Power-on self-test
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.
[1] POST processes may set the initial state of the device from firmware and detect if any hardware components are non-functional.
This design by IBM was modeled after their larger mainframe systems, which would perform a complete hardware test as part of their cold-start process.
With boot times more of a concern 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.
During the POST, the BIOS must integrate multiple competing, changing, and even mutually exclusive standards and initiatives for the matrix of hardware and operating systems the PC is expected to support, although at most only simple memory tests and the setup screen are displayed.
In the case of a hard reboot, the northbridge will direct a code fetch request to the BIOS located on the system flash memory.
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.
If no key is pressed, the POST will proceed on to the boot sequence required to load the installed operating system.
Many modern BIOS and UEFI implementations show a manufacturers logo during POST and hide the classic text screens unless an error occurs.
The original IBM BIOS made POST diagnostic information available by outputting a number to I/O port 0x80 (a screen display was not possible with some failure modes).
Using a logic analyzer or a dedicated POST card—an interface card that shows port 0x80 output on a small display—a technician could determine the origin of the problem.
Later BIOSes used a sequence of beeps from the motherboard-attached PC speaker (if present and working) to signal error codes.
[9] A4000 presents just a light gray screen during its boot time (it just occurs in 2 or max 3 seconds) The keyboards of historical Amiga models are not proprietary as it happened in early computer ages, but more pragmatically it was based on international standard ANSI/ISO 8859-1.
The keyboard itself was an intelligent device and had its own processor and 4 kilobytes of RAM for keeping a buffer of the sequence of keys that were being pressed, thus can communicate with the user if a fault is found by flashing its main LED in sequence: Many embedded systems such as those in major appliances, avionics, communications, or medical equipment have built-in self-test routines that are automatically invoked at power-on.
