What is Power on Self Test (POST)? How Does It Work

What is Power on Self Test (POST)
What is Power on Self Test (POST)

Every time a computer is turned on, its basic input/output system (BIOS) performs a diagnostic procedure known as the Power On Self Test (POST). Prior to loading the operating system, POST checks and confirms that all of the computer’s hardware components are performing properly and communicating with the motherboard.

What is the power on self test?

Every time a computer is turned on, its fundamental input/output system runs a self-diagnostic procedure known as the Power On Self Test. Prior to loading the operating system, POST checks and confirms that all of the computer’s hardware components are performing properly and communicating with the motherboard.

The BIOS runs a number of checks during POST on hardware elements such the RAM, CPU, hard drive, keyboard, and video card, and reports the results on the screen in the form of diagnostic messages and codes. If any problems are discovered, the BIOS will either try to fix them or display an error message describing what they are and where they happened.

The BIOS will initialize the machine and turn control over to the boot loader, which loads the operating system, if all components pass the POST test. The computer might not start or start with restricted capability if the POST procedure fails.

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How is the power on self test performed?

The Power On Self Test (POST) is performed as follows:

  1. The computer is powered on: Immediately upon powering up the computer, the BIOS firmware is activated and set into motion.
  2. Initial tests: The BIOS performs a variety of initial tests, such as examining the video card, keyboard and system clock.
  3. RAM test: The BIOS initiates a memory test to ensure that the RAM is functioning properly and all addressable areas of memory are accessible.
  4. CPU test: The BIOS verifies that your CPU is working efficiently and communicating accurately with the motherboard, so you can have peace of mind knowing it’s in good shape.
  5. Hard drive test: The BIOS executes a test on the hard drive to guarantee it is working adequately, and that both data read and write operations can be conducted.
  6. Display test: The BIOS reveals crucial diagnostic data and messages onscreen, ensuring you are updated with the status of your tests.
  7. Error handling: During the POST process, if any issues arise, the BIOS will not only attempt to rectify them but also display an error message delineating where and what kind of issue has occurred.
  8. Initialization: When all elements successfully pass the POST test, the BIOS initiates the system and shifts control to the boot loader. At this time, it loads and starts up your operating system.

Every time your computer is powered up, the POST process is immediately and automatically initiated; usually taking under a few seconds to complete. Before any operating systems are installed, POST verifies that all hardware components operate correctly as well as interact with the motherboard appropriately in order for it to begin and run efficiently.

Why is power on self test important?

Power On Self Test (POST) is important for several reasons:

  1. Hardware verification: POST assists in making sure that all of the computer’s hardware is operating properly and communicates with the motherboard.
  2. Early problem detection: Before the operating system is loaded and during startup, POST can identify hardware component faults, allowing the user to fix the problem before it worsens.
  3. Enhanced dependability: POST helps to ensure that the computer can start and run smoothly by checking that all hardware components are in good working order before the operating system is loaded, enhancing overall reliability.
  4. POST assists in ensuring compatibility between all hardware parts and the motherboard, which is crucial for the stable operation of the computer.

In conclusion, POST is a crucial stage in a computer’s startup procedure that helps verify that all hardware parts are operating properly and that the computer can start and run smoothly. This makes it simpler to troubleshoot potential issues as well as increase the computer’s stability and reliability.

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What kind of errors in the power on self test?

There are several types of errors that can occur during the Power On Self Test (POST):

  1. Memory errors: POST is able to detect problems with the computer’s RAM, such as incorrect memory addresses and defective memory chips.
  2. Problems with the CPU: POST can detect CPU issues like a damaged processor or an incorrect clock speed.
  3. The hard drive may have problems with data read/write operations, a broken disc, or a misconfigured disc, among other things. These problems are recognisable by POST.
  4. Using POST, it is possible to identify keyboard errors such a damaged or disconnected keyboard.
  5. Problems with the visual card: POST can detect problems with the video card, such as incorrect video memory settings, missing or outdated drivers, or a broken video card.
  6. A motherboard, power supply, or peripheral device issue may be discovered by POST.

The BIOS will typically show a diagnostic message or error code on the screen, explaining what the issue is and where it happened, if any faults are found during POST. You can use this information to troubleshoot problems. But occasionally, POST might not be able to identify or fix the issue, and the computer might either not start at all or start with a very limited set of capabilities.

How to Resolve Errors Detected on POST?

To resolve errors detected during a Power On Self Test (POST), you can try the following steps:

  1. Check for error messages or codes: The BIOS may display a diagnostic message or error code on the screen indicating the problem. Check for any messages or codes and search for their meanings online to determine the cause of the error.
  2. Check connections: Ensure that all cables and peripheral devices are connected properly and securely. If a device is not connected, reconnect it and see if the error disappears.
  3. Reseat hardware components: If a hardware component is not functioning properly, try removing it and reinserting it to see if this resolves the issue.
  4. Update drivers: Check for updated drivers for any hardware components that are causing errors and install them if available.
  5. Replace hardware components: If a hardware component is damaged or malfunctioning, it may need to be replaced.
  6. Use system restore: If the error is related to software or the operating system, you may be able to use system restore to return the computer to a previous state where it was working correctly.
  7. Consult a professional: If you are unable to resolve the issue, consider consulting a professional or taking the computer to a repair shop for further assistance.


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