Power On Self Test (POST)

What is Power on Self Test (POST)?

A Power on Self Test, commonly referred to as POST, refers to the series of tests conducted by the software or firmware as soon as a computer is switched on to find out whether or not the hardware of the computer is functioning properly. It is executed irrespective of the operating system.

Technically, it refers to the systematic check of the basic devices of the computer where the BIOS displays the codes usually at the bottom right corner of the screen when the test progresses enough to initialize the video editor.

Understanding Power on Self Test (POST)

What is Power on Self Test (POST)

The Power-On Self Test is actually a process which involves a series of built-in analytical tests performed by a computer when it is turned on.

This test is always the first step of booting a device and it will run when you just switch it on for the first time after a few days or restart it regardless of whether the operating system is installed or not because it is handled by the BIOS.

This operation is usually carried out by the firmware of the computer which can be The Basic Input Output system or BIOS, the Unified Extensible Firmware Interface or UEFI or any other system.

It is actually done to determine whether or not the following components of the computer system are functioning properly:

In addition to the above, checks are also performed by POST on keyboard, Direct Memory Access, interrupt controller and timer.

Usually, in most of the cases this particular series of tests are handled and the error message, if any, is sent by the BIOS of the computer. However, the preliminary tests are carried out by the Read Only Memory or ROM.

If there is any issue found during POST with the hardware the BIOS will send an error message saying that the specific faulty hardware is not functioning properly.

It will also send a similar message if a particular hardware device is not identified during the test.

This error message can be sent in two specific forms such as:

Other than that, the results of the POST may also be displayed on the panel of a device that is a part of it, as an output to some external device, and may even be stored to be retrieved in the future by any analytical tool.

If some information is stored in the ROM, it can also test every single byte of the memory to determine the pattern.

When the POST error codes are displayed on the monitor of the computer, it may scroll off too quickly off the screen to read.

The best alternative to this method is to display the codes by redirecting the result of the console to a serial port.

However, the fact that these tests are initiated before the video card is activated, the error message may not always be displayed on the computer screen.

This means that in most of the cases you will usually receive different types of beep codes.

For example, in a MAC, the beep codes can be as follows:

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These codes help in troubleshooting any error because these are quite descriptive and have different meanings for different numbers of beeps.

Usually, the beep codes describe following issues and are certainly not limited to:

When such errors occur, depending on the type, it may even stop the boot up process and may not resume until it is rectified.

If there is any issue detected with any particular device during POST, it may not be allowed to function. This ensures safety of the entire system.

Typically, a POST error message can be simple or drastic. For example, if you receive an error 161 message, it will indicate a dead battery on the system board.

On the other hand, a drastic message for example would be displayed when the motherboard is not able to detect the Random Access Memory of the computer system.

If POST is successful, bootstrapping happens and the operating system is activated.

Importance of POST

POST is very important for the computer system because it happens before the system starts to function. It helps in determining whether or not all of its components are functioning properly to ensure safety of the computer and in troubleshooting the issues as well.

Being a pre-boot sequence or test, bootstrapping is enabled only after the successful completion of POST. This initializes the operating system and the computer system starts to function.

The messages sent by the BIOS after the POST helps in troubleshooting different issues that can be viewed in different ways depending on the type of the hardware that is creating the particular issue.

It can be that the system turns on perfectly but things may get stuck before the boot process can be initiated with or without any hint given or error messages displayed on the computer monitor.

POST also helps in performing different steps of troubleshooting the issue such as:

The role of POST in the pre-boot sequence does not end there. By helping in finding and loading the operating system, these tests indirectly ensure proper interaction between all the hardware units of the computer system.

Some of the other major responsibilities of POST can be summarized as follows:

And, in the later versions of BIOS, a POST also performs some other vital functions such as:

The POST codes help the computer systems as well as the user to respond or take proper actions after encountering specific errors. These actions include:

The importance of POST is not restricted to the computers only but it is also necessary for some other appliances such as medical equipment and other embedded systems such as those found in communications, avionics, and others.

POST Steps

The steps followed for the POST steps may vary according to the hardware architecture of the computer system as well as the firmware installed in it.

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In general, it involves validating the capability of all the hardware required and ensuring the apps and operating system run properly.

Ideally, you can say that POST is typically the BIOS startup program that consists of different aspects and is done in different steps. The basic steps of the test involve:

Typically, when the system is switched on, the following steps are followed:

After the results of these tests are obtained, these are compared with the hardware devices. These devices can be varied and many and include but not limit to those mentioned above.

When the comparison is done, the ROM allocates system resources according to the need as needed which is done primarily with an intention to set up the perfect environment that is necessary for the operating system to function.

When this is done and all of the tests are completed, POST will typically alert the operating system by emitting one or more beeps, if the OS is installed. Even if the operating system is not installed, POST will always happen. This step will however depend on the type of the system.

POST Error Codes

There are different codes of POST errors that are usually represented by a number of beeps so that you know its status. Depending on the number of beeps you hear there can be no error or errors in display, RAM, CPU, keyboard and others.

Usually, the different POST errors are pointed out by different beeps combinations you hear when you start your computer.

Here is however some of the most common POST error codes that are indicated through beep codes:

These beep codes, as said earlier, can vary from company to company because there is no universal standard for it. Therefore, it is essential that you look for and know the beep codes for your particular computer system.

Though the POST codes are very short lived and are often unreadable or the digits cannot be distinguished easily, there are a few specific tests that take a long time. You can read them by looking at the LEDs through the front panel. These codes and their meanings, in that order, are:

POST Issues and Meanings

The descriptions of every POST code will usually appear as a four digit string at the bottom right corner of the BIOS screen. Two digits represent the output of the primary I/O port 80 and other two represent that of the secondary I/O port 81.

A few POST codes from the primary I/O port 80 can be seen on the LEDs of the front panel of a server node.

Here is a list of some of the issues, their meanings as well as the pause, warning, halt or no actions taken by the system against each error.

In this list the first two digits of the code are from port 81 and the last two are from port 80.

All these in the order error code, error message and response:

How to Enable POST?

The process to enable POST is pretty simple but may vary depending on the BIOS available. You will typically have to use the Function key F2 to start and F10 to exit with a few steps to follow in the interim.

What Will Happen If POST Fails?

If POST fails, it will produce a beep code which will indicate the specific error, as mentioned above.

Sometimes, depending on the type of issue, the BIOS will send a simple error message for you to take the necessary action and sometimes the computer will not boot at all.

Where is POST Stored?

POST is an analysis process that happens even before the drive and operating system starts working and therefore is stored in the ROM BIOS of the motherboard. This allows faster access.


As it is clear from this article, the POST process is very important for booting a system up as well as for its safety.

It happens within a few seconds after the computer system is turned on and it detects hardware issues before the operating system is loaded for the computer to start working.