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Power-on self-test (POST) is the common term for a computer, router or printer’s pre-boot sequence. The same basic sequence is present on all computer architectures. It is the first step of the more general process called initial program load (IPL), booting, or bootstrapping. The term POST has become popular in association with and as a result of the proliferation of the PC. It can be used as a noun when referring to the code that controls the pre-boot phase or when referring to the phase itself. It can also be used as a verb when referring to the code or the system as it progresses through the pre-boot phase. Alternatively, this may be called “POSTing.”

For embedded systems power-on self-test (POST) refers to the testing sequence that occurs when a system is first powered on. POST is software written to initialize and configure a processor and then execute a defined series of tests to determine if the computer hardware is working properly. Any errors found during the self-test are stored or reported through auditory or visual means, for example through a series of beeps, flashing LEDs or text displayed on a display. Once the POST sequence completes, execution is handed over to the normal boot sequence which typically runs a boot loader or operating system. POST for embedded systems has been around since the earliest days of computer systems.

On power up, 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 the integrity of the BIOS code itself
• find, size, and verify system main memory
• discover, initialize, and catalog all system buses and devices
• pass control to other specialized BIOSes (if and when required)
• provide a user interface for system’s configuration
• identify, organize, and select which devices are available for booting
• construct whatever system environment that is required by the target OS

The BIOS will begin its POST duties 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 northbridgewill 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.

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 savings 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. However, the average user still knows the POST and BIOS only through its simple visible memory tests and setup screen.