A power-on self-test (POST)

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.

This article mainly deals with personal computers, but many other embedded systems such as those in major appliances, avionics, communications, or medical equipment also have self-test routines which are automatically invoked at power-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 and only once they complete successfully is the bootstrap loader code invoked to load an operating system.

In IBM PCcompatible computers, the main duties of POST are handled by the BIOS, which may hand some of these duties to other programs designed to initialize very specific peripheral devices, notably for video and SCSI initialization. These other duty-specific programs are generally known collectively as option ROMs or individually as the video BIOS, SCSI BIOS, etc.

The principal duties of the main BIOS during POST are as follows:

·         verify CPU registers

·         verify the integrity of the BIOS code itself

·         verify some basic components like DMA, timer, interrupt controller

·         find, size, and verify system main memory

·         initialize BIOS

·         pass control to other specialized extension BIOSes (if installed)

·         identify, organize, and select which devices are available for booting

The functions above are served by the POST in all BIOS versions back to the very first. In later BIOS versions, POST will also:

·         discover, initialize, and catalog all system buses and devices

·         provide a user interface for system's configuration

·         construct whatever system environment is required by the target operatingsystem