POST (Power-On Self-Test)

A power-on self-test (POST) is a process performed by firmware or software routines immediately after a computer or other digital electronic device is powered on.

The results of the POST may be displayed on a panel that is part of the device, output to an external device, or stored for future retrieval by a diagnostic tool. Since a self-test might detect that the system’s usual human-readable display is non-functional, an indicator lamp or a speaker may be provided to show error codes as a sequence of flashes or beeps. In addition to running tests, the POST process may also set the initial state of the device from firmware.

In the case of a computer, the POST routines are part of a device’s pre-boot sequence; if they complete successfully, the bootstrap loader code is invoked to load an operating system.

Progress and Error Reporting

The original IBM BIOS made POST diagnostic information available by outputting a number to I/O port 80 (a screen display was not possible with some failure modes). Both progress indication and error codes were generated; in the case of a failure which did not generate a code, the code of the last successful operation was available to aid in diagnosing the problem.

Using a logic analyzer or a dedicated POST Card‍—‌an interface card that shows port 80 output on a small display‍—‌a technician could determine the origin of the problem. Once an operating system is running on the computer the code displayed by such a board may become meaningless, since some OSes, e.g. Linux, use port 80 for I/O timing operations. The actual numeric codes for the possible stages and error conditions differ from one BIOS supplier to another. Codes for different BIOS versions from a single supplier may also vary, although many codes remain unchanged in different versions.

Later BIOSes used a sequence of beeps from the motherboard-attached PC speaker (if present and working) to signal error codes. Some vendors developed proprietary variants or enhancements, such as MSI’s D-Bracket. POST beep codes vary from manufacturer to manufacturer.

Information on numeric and beep codes is available from manufacturers of BIOSes and motherboards. There are websites which collect codes for many BIOSes.

CompTIA A+ POST Beep Code

Beeps Meaning
Steady, short beeps Power supply may be bad
Long continuous beep tone Memory failure
Steady, long beeps Power supply bad
No beep Power supply bad, system not plugged in, or power not turned on
No beep If everything seems to be functioning correctly there may be a problem with the ‘beeper’ itself. The system will normally beep one short beep.
One long, two short beeps Video card failure


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