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Home > BIOS Systems > BIOS Index Pages > Introduction To The Basic Input Output System - How The Power On Self Test Routine Works In A PC
This page provides an understanding of what the Power On Self Test (POST) Routine is all about and gives real world examples of the screens you see each time the BIOS is invoked on a personal computer.
On This Page:
Page Overview:
Descriptions of Tests
The Video Card P.O.S.T
BIOS Version Test
motherboard Identification
CPU Test
The Memory Count Test
The Power On Self Test - Summary Screen
P.O.S.T Failures
Introduction To The Basic Input Output System The Power On Self Test (POST Routine) CMOS Settings What's On Board?
How Operating Systems Work BIOS Upgrades Page BIOS Beep Codes
Video Demonstration
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How The Power On Self Test Routine Works In A PC

The Power On Self Test (P.O.S.T) is a routine test invoked by the system BIOS each time the computer is powered on or rebooted.

Several key devices are checked by the BIOS to ensure they are functioning properly. Some of the items tested during the P.O.S.T routine include a basic keyboard test, a memory count, floppy, hard disk drive, and optical storage drive initializations.

The fundamental purpose of these tests are to determine if the system is properly recognizing each device installed in the computer. If the BIOS cannot recognize the devices properly, the operating system will also have problems controlling the hardware.

If a motherboard has on-board devices such as sound and Ethernet controllers, the BIOS also performs an identification check and information is displayed on screen indicating whether the device passed or failed the check. If a device fails the check, various errors might be noted on this screen usually requiring some type of user intervention before the BIOS can and over control to the operating system.

The CMOS chip (Complimentary Metal Oxide Semi Conductor) chip stores information neccessary for the BIOS to complete its duties.
Figure 1.1 - A Typical BIOS Chip On A Motherboard

The CMOS chip (Complimentary Metal Oxide Semi Conductor) chip stores information necessary for the BIOS to complete its duties.
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Descriptions of Tests

The Power On Self Test - Summary Screen

The following illustration shows you a specific BIOS screen that shows each time the computer is powered on. This screen is part of a QDI motherboard / BIOS ROM chip and uses a Pentium III based processor. It is listed here for your reference only.

Award Modular BIOS v4.51pg, An Energy Star AllY
SEC. MASTER DISK: LBA,UDMA33,2112MB SDRAM at bank: 0 1 2


0 7 1 1106 0571 IDE CONTROLLER 14
0 7 2 1106 3038 SERIAL BUS CONTROLLER 10
0 9 0 10B7 9050 NETWORK CONTROLLER 12
0 10 0 1102 0002 MULTIMEDIA DEVICE 5
0 10 1 1102 7002 INPUT DEVICE NA
Verifying Dmi Pool Data ................
Figure 1.2 - The First Screen You Will See When Attempting To POST A Computer On A Pentium III Based Motherboard

As you can tell the POST routine is almost performing an inventory of hardware components each time a computer is powered on.

The BIOS version and other information is usually displayed at the top of the window, while the storage mediums are the next to be checked and information displayed on screen.

On the right near the top of this image, the display type, serial ports, parellel ports, and memory checks are invoked to tell you whether there is RAM installed. The POST tells you were the memory is installed in the by detailing which banks the memory chips are being used in.

Near the middle of the window shows you the hard disk drive testing done. Information is relayed including telling you whether the particular hard drives it tested are S.M.A.R.T compatible. S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology; often written as SMART) is a monitoring system included in computer hard disk drives (HDDs) and solid-state drives (SSDs) that detects and reports on various indicators of drive reliability, with the intent of enabling the anticipation of hardware failures.

When S.M.A.R.T. data indicates a possible imminent drive failure, software running on the host system may notify the user so stored data can be copied to another storage device, preventing data loss, and the failing drive can be replaced.

The POST tells you that there are two channels, a primary master, and secondary master. Each channel can support up to two devices in a master and slave configuration mode.

Right below the hard drive information shows you the PCI Device listing or a listing of all PCI cards installed in the machine. Because I am using an older motherboard in this tutorial, there are only PCI devices listed. On more modern machines, PCI Express would normally show as the type of interface being used.

The final area on this screen to be explained includes the verification of DMI pool data. DMI or Desktop Management Interface is a layer of abstraction between system components and the software that manages them. The System Management BIOS (SMBIOS) is an extension of the Basic Input Output System (BIOS) that formulates and delivers this information to the operating system. The pool data is the information. In short, when the BIOS is "Verifying DMI pool data" it is verifying the table of data it sends to the operating system (Windows, etc.). If it isn't successful, it should return an error. Wait a reasonable period of time for it to finish. It may make take some time or it may be stuck. Often times the POST routine will hang at this point indicating a failure in a particular hardware component. If you just installed a new hard drive, setup the BIOS to automatically configure the new information. Often times you can reset configuration data in the CMOS setup or the CMOS settings may have become corrupted. This tutorial will not focus on troubleshooting these types of errors. I merely want you to know about the DMI pool test when the POST is activated.

The next section walk you through how to troubleshoot P.O.S.T failures and what you can do if you don't see any video on your screen when your system is powered on.

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P.O.S.T Failures

The POST routine can fail in any number of ways. For example, the POST could not display at all indicating a problem with the CPU or video card.

During the keyboard check if the keyboard fails to respond upon boot time, the system BIOS will prompt the user with an error message similar to the following: "Keyboard Error Press F1 to continue”. Normally checking the keyboard input to ensure a keyboard is properly connected resolves the problem after hard rebooting the system.

Here are some general troubleshooting steps to complete when the Power On Self Test fails to do its jobs.

Remove new hardware

If any new hardware has been recently added to the computer, remove that hardware to make sure it is not the cause of your issue. If your computer works after removing the new hardware, it can mean a few things. Either the new hardware is not compatible with your computer, a system setting needs to be changed, or the new hardware is defective.

Remove any disks or USB devices

Remove any disks, CDs, or DVDs that are in the computer. If any USB devices (iPods, drives, phones, etc.) are connected, disconnect all of them as well. Reboot the computer and see if anything changes.

Disconnect external devices

Remove everything from the back of the computer, except the power cable. Turn on the computer and see if it beeps normally. If the computer has never beeped, keep the monitor or display connected to see if any change occurs.

Reconnect and check power cords

If the computer is not getting enough power or the power is getting interrupted, the computer can encounter problems. Disconnect your power cables from any power strip or UPS and connect the computer directly to a known good wall outlet.

Identify beep code

If you are receiving a sequence of beeps, see the beep code page for a listing of different beep codes and their explanation. You can also check your motherboard or computer documentation for information on the beep codes. These beep codes are meant to help identify which computer component is failing or bad. If your beep code is not listed, continue troubleshooting.

Check all fans

Make sure all fans are running on the computer. If a fan has failed (especially the heat sink fan for the CPU), your computer could be overheating or detecting the fan failure, causing the computer not to boot.

Check all cables

Verify that all the cables are securely connected to the computer and that there are no loose cables by firmly pressing in each cable.

  • All disk drives should have a data cable and power cable connected to them.
  • Your power supply should have at least one cable going to the motherboard. Many motherboards may also have additional cables connected to them to supply power to the fans.

Disconnect all expansion cards

If the above recommendations still have not resolved the irregular POST, disconnect the riser board (if applicable) and each of the expansion cards. If this fixes the problem or allows the computer to POST, connect one card at a time until you determine which card is causing the problem.

Disconnect all drives

If you are unable to diagnose the problem by the beep code (or you do not hear a beep code), power off the computer. Then, disconnect any IDE, SATA, SCSI, or other data cables from the motherboard. When they are disconnected, try booting the computer again.

If this resolves your irregular POST or generates error messages, reconnect each device until you determine which device or cable is causing the issue. In some situations, it can also be a loose cable connection that causes the issue.

Remove the RAM

If you continue to experience the same problem with all the above hardware removed, remove the RAM from the motherboard and turn on the computer. If the computer has a different beep code or if your computer was not beeping and is now beeping, turn off your computer and try the below suggestions. Make sure to turn off the computer each time you are adding and removing the memory and then turning the computer back on to see if the suggestion resolves the issue.

  1. Re-insert the memory into the same slot.
  2. If you have more than one stick of memory, remove all but one stick of memory and try rotating through each stick.
  3. Try one stick of memory in each slot.

If you can get the computer to boot with one or more of the sticks of memory installed, it is likely you are dealing with some bad memory. Try to identify which stick of memory is bad and replace it.

If you can get the memory to work in one slot but not another slot, the motherboard is likely defective. You can either workaround the issue by running the memory in a different slot that does work or replace the motherboard.

Power cycle the computer

In some situations, a computer may have power related issues often caused by either the power supply or the motherboard. To help determine if this is the cause of your issue, try turning the computer on, off, and back on as fast as possible, making sure the computer power light goes on and off each time. In some situations, you may be able to temporarily get the computer to boot.

Warning: Try this method only as a temporary workaround or as a last resort to get any valuable information off of the computer.

Disconnect and reconnect the CPU

For users who are more comfortable working inside their computer, one last recommendation is to reseat the CPU by removing it and re-inserting it into the socket. You could also try applying fresh thermal compound between the CPU and the heat sink.

Loose BIOS chip

If your motherboard has a BIOS chip, it can become loose over time due to heat expansion and cause the computer to give an irregular POST. Gently press down on the BIOS chip to make sure it has not become loose.

Bad motherboard, CPU, RAM, or power supply

If you have tried all of the above recommendations and continue to have the same issue, it is likely that you have a bad motherboard, power supply, CPU, or RAM stick. The next step would be either to replace these components or have the computer serviced. If you plan on doing the repairs yourself, we suggest that you replace or swap in parts from another computer that is known to work. The order in which you should replace these parts is the motherboard first, then the RAM, the CPU, and finally, the power supply.

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The Video Card P.O.S.T

The first test that the BIOS completes is a video card test. This performs a basic initialization of a video card to determine if it is working properly.

A video card P.O.S.T routine carries out its own set of tests and instructions that need to be run during the boot-up process. The system BIOS will yield to the video BIOS and wait until the video BIOS completes its duties. After that process completes, the system BIOS will kick back in and continue to check other elements in the system.

The illustration below shows a sample video card test.

Nvidia Geforce2 MX VGA BIOS
Nvidia Corporation
Version 3.11.11
Copyright (c) 2000
64mb DDR Video Memory

Figure 1.3 - The VGA Test Within The POST Routine

If the screen shows nothing when powered up then a problem is with the the video card, most often the video card has become dislodged from the socket on the motherboard, the cable connecting the monitor to the video card is not connected properly, or the video card has broken down and needs replacement.

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BIOS Version Test

This test will determine the exact version of the BIOS being used. It will also display the manufacturer of the ROM chip where the BIOS settings are stored in addition to a year the BIOS file was created.

The illustration below shows a sample BIOS Version test.


Figure 1.4 - Information Displayed During The BIOS Recognition Self Test
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Motherboard Identification

This test immediately follows the video card test normally displaying a string of characters known as an identification string used to identify the model and make of motherboard installed in a PC.

The illustration below shows the information during the BIOS identification and motherboard version tests.

GA-6VXE7+ F4
Figure 1.5 - Information Displayed During The BIOS Version and Motherboard Identification Tests

The string indicates that a Gigabyte brand of motherboard is being used and the model number of the motherboard is a 693A which translates into an Apollo based motherboard manufactured by Gigabyte.

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CPU Test

The most important P.O.S.T routine completed by a system BIOS is a CPU test. This test determines the model and speed of the installed processor. It may also show basic information about the processor including an id representing the family of processor being used. The speed at which the processor is running is normally displayed in either Mhz or Ghz.

The figure below illustrates a typical CPU test.


Figure 1.6 - Information Displayed During The BIOS CPU Test

As you may noticed the particular CPU being used is an Intel branded Celeron running at 850 megahertz.

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Memory Count Test

This test determines how much physical memory is installed in the computer. The test will display the quantity of Random Access Memory as a value in K, KB, or MB.

If a system is equipped with 2 sticks of RAM each totalling 128 Megabytes, the P.O.S.T report would show 294912K of physical ram.

Figure 1.5 illustrated below shows a typical memory test.

Figure 1.7 - Information Displayed During The Memory Count Test
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Final Post Tests

The figure shown below illustrates the final P.O.S.T report before the BIOS calls an operating system. The P.O.S.T routine displays a summary of all hardware installed in the computer in addition to the mapping of PCI based devices.

GA-6VXE7+ F4
<CPU ID:0686, PATCH ID:0008>


Figure 1.8 - The POST Summary Screen
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