Power-On Self Test

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What Does Power-On Self Test Mean?

A power-on self-test (POST) is a succession of built-in diagnostic tests performed when turning on a computer. This series of tests determines proper functioning of the following:

  • Random access memory (RAM)
  • Disk drives
  • Hard drives
  • Central processing unit (CPU)
  • All other hardware devices

Techopedia Explains Power-On Self Test

In many cases, with IBM-compatible or PC computers, the POST is run by a computer’s basic input/output system (BIOS). The initial tests, which are executed by the read-only memory (ROM) BIOS startup program, include reading configuration information stored in the complementary metal-oxide-semiconductor (CMOS) chip, dual inline package (DIP) switches and jumpers. This information is then compared to hardware devices, such as the CPU, hard drive, disc drives and video card. Then, ROM BIOS assigns system resources as needed. These set up the environment required by the operating system (OS). After completion of these tests, POST generally alerts the OS with one or more beeps, depending on the system.

The BIOS issues an error message if hardware is not working properly or if it is not identified. The error message consists of text on the display screen or a series of coded beep sounds. Because POST is initiated prior to activating the video card, a display screen message is not typical. There are a variety of beep codes that are properly descriptive for troubleshooting the error. A beep code may indicate a parity error, base memory read/write (R/W) error, memory refresh timer error, display memory error, motherboard timer not functioning, cache memory failed or numerous other errors.

Sometimes, an error stops the boot process until the error is corrected, and a device with an error is not allowed to run, ensuring safety. An error message can be basic. For example, error 161 means the system board has a dead battery. Sometimes a POST error can be drastic, such as when the motherboard does not detect a RAM component.

POST is part of a devices pre-boot sequence. When POST is successfully finalized, bootstrapping is enabled. Bootstrapping starts the initialization of the OS. Examples of a bootstrapping program are the Linux Loader, Windows NT Loader and Linux’s Grand Unified Bootloader.


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Margaret Rouse
Technology Expert
Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.