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Concept
Power-on self-test
Summary
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 POSTs on 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; if they complete successfully, the bootstrap loader code is invoked to load an operating system.
Booting#Boot sequence of IBM PC compatibles
In IBM PC compatible computers, the main duties of POST are handled by the BIOS/UEFI, 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
initialize, 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.
Official source
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