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 operating system

(In early BIOSes, POST did not organize or select boot devices, it simply identified floppy or hard disks, which the system would try to boot in that order, always.)

The BIOS begins its POST when the CPU is reset. The first memory location the CPU tries to execute is known as the reset vector. In the case of a hard reboot, the northbridge will direct this code fetch (request) to the BIOS located on the system flash memory. For a warm boot, the BIOS will be located in the proper place in RAM and the northbridge will direct the reset vector call to the RAM. (In earlier PC systems, before chipsets were standard, the BIOS ROM would be located at an address range that included the reset vector, and BIOS ran directly out of ROM. This is why the motherboard BIOS ROM is in segment F000 in the conventional memory map.)

During the POST

flow of a contemporary BIOS, one of the first things a BIOS should do is determine the reason it is executing. For a cold boot, for example, it may need to execute all of its functionality. If, however, the system supports power saving or quick boot methods, the BIOS may be able to circumvent the standard POST device discovery, and simply program the devices from a preloaded system device table.

The POST flow for the PC has developed from a very simple, straightforward process to one that is complex and convoluted. During POST, the BIOS must integrate a plethora of competing, evolving, and even mutually exclusive standards and initiatives for the matrix of hardware and OSes the PC is expected to support, although at most only simple memory tests and the setup screen are displayed.

In earlier BIOSes, up to around the turn of the millennium, the POSTwould perform a thorough test of all devices, including a complete memory test. This design by IBM was modeled after their larger (e.g. mainframe) systems, which would perform a complete hardware test as part of their cold-start process. As the PC platform evolved into more of a commodity consumer device, the mainframe- and minicomputer-inspired high-reliability features such as parity memory and the thorough memory test in every POST were dropped from most models. The exponential growth of PC memory sizes, driven by the equally exponential drop in memory prices, thanks to Moore's Law, was also a factor in this, as the duration of a memory test using a given CPU is directly proportional to the memory size.

The original IBM PC could be equipped with as little as 16 KiB of RAM and typically had between 64 and 640 KiB; depending on the amount of equipped memory, the computer's 4.77 MHz 8088 required between five seconds and 1.5 minutes to complete the POST and there was no way to skip it. Beginning with the IBM XT, a memory count was displayed during POST instead of a blank screen.^[1] A modern PC with a bus rate of around 1 GHz and a 32-bit bus might be 2000x or even 5000x faster, but it might have more than 3 GB of memory—5000x more. With people being more concerned with boot times now than in the 1980s, the 30 to 60 second memory test adds undesirable delay for a benefit of confidence that is not perceived to be worth that cost by most users. Most clone PC BIOSes allowed the user to skip the POST RAM check by pressing a key, and more modern machines often performed no RAM test at all unless it was enabled via the BIOS setup. In addition, modern DRAM is significantly more reliable than DRAM was in the 1980s.

As part of the starting sequence the POST routines may display a prompt to the user for a key press to access built-in setup functions of the BIOS. This allows the user to set various options particular to the mother board before the operating system is loaded. If no key is pressed, the POST will proceed on to the boot sequence required to load the installed operating system.