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(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. 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 the case of a hard reboot, the northbridge will direct this code fetch (request) to the BIOS located on the system flash memory. The first memory location the CPU tries to execute is known as the reset vector.

The BIOS begins its POST when the CPU is reset. (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.)

construct whatever system environment is required by the target operating system.

provide a user interface for system's configuration.

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

The functions above are served by the POST in all BIOS versions back to the very first.

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

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

initialize, size, and verify system main memory.

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

verify the integrity of the BIOS code itself.

The principal duties of the main BIOS during POST are as follows: These other duty-specific programs are generally known collectively as option ROMs or individually as the video BIOS, SCSI BIOS, etc. In IBM PC compatible 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. See also: Booting § Boot sequence of IBM PC compatibles

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. In addition to running tests, the POST process may also set the initial state of the device from firmware. 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. 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. 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. Summary screen after POST and before booting an operating system (AMI BIOS)Ī 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.