Power-on self-test

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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."

General internal workings

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 northbridge 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 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.

Fundamental structure

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 runtime code, the runtime BIOS, or the runtime 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 runtime code remains resident in memory. This division may be a misleading oversimplification, however, as many Runtime functions are executed while the system is POSTing.


Error reporting
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.

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, system board problem, disconnected cpu, or disconnected speaker,
Continuous beep - Power supply, system board, or keyboard problem
Repeating short beeps - Power supply or system board problem or keyboard
Repeating long beeps - memory etc is unseated
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 KiB 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

Reference: AMIBIOS8 Check Point and Beep Code List, version 1.9, last updated 11 October 2007

POST beep codes on CompTIA A+ Hardware Core exam

These POST beep codes are covered specifically on the CompTIA A+ Core Hardware Exam:Beeps    Meaning
Steady, short beeps    Power supply may be bad
Long continuous beep tone    Memory failure
Steady, long beeps    Power supply bad
No beep    Power supply bad, system not plugged in, or power not turned on
No beep    If everything seems to be functioning correctly there may be a problem with the 'beeper' itself.
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

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 startup 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:[1]
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

New World Macs (1999 onward) and Intel-based Macs

The beep codes were revised in October 1999,[2] and have been the same since. In addition, on some models, the power LED would flash in cadence.
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


External links
AMIBIOS8 Check Point and Beep Code List, Version 1.71
AwardBios Version 4.51PG - POST Codes and Error Messages
PhoenixBIOS 4.0 - Revision 6.0 POST Tasks and Beep Codes
Beep Code list & meaning tutorial
Bios Central - AMI/AMIT BIOS Post Codes
Power On Self Test Beep Codes for AMI and Phoenix BIOS - from PC Hell.
Computer Hardware - Additional information on computer POST / Beep Codes - from.
Phoenix ISA/MCA/EISA BIOS Beep Codes