How to use POST Sequence to Troubleshoot Windows Start-up Problems

Troubleshooting Windows Start-up with POST

Picture the scene. You turn your computer on one morning. The monitor is not switching on and it's issuing a string of beeps and other expletives. It’s the day before your final deadline on an important project and you just have to be able to access your documents.

Your only option is to take it into the local computer repair store and wait two weeks while it's being repaired, or try and fix the problem yourself.

Fixing your own computer seems like a task reserved for the experts, and sometimes it is, but there are many repairs which you can do yourself. Also knowing something about the subject will help to prevent you from getting taken for a ride, when the “supposed expert” behind the counter wants you to buy a whole new computer, when all that is needed is an inexpensive memory module.

But where do you start? The key to discovering what is wrong with your computer is to understand how the POST sequence on your computer works.

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The term POST is an acronym for “Power On Self Test”. Every computer completes a series of tests when it first starts up. Every circuit inside the computer is scanned and a series of beeps issued via the internal speaker of the computer to indicate system performance.

When everything is working correctly you will generally hear a single short beep to indicate that everything is functioning as it should be. More than one beep will normally indicate a problem of some kind.

The sequence of beeps used to signal an error can be quite different depending on the BIOS chip used within the computer. This is not much of an issue though since the error codes are usually well documented and easy to figure out if you know what to look for. I have included a list of the more common BIOS manufacturers and the POST codes they employ at the end of this document.

The term BIOS is an acronym for “Basic Input Output System” and it refers to a chip on the computer’s motherboard (the main circuit board in the computer). This chip is responsible for handling all of the communication between the computer and its peripherals. One of the tasks of the BIOS is to perform the POST sequence at startup. During the POST sequence the main BIOS is responsible for -

* Verifying its integrity;
* Finding the size of and verifying the main system memory;
* Discovering, initializing and cataloging the system buses and peripherals;
* Passing control to other specialized BIOS chips on peripheral cards and devices;
* Identifying, organizing and selecting the devices that are available for booting.

As mentioned before if a problem occurs during the POST then most computers will issue an error code indicating what the problem might be. Below is a simple table from the A+ hardware exam that can help you to troubleshoot the problem at hand.

Table 1.1 Comptia A+ Post Codes

Beeps Meaning

Steady, short beeps. A bad power supply.
Long, continuous beep tone. Memory failure.
Steady, long beeps. Bad power supply.
No beep. Bad power supply, system not plugged in or power
not turned on.
No beep. If everything seems to be functioning correctly
then the problem might be with the "beeper"
itself.
One long, two short beeps. Video card failure.

As mentioned before it's important to note that different BIOS manufacturers use different error codes. See Appendix A for a list of common BIOS manufacturers and the meaning of their individual POST codes.

Some machines also write a value to an area of memory (80h) which can be read with a special device called a “POST card”. The value returned can then be compared to a chart from the manufacturer’s website to determine why the POST sequence failed.

Once you have used the POST to narrow down the issue to a couple of potential suspects you can test your theory by swapping out the suspect components with components from an equivalent machine. Swap only one device at a time and slowly narrow it down to the actual fault.

Always make a careful note of cables and jumper settings before you start, especially if you do not have the manual that came with the system. I sometimes take some close up shots with my digital camera before starting the job.

Before you start testing other components try pushing down gently on every chip on the motherboard. Computers tend to get quite hot inside and sometimes the constant heating and cooling taking place inside the computer can cause chips to work themselves loose. The term used for this is chip creep, and though it's becoming rarer it's worth checking for.

Be sure to earth yourself by holding onto the metal case of the machine with your free hand. This is to prevent you accidentally damaging the computer through static discharge. Also make sure the computer is unplugged from the wall before trying this. This is to prevent you from accidentally giving yourself a nasty shock.

Never open the power supply unit even when the computer is unplugged. The capacitors inside can hold a charge for a very long time and can kill or seriously injure you if you accidentally touch them. Rather buy a new power supply unit unless you're totally confident that you know what you’re doing.

Understanding the POST sequence is integral in diagnosing and troubleshooting computer faults. I hope that the information presented here will assist you to troubleshoot your computer the next time it decides to go on strike.

APPENDIX A: POPULAR BIOS MANUFACTURERS AND THEIR POST CODES

Table 1.2 IBM BIOS

Legend
. = Short Beep
- = Long Beep

Sequence Sound Fault Description

One short . Normal Post. System OK.
Two short .. Post error.
None Power supply, system board.
Continuous __________ Power supply, system board.
Repeating ...... Power supply, system board.
One long, one short -. System board.
One long, two short -.. Display adapter.
One long, three short -... Display adapter.
Three long --- Keyboard.

Table 1.3 AMI BIOS
Sequence Sound Fault Description

One short . DRAM refresh failure.
Two short .. Parity circuit failure.
Three short ... RAM Failure.
Four short .... System Timer failure.
Five short ..... Processor failure.
Six short ...... Keyboard gate A20 error.
Seven short ....... Virtual mode exception error.
Eight short ........ Display memory read write failure.
Nine short ......... ROM BIOS checksum failure.
Ten short .......... CMOS shutdown read write error.
Eleven short ........... Cache memory error.
One long, three short -... Conventional/extended memory failure.
One long, eight short -........ Display/retrace test failed.

Table 1.4 Phoenix BIOS
Sequence Sound Fault Description

1-1-4-1 . . . .... . Cache error (level 2).
1-2-2-3 . .. .. ... BIOS ROM checksum error.
1-3-1-1 . ... . . DRAM refresh test.
1-3-1-3 . ... . ... Keyboard controller test
1-3-4-1 . ... .... . RAM Failure on address line xxxx (check memory)
1-3-4-3 . ... .... ... RAM Failure on data bits xxxx of low byte of memory bus
1-4-1-1 . .... . . RAM Failure on data bits xxxx of high byte of memory bus
2-1-2-3 .. . .. ... ROM copyright notice
2-2-3-1 .. .. ... . Test for unexpected interrupts

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