them. Test the RAM.
3
Base 64K RAM failure
Place RAM modules correctly in slot, or change them. If need be, update the BIOS.
4
System timer not operational
The motherboard must be sent for repairs.
5
Processor Error
Check that the processor is correctly plugged in, and that the fan is working. If need be, change it.
6
Gate A20 failure
Check that the keyboard is correctly plugged in, and that no keys are depressed. If need be, test with another keyboard.
7
Processor exception interrupt error
The motherboard must be sent for repairs.
8
Display memory read/write failure
Check that the graphics card is correctly placed in its slot. If need be, test with another video card.
9
ROM checksum error
The BIOS chip must be replaced or updated.
10
CMOS shutdown register read/write error
The motherboard must be sent for repairs.
11
Cache memory problem
Check that the processor is correctly plugged in, and that the fan is working. If need be, change it. Place the RAM modules correctly in their slots, or replace them.
Meaning of beeps in a Phoenix BIOS
# of beeps
Meaning
How to resolve the problem
1-3-1-1
DRAM Refresh error
Place the RAM modules correctly in their slots, or replace them.
1-2-2-3
ROM checksum error
Place the RAM modules correctly in their slots, or replace them.
1-3-1-3
Keyboard Controller Error
Place the keyboard correctly in its slot, or replace it.
1-3-4-1
RAM error
Place the RAM modules correctly in their slots, or replace them.
1-3-4-3
RAM error
Place the RAM modules correctly in their slots, or replace them.
1-4-1-1
RAM error
Place the RAM modules correctly in their slots, or replace them.
2-2-3-1
Unexpected interrupt
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:
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 reinitialized 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 initialization 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
Reinitializing 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.
This document, titled « BIOS », is available under the Creative Commons license. Any copy, reuse, or modification of the content should be sufficiently credited to CCM (ccm.net).
