The Power On Self Test

When the system is powered on, the BIOS will perform diagnostics and initialize

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system components, including the video system. (This is self-evident when the

screen first flicks before the Video Card header is displayed). This is commonly

referred as POST (Power-On Self Test).

Afterwards, the computer will proceed its final boot-up stage by calling the

operating system. Just before that, the user may interrupt to have access to


To allow the user to alter the CMOS settings, the BIOS provides a little program,

SETUP. Usually, setup can be entered by pressing a special key combination (DEL,

ESC, CTRL-ESC, or CRTL-ALT-ESC) at boot time (Some BIOSes allow you to enter

setup at any time by pressing CTRL-ALT-ESC). The AMI BIOS is mostly entered by

pressing the DEL key after resetting (CTRL-ALT-DEL) or powering up the computer.

You can bypass the extended CMOS settings by holding the key down during

boot-up. This is really helpful, especially if you bend the CMOS settings right

out of shape and the computer won't boot properly anymore. This is also a handy

tip for people who play with the older AMI BIOSes with the XCMOS setup. It

allows changes directly to the chip registers with very little technical


A Typical BIOS POST Sequence

Most BIOS POST sequences occur along four stages:

1. Display some basic information about the video card like its brand,

video BIOS version and video memory available.

2. Display the BIOS version and copyright notice in upper middle screen.

You will see a large sequence of numbers at the bottom of the screen. This

sequence is the .

3. Display memory count. You will also hear tick sounds if you have enabled

it (see Memory Test Tick Sound section).

4. Once the POST have succeeded and the BIOS is ready to call the operating

system (DOS, OS/2, NT, WIN95, etc.) you will see a basic table of the system's


A· Main Processor: The type of CPU identified by the BIOS. Usually Cx386DX,

Cx486DX, etc..

A· Numeric Processor: Present if you have a FPU or None on the contrary. If

you have a FPU and the BIOS does not recognize it, see section Numeric Processor

Test in Advanced CMOS Setup.

A· Floppy Drive A: The drive A type. See section Floppy drive A in Standard

CMOS Setup to alter this setting.

A· Floppy Drive B: Idem.

A· Display Type: See section Primary display in Standard CMOS Setup.

A· AMI or Award BIOS Date: The revision date of your BIOS. Useful to

mention when you have compatibility problems with adaptor cards (notably fancy


A· Base Memory Size: The number of KB of base memory. Usually 640.

A· Ext. Memory Size: The number of KB of extended memory.

In the majority of cases, the summation of base memory and extended memory does

not equal the total system memory.

For instance in a 4096 KB (4MB) system, you will have 640KB of base memory and

3072KB of extended memory, a total of 3712KB.

The missing 384KB is reserved by the BIOS, mainly as shadow memory (see Advanced

CMOS Setup).

A· Hard Disk C: Type: The master HDD number. See Hard disk C: type section

in Standard CMOS Setup.

A· Hard Disk D: Type: The slave HDD number. See Hard disk D: type section

in Standard CMOS Setup.

A· Serial Port(s): The hex numbers of your COM ports. 3F8 and 2F8 for COM1

and COM2.

A· Parallel Port(s): The hex number of your LTP ports. 378 for LPT1.

A· Other information: Right under the table, BIOS usually displays the size

of cache memory.

Common sizes are 64KB, 128KB or 256KB. See External Cache Memory section in

Advanced CMOS Setup.


During the POST routines, which are performed each time the system is powered on,

errors may occur. Non-fatal errors are those which, in most cases, allow the

system to continue the boot up process. The error messages normally appear on

the screen.

Fatal errors are those which will not allow the system to continue the boot-up

procedure. If a fatal error occurs, you should consult with your system

manufacturer or dealer for possible repairs. These errors are usually

communicated through a series of audible beeps. The numbers on the fatal error

list correspond to the number of beeps for the corresponding error. All errors

listed, with the exception of #8, are fatal errors. All errors found by the BIOS

will be forwarded to the I/O port 80h.

A· 1 beep: DRAM refresh failure. The memory refresh circuitry on the

motherboard is faulty.

A· 2 beeps: Parity Circuit failure. A parity error was detected in the base

memory (first 64k Block) of the system.

A· 3 beeps: Base 64K RAM failure. A memory failure occurred within the

first 64k of memory.

A· 4 beeps: System Timer failure. Timer #1 on the system board has failed

to function properly.

A· 5 beeps: Processor failure. The CPU on the system board has generated an


A· 6 beeps: Keyboard Controller 8042-Gate A20 error. The keyboard

controller (8042) contains the gate A20 switch which allows the computer to

operate in virtual mode.

This error message means that the BIOS is not able to switch the CPU

into protected mode.

A· 7 beeps: Virtual Mode (processor) Exception error. The CPU on the

motherboard has generated an Interrupt Failure exception interrupt.

A· 8 beeps: Display Memory R/W Test failure. The system video adapter is

either missing or Read/Write Error its memory is faulty. This is not a fatal


A· 9 beeps: ROM-BIOS Checksum failure. The ROM checksum value does not

match the value encoded in the BIOS. This is good indication that the BIOS ROMs

went bad.

A· 10 beeps: CMOS Shutdown Register. The shutdown register for the CMOS

memory Read/Write Error has failed.

A· 11 beeps: Cache Error / External Cache Bad. The external cache is faulty.


A· 2 short beeps: POST failed. This is caused by a failure of one of the

hardware testing procedures.

A· 1 long & 2 short beeps: Video failure. This is caused by one of two

possible hardware faults. 1) Video BIOS ROM failure, checksum error encountered.

2) The video adapter installed has a horizontal retrace failure.

A· 1 long & 3 short beeps: Video failure. This is caused by one of three

possible hardware problems. 1) The video DAC has failed. 2) the monitor

detection process has failed. 3) The video RAM has failed.

A· 1 long beep: POST successful. This indicates that all hardware tests

were completed without encountering errors.

If you have access to a POST Card reader, (Jameco, etc.) you can watch the

system perform each test by the value that's displayed. If/when the system hangs

(if there's a problem) the last value displayed will give you a good idea where

and what went wrong, or what's bad on the system board. Of course, having a

description of those codes would be helpful, and different BIOSes have different

meanings for the codes. (could someone point out FTP sites where we could have

access to a complete list of error codes for different versions of AMI and Award


BIOS Error Messages

This is a short list of most frequent on-screen BIOS error messages. Your system

may show them in a different manner. When you see any of these, you are in

trouble - Doh! (Does someone has any additions or corrections?)

A· "8042 Gate - A20 Error": Gate A20 on the keyboard controller (8042) is

not working.

A· "Address Line Short!": Error in the address decoding circuitry.

A· "Cache Memory Bad, Do Not Enable Cache!": Cache memory is defective.

A· "CH-2 Timer Error": There is an error in timer 2. Several systems have

two timers.

A· "CMOS Battery State Low" : The battery power is getting low. It would be

a good idea to replace the battery.

A· "CMOS Checksum Failure" : After CMOS RAM values are saved, a checksum

value is generated for error checking. The previous value is different from the

current value.

A· "CMOS System Options Not Set": The values stored in CMOS RAM are either

corrupt or nonexistent.

A· "CMOS Display Type Mismatch": The video type in CMOS RAM is not the one

detected by the BIOS.

A· "CMOS Memory Size Mismatch": The physical amount of memory on the

motherboard is different than the amount in CMOS RAM.

A· "CMOS Time and Date Not Set": Self evident.

A· "Diskette Boot Failure": The boot disk in floppy drive A: is corrupted

(virus?). Is an operating system present?

A· "Display Switch Not Proper": A video switch on the motherboard must be

set to either color or monochrome.

A· "DMA Error": Error in the DMA (Direct Memory Access) controller.

A· "DMA #1 Error": Error in the first DMA channel.

A· "DMA #2 Error": Error in the second DMA channel.

A· "FDD Controller Failure": The BIOS cannot communicate with the floppy

disk drive controller.

A· "HDD Controller Failure": The BIOS cannot communicate with the hard disk

drive controller.

A· "INTR #1 Error": Interrupt channel 1 failed POST.

A· "INTR #2 Error": Interrupt channel 2 failed POST.

A· "Keyboard Error": There is a timing problem with the keyboard.

A· "KB/Interface Error": There is an error in the keyboard connector.

A· "Parity Error ????": Parity error in system memory at an unknown address.

A· "Memory Parity Error at xxxxx": Memory failed at the xxxxx address.

A· "I/O Card Parity Error at xxxxx": An expansion card failed at the xxxxx


A· "DMA Bus Time-out": A device has used the bus signal for more than

allocated time (around 8 microseconds).

If you encounter any POST error, there is a good chance that it is an HARDWARE

related problem. You should at least verify if adaptor cards or other removable

components (simms, drams etc...) are properly inserted before calling for help.

One common attribute in human nature is to rely on others before investigating

the problem yourself.