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