Power-on self-test

When power is turned on, POST (Power-On Self-Test) is the diagnostic testing sequence that a computer's basic input/output system (or "starting program") runs to determine if the computer keyboard,
random access memory, disk drives, and other hardware are working correctly.
If the necessary hardware is detected and found to be operating properly, the computer begins to boot. If the hardware is not detected or is found not to be operating properly, the BIOS issues an error message which may be text on the display screen and/or a series of coded beeps, depending on the nature of the problem.The pattern of beeps may be a variable numbers of short beeps or a mixture of long and short beeps, depending on what type of BIOS is installed.
The patterns of beeps contain messages about the nature of the problem detected. For example, if the keyboard is not detected, a particular pattern of beeps will inform you of that fact. An error found in the POST is usually fatal (that is, it causes current program to stop running) and will halt the boot process, since the hardware checked is absolutely essential for the computer's functions.

General internal workings

• verify the integrity of the BIOS code itself

• determine the reason POST is being executed

• find, size, and verify system main memory

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

• pass control to other specialized BIOS-es (if and when required)

• provide a user interface for systems configuration

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

• construct whatever system environment that is required by the target OS

Fundamental structure/Error reporting/Original IBM POST error codes

• 1 short beep - Normal POST - system is OK

• 2 short beeps - POST error - error code shown on screen

• No beep - Power supply or system board problem

• Continuous beep - Power supply, system board, or keyboard problem

• Repeating short beeps - Power supply or system board problem or keyboard

• 1 long, 1 short beep - System board problem

• 1 long, 2 short beeps - Display adapter problem (MDA, CGA)

• 1 long, 3 short beeps - Enhanced Graphics Adapter (EGA)

• 3 long beeps - 3270 keyboard card

POST AMI BIOS beep codes

• 1 - Memory refresh timer error

• 2 - Parity error in base memory (first 64 KB block)

• 3 - Base memory read/write test error

• 4 - Mother board timer not operational

• 5 - Processor error

• 6 - 8042 Gate A20 test error (cannot switch to protected mode)

• 7 - General exception error (processor exception interrupt error)

• 8 - Display memory error (system video adapter)

• 9 - AMI BIOS ROM checksum error

• 10 - CMOS shutdown register read/write error

• 11 - Cache memory test failed

POST beep codes on CompTIA A+ Hardware Core exam Beeps Meaning

• Steady,short beeps Power supply may be bad

• Long continuous beep tone Power supply bad or not plugged into correctly

• Steady, long beeps Power supply bad

• No beep Power supply bad, system not plugged in, or power not turned on

• One long, two short beeps Video card failure

IBM POST diagnostic code descriptions

• 100 to 199 - System boards

• 200 to 299 - Memory

• 300 to 399 - Keyboard

• 400 to 499 - Monochrome display

• 500 to 599 - Color/graphics display

• 600 to 699 - Floppy-disk drive or adapter

• 700 to 799 - Math coprocessor

• 900 to 999 - Parallel printer port

• 1000 to 1099 - Alternate printer adapter

• 1100 to 1299 - Asynchronous communication device, adapter, or port

• 1300 to 1399 - Game port

• 1400 to 1499 - Color/graphics printer

• 1500 to 1599 - Synchronous communication device, adapter, or port

• 1700 to 1799 - Hard drive and/or adapter

• 1800 to 1899 - Expansion unit (XT)

• 2000 to 2199 - Bisynchronous communication adapter

• 2400 to 2599 - EGA system-board video (MCA)

• 3000 to 3199 - LAN adapter

• 4800 to 4999 - Internal modem

• 7000 to 7099 - Phoenix BIOS chips

• 7300 to 7399 - 3.5-inch disk drive

• 8900 to 8999 - MIDI adapter

• 11200 to 11299 - SCSI adapter

• 21000 to 21099 - SCSI fixed disk and controller

•  21500 to 21599 - SCSI CD-ROM system

Macintosh POST

• 1 beep = No RAM installed/detected

• 2 beeps = Incompatible RAM type installed (for example, EDO)

• 3 beeps = No RAM banks passed memory testing

• 4 beeps = Bad checksum for the remainder of the boot ROM

• 5 beeps = Bad checksum for the ROM boot block

• 1 beep = no RAM installed

• 2 beeps = incompatible RAM types

• 3 beeps = no good banks

• 4 beeps = no good boot images in the boot ROM (and/or bad sys config block)

• 5 beeps = processor is not usable

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:

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 north-bridge 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 north-bridge 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 reloaded 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.
In the case of the IBM PC compatible machines, the main BIOS is divided into two basic sections. The POST section, or POST code, is responsible for the tasks mentioned above, and the environment POST constructs for the OS is known as the run-time code, the run-time BIOS, or the run-time footprint. Primarily these two divisions can be distinguished in that POST code should be flushed from memory before control is passed to the target OS while the run-time code remains resident in memory. This division may be a misleading oversimplification, however, as many Run-time functions are executed while the system is Posting.
The original IBM BIOS reported errors detected during POST by outputting a number to a fixed I/O port address, 80. Using a logic analyzer or a dedicated POST card, an interface card that shows port 80 output on a small display, a technician could determine the origin of the problem. (Note that once an operating system is running on the computer, the code displayed by such a board is often meaningless, since some OSes, e.g. Linux, use port 80 for I/O timing operations.) In later years, BIOS vendors used a sequence of beeps from the motherboard-attached loudspeaker to signal error codes.
These POST beep codes are covered specifically on the CompTIA A+ Core Hardware Exam:

Apple's Macintosh computers also perform a POST after a cold boot. In the event of a fatal error, the Mac will not make its start-up chime.
Old World Macs (until 1998)
Macs made prior to 1998, upon failing the POST, will immediately halt with a "death chime," which is a sound that varies by model; it can be a beep, a car crash sound, the sound of shattering glass, a short musical tone, or more. On the screen will be the Sad Mac icon, along with two hexadecimal strings, which can be used to identify the problem.
New World Macs (1998-1999)
When Apple introduced the iMac in 1998, it was a radical departure from other Macs of the time. The iMac began the production of New World Macs, as they are called; New World Macs, such as the iMac, Power Macintosh G3 (Blue & White), Power Mac G4 (PCI Graphics), PowerBook G3 (bronze keyboard), and PowerBook G3 (FireWire), load the Mac OS ROM from the hard drive. In the event of a fatal error, they give these beeps:
New World Macs (1999 onward) and Intel-based Macs
The beep codes were revised in October 1999, and have been the same since. In addition, on some models, the power LED would flash in cadence.