What Is POST? (Power On Self Test Definition)


What is POST?

The Power On Self Test (POST) is a series of diagnostic tests that a computer system performs during startup. It is a crucial part of the boot process as it checks the essential hardware components to ensure they are functioning correctly before the operating system is loaded.

When you power on your computer, the system firmware, such as the Basic Input/Output System (BIOS) or the Unified Extensible Firmware Interface (UEFI), triggers the POST. The POST then begins assessing various hardware components, including the processor, memory, storage devices, and input/output ports.

The primary purpose of POST is to detect any potential hardware errors or failures that could hinder the system’s proper functioning. By retrieving and analyzing information from the hardware components, the POST can determine if they are operating within expected parameters. If any issues are identified during the diagnostic tests, the POST will display error codes or error messages to inform the user of the problem.

POST serves as an important tool in troubleshooting computer issues. It helps identify faulty hardware, such as a malfunctioning RAM module or a disconnected hard drive, that could lead to system instability or failures. By isolating the problematic component, users and technicians can take appropriate actions to resolve the issue, such as replacing the faulty hardware or reseating the loose connections.

Additionally, the POST is designed to be able to detect and resolve certain types of hardware conflicts, such as resource conflicts between devices. This feature ensures that computer systems can allocate resources properly and avoid conflicts that could cause system crashes or performance degradation.

Overall, the POST plays a critical role in ensuring the proper functioning of a computer system. By effectively diagnosing hardware issues and preventing potential problems, it helps provide a stable and reliable computing experience. Understanding the significance of the POST can empower users to troubleshoot and resolve any startup issues they may encounter, ensuring smooth operation and optimal performance of their computer systems.

Why is POST important?

The Power On Self Test (POST) is an essential component of the computer boot process, and its importance cannot be overstated. Here are several reasons why POST is crucial:

  • Hardware diagnostics: POST performs a series of diagnostic tests on the hardware components of the computer, including the CPU, memory, storage devices, and peripherals. By checking these components for errors or failures, POST helps identify potential issues that could affect the system’s performance and stability.
  • Error detection: POST is designed to detect and report any errors that occur during the hardware diagnostics. It displays error codes or error messages that indicate the nature of the problem, allowing users and technicians to troubleshoot and resolve the issues more effectively.
  • Troubleshooting guidance: When a computer encounters a startup problem, the error codes or messages provided by POST act as a roadmap for troubleshooting. By indicating the specific component or area where the problem lies, POST helps users and technicians narrow down the possible causes and take appropriate actions for resolution.
  • Preventing system crashes: By identifying and resolving hardware issues, POST helps prevent system crashes and unexpected shutdowns. This ensures a more stable and reliable computing experience, minimizing disruptions and data loss.
  • Resource allocation: POST helps detect and resolve hardware conflicts, such as resource conflicts between devices. By ensuring proper resource allocation, POST helps avoid conflicts that could lead to system crashes, performance degradation, or hardware malfunctions.
  • System integrity: The reliability of the entire computer system depends on the individual components functioning properly. POST ensures that each essential hardware component is tested and verified, providing confidence in the system’s overall integrity.

How does POST work?

The Power On Self Test (POST) is triggered when you power on your computer. It is performed by the system firmware, such as the Basic Input/Output System (BIOS) or the Unified Extensible Firmware Interface (UEFI). Here’s a breakdown of how POST works:

  1. Initialization: When the computer receives power, the system firmware initiates the POST process. It initializes the hardware components and prepares them for testing.
  2. Hardware detection: The POST then scans and identifies the hardware components connected to the computer. It detects the presence of the CPU, memory modules, storage devices, graphics card, and other peripherals.
  3. Diagnostic tests: Once the hardware is detected, the POST begins a series of diagnostic tests. These tests involve checking the functionality and integrity of the hardware components. For example, it may test the CPU by sending specific signals and evaluating its response, or verify the memory modules by writing and reading data to ensure proper operation.
  4. Error checking: During the diagnostic tests, the POST monitors the performance of each hardware component. It compares the results of the tests against predefined criteria to determine if any errors or failures are present. If an issue is detected, the POST generates an error code or displays an error message to inform the user of the problem.
  5. Error reporting: If the POST identifies an error, it communicates the information to the user through error codes or messages. These codes or messages provide details about the nature of the problem, such as a specific hardware component that has failed or an invalid hardware configuration.
  6. Completion and handover: Once the POST has completed the diagnostic tests, it hands control over to the operating system. If no errors were detected, the operating system continues to load and the computer proceeds to the login screen or desktop environment.

The entire POST process occurs within a matter of seconds, and its duration may vary depending on the complexity of the hardware configuration. It is worth noting that some BIOS or UEFI firmware versions allow users to customize the POST process by enabling or disabling specific diagnostic tests or adjusting the error reporting options.

Understanding how the POST works can provide insights into the startup process of a computer and help users diagnose and troubleshoot hardware-related issues more effectively.

What does POST test?

The Power On Self Test (POST) is a diagnostic procedure that extensively tests various hardware components during the computer boot process. It ensures that critical parts of the system are functioning correctly before the operating system is loaded. Here are the main hardware components that POST tests:

  1. CPU (Central Processing Unit): The POST verifies the CPU’s functionality by sending and receiving signals to and from the processor. This includes checking the speed, cache, and basic instruction processing capabilities.
  2. Memory (RAM): POST tests the system memory by performing a series of read and write operations. It checks for errors such as faulty memory modules or incorrect memory timings.
  3. Hard drives and SSDs: POST checks the presence and accessibility of storage devices, including hard drives and solid-state drives (SSDs). It ensures that the system can communicate with these devices properly.
  4. Video/Graphics Card: The POST examines the graphics card to ensure it is functioning correctly. It verifies the card’s ability to output video signals, the memory on the card, and any associated hardware accelerators.
  5. Peripherals and Input/Output (I/O) devices: POST tests peripheral devices such as keyboards, mice, and USB ports to confirm their proper operation. It ensures that the system can communicate with these input/output devices effectively.
  6. BIOS/UEFI firmware: The POST verifies the integrity and stability of the system firmware, such as the Basic Input/Output System (BIOS) or the Unified Extensible Firmware Interface (UEFI). It ensures that the firmware is properly loaded and functioning correctly.

During the testing process, the POST retrieves information and data from each component and compares it to expected values. If any errors or discrepancies are detected, the POST will display error codes or messages to indicate the nature of the problem.

It’s important to note that the specific components tested by POST can vary depending on the computer’s hardware configuration and the firmware version. Additionally, some advanced BIOS or UEFI versions allow users to configure or skip certain tests, depending on their needs or preferences.

By testing these critical hardware components, the POST ensures that the computer system is in a stable and functional state before the operating system takes control. Identifying potential hardware issues early on allows users to address them promptly and maintain a smooth computing experience.

Common POST Error Codes

During the Power On Self Test (POST), if any errors or issues are detected with the hardware components, the POST will generate specific error codes or error messages to indicate the nature of the problem. Here are some common POST error codes and their possible meanings:

  1. Beep codes: Many motherboards use beep codes to indicate various errors. For example, a continuous beep might indicate a memory failure, while a series of short beeps could indicate a CPU problem. The specific beep patterns may vary depending on the motherboard manufacturer.
  2. Memory-related errors: These errors often indicate an issue with the RAM modules. Common memory-related error codes include “Memory Parity Error,” “Bad Memory,” or “Memory Configuration Error.” This could be due to faulty memory modules, incorrect memory timings, or mismatches between different RAM modules.
  3. Hard drive errors: If the POST encounters errors with the hard drives or SSDs, it may display messages like “No Bootable Device Found,” “Disk Boot Failure,” or “Invalid System Disk.” These errors can be caused by a faulty hard drive, loose cables, or incorrect boot settings.
  4. Video/Graphics card issues: If there are problems with the video or graphics card, the POST may display error codes such as “No Video Signal,” “Graphics Card Not Detected,” or “Video Memory Error.” This could be due to a defective card, improper installation, or incompatible drivers.
  5. Keyboard or mouse errors: Issues with the keyboard or mouse can trigger error messages like “Keyboard Error” or “Mouse Not Detected.” These errors may indicate loose connections, faulty peripherals, or incorrect BIOS settings.
  6. BIOS/UEFI errors: If there are errors in the system firmware (BIOS/UEFI), the POST may display messages such as “BIOS ROM Checksum Error” or “Invalid System Configuration.” This could be caused by a corrupted firmware, incorrect settings, or outdated BIOS/UEFI version.

It is crucial to consult the motherboard or system manufacturer’s documentation to identify the specific meaning behind the error codes or messages. They often provide detailed explanations for each error, along with troubleshooting steps to resolve the issue.

Keep in mind that the error codes may vary depending on the BIOS/UEFI firmware version or the manufacturer of the motherboard. Consulting the motherboard manual or the manufacturer’s support website can help users troubleshoot and resolve common POST error codes effectively.

Troubleshooting POST Errors

Encountering errors during the Power On Self Test (POST) can be frustrating, but they often provide valuable clues to help diagnose and resolve hardware issues. Here are some troubleshooting steps to follow when dealing with POST errors:

  1. Refer to the error codes or messages: First, carefully read and note down the specific error codes or messages displayed by the POST. These codes provide essential information about the nature of the problem and can guide your troubleshooting efforts.
  2. Consult the motherboard manual: Check the motherboard manual or the manufacturer’s support website for a comprehensive list of error codes and their meanings. This will help you narrow down the possible causes and take appropriate actions to resolve the issue.
  3. Ensure proper hardware connections: Check that all hardware components, such as the RAM modules, hard drives, and power cables, are firmly and correctly connected. Loose or improperly seated components can cause errors during the POST. Reconnecting or reseating them may resolve the issue.
  4. Test hardware components: If possible, test individual hardware components, such as RAM modules or hard drives, in separate systems or using diagnostic tools. This will help identify if any specific component is causing the POST errors. Replace or repair the faulty components as necessary.
  5. Update BIOS/UEFI firmware: Check if your motherboard manufacturer provides any firmware updates. Updating the BIOS/UEFI firmware can fix bugs, improve compatibility, and address known issues that may be causing the POST errors.
  6. Reset BIOS/UEFI settings: Sometimes, incorrect BIOS/UEFI settings can lead to POST errors. To troubleshoot this, reset the BIOS/UEFI settings to their default values. This can usually be done by removing the CMOS battery for a few minutes or using the motherboard’s jumper pins. Consult the motherboard manual for specific instructions.
  7. Disconnect unnecessary devices: Temporarily disconnect any unnecessary peripherals or devices connected to the computer, such as external hard drives, printers, or USB devices. Sometimes, conflicts or compatibility issues with these devices can cause POST errors.
  8. Seek professional assistance: If you have tried the above steps and are still unable to resolve the POST errors, it may be beneficial to seek assistance from a computer technician or your motherboard manufacturer’s support team. They can provide more specialized guidance and assistance based on your specific hardware configuration.

Remember to document the steps you have taken and the changes you have made during the troubleshooting process. This information can be helpful for technicians if you need to seek further assistance.

Troubleshooting POST errors requires patience and attention to detail. By following these steps and carefully investigating the error codes or messages, you can effectively diagnose and resolve hardware issues to ensure a smooth booting process for your computer.


While they are closely related, the Power On Self Test (POST) and the Basic Input/Output System (BIOS) serve different functions in the computer boot process. Here’s a comparison between POST and BIOS:


The POST is a series of diagnostic tests performed during the boot process. Its main purpose is to check the integrity and functionality of various hardware components, including the CPU, memory, storage devices, and peripherals. POST retrieves information from these components and compares it against expected values to detect any errors or failures. It generates error codes or error messages to indicate the nature of the problem. The POST helps ensure that the computer system is in a stable and functional state before handing control to the operating system to continue the boot process.


The BIOS, or the newer UEFI (Unified Extensible Firmware Interface), is firmware embedded in the computer’s motherboard. It provides a set of low-level software instructions and functions that enable communication between the hardware and the operating system. The BIOS/UEFI initializes the hardware components, configures the system settings, and provides a basic interface for accessing and modifying settings, such as changing boot priority or adjusting CPU and memory parameters. The BIOS/UEFI also has the ability to load the operating system from the bootable storage device specified in the boot order.

In summary, the POST is a diagnostic process that checks hardware components, while the BIOS/UEFI is a firmware that provides system initialization, configuration, and the ability to load the operating system. The POST occurs before the BIOS/UEFI takes control of the boot process.

It’s worth noting that the POST is an integral part of the BIOS/UEFI firmware. The BIOS/UEFI triggers the POST during the boot process and relies on its results to determine if the system is ready for the operating system to load. If the POST encounters errors, it may affect the normal execution of the BIOS/UEFI and can lead to issues with booting the operating system.

The Evolution of POST

The Power On Self Test (POST) has undergone significant evolution over the years to keep up with advancements in computer hardware and firmware technology. Here’s a glimpse into the evolution of POST:

Early PC era:

In the early days of personal computing, POST primarily consisted of a series of beep codes. These codes signified different error conditions encountered during the hardware check process. The user had to refer to the system manual to interpret the beeps and understand the nature of the problem.

Graphical POST:

With the introduction of graphical user interfaces (GUI), POST evolved to display error messages or codes on a computer monitor. This made it easier for users to identify and troubleshoot issues. Instead of relying solely on audio cues, users could now visually analyze the POST results.

Error code standardization:

To simplify troubleshooting, there was a movement towards standardizing the error codes produced by the POST. Vendors began adopting common error code formats, ensuring consistency across different computer systems. This helped technicians and users quickly identify and address hardware-related issues.

Expansion of diagnostic tests:

As computer hardware became more complex, POST expanded its diagnostic tests to cover a wider range of components. It began checking additional hardware, including the video card, input/output ports, network connectivity, and more. This allowed for comprehensive hardware diagnosis during the boot process.

Enhanced error reporting:

Modern POST implementations now provide detailed error messages, often accompanied by suggestions or recommendations for troubleshooting. These error messages offer more specific information about the detected problems, helping users pinpoint and resolve issues more effectively.

Integration with BIOS/UEFI:

The integration of POST with the system firmware, such as the Basic Input/Output System (BIOS) or the Unified Extensible Firmware Interface (UEFI), has improved its capabilities. POST now works in conjunction with the firmware to enhance system initialization, configuration, and the loading of the operating system.

Self-recovery mechanisms:

Recent advancements have introduced self-recovery mechanisms to the POST. If a minor error is detected during the POST, it may attempt to automatically fix the issue before displaying an error message. These self-recovery mechanisms help streamline the boot process and reduce the need for manual intervention.

Overall, the evolution of POST has contributed to a more user-friendly and efficient boot process. With improved error reporting, better diagnostic tests, and enhanced integration with the system firmware, POST continues to play a vital role in ensuring the smooth startup of computer systems.


The Power On Self Test (POST) and the Unified Extensible Firmware Interface (UEFI) are closely intertwined in the modern boot process of computers. Here’s a closer look at the relationship between POST and UEFI:

POST within UEFI:

UEFI is an advanced firmware interface that has largely replaced the traditional Basic Input/Output System (BIOS). UEFI provides more features and flexibility compared to BIOS, and one of its core functionalities is initiating and executing the POST.

Improved hardware initialization:

With UEFI, the initialization of hardware components during the POST phase is more efficient and streamlined. UEFI interacts directly with the hardware, allowing for faster and more accurate detection and initialization of devices. This improves system boot times and overall performance.

Graphical user interface:

UEFI firmware often provides a graphical user interface (GUI) for configuring system settings, including those related to the POST. This GUI allows users to customize various aspects of the POST, such as enabling or disabling specific diagnostic tests, adjusting error reporting options, or setting the boot priorities.

Advanced error reporting:

UEFI enhances the error reporting capabilities of the POST. It provides more detailed error messages or codes, often accompanied by suggested troubleshooting steps. This helps users and technicians quickly identify and resolve issues that arise during the POST process.

Secure Boot:

UEFI introduced a security feature known as Secure Boot. It ensures that only digitally signed operating system bootloaders and device drivers are allowed to run during the boot process. Secure Boot protects against the execution of malicious code or unauthorized components, enhancing system security and mitigating potential tampering at boot time.

Compatibility with legacy systems:

UEFI is designed to be backward compatible with legacy systems that use BIOS firmware. This allows UEFI-based systems to emulate BIOS functionality, including running the traditional POST process, for compatibility with older software and operating systems.

Extensive firmware configuration:

UEFI provides extensive firmware configuration options beyond the scope of the traditional BIOS. Users can access UEFI settings to adjust various system parameters, such as fan control, CPU clock speed, memory timings, and more. These customization options can affect the behavior and performance of the POST process.