Power-On Self-Test (POST) refers to routines run immediately after power is applied, by nearly all electronic devices. Perhaps the most widely-known usage pertains to computing devices (personal computers, PDAs, networking devices such as routers, switches, intrusion detection systems and other monitoring devices). Other devices include kitchen appliances, avionics, medical equipment, laboratory test equipment—all embedded devices. The routines are part of a device's pre-boot sequence. Once POST completes successfully, bootstrapping code is invoked.
POST includes routines to set an initial value for internal and output signals and to execute internal tests, as determined by the device manufacturer. These initial conditions are also referred to as the device's state. They may be stored in firmware or included as hardware, either as part of the design itself, or they may be part of semiconductor substrate either by virtue of being part of a device mask, or after being burned into a device such as a Programmable Logic Array (PLA).
Test results may either be displayed on a panel that is part of the device, or output via bus to an external device. They may also be stored internally, or may exist only until the next power-down. In some cases, such as in aircraft and automobiles, only the fact that a failure occurred may be displayed (either visibly or to an on-board computer) but may also upload detail about the failure(s) when a diagnostic tool is connected.
POST protects the bootstrapped code from being interrupted by faulty hardware. Diagnostic information provided by a device, for example when connected to an engine analyzer, depends on the proper function of the device's internal components. In these cases, if the device is not capable of providing accurate information—which ensures that the device is safe to run—subsequent code (such as bootstrapping code) may not be permitted to run.
Contents |
In IBM PC compatible computers, 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 begins its POST 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 saving 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, although at most only simple memory tests and the setup screen are displayed.
The original IBM BIOS made POST diagnostic information available by outputting a number to I/O port 80 (a screen display was not possible with some failure modes). Both progress indication and error codes were generated; in the case of a failure which did not generate a code, the code of the last successful operation was available to aid in diagnosing the problem. 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. Once an operating system is running on the computer the code displayed by such a board may become meaningless, since some OSes, e.g. Linux, use port 80 for I/O timing operations. The actual numeric codes for the possible stages and error conditions differ from one BIOS supplier to another. Codes for different BIOS versions from a single supplier may also vary, although many codes remain unchanged in different versions.
Later BIOSes used a sequence of beeps from the motherboard-attached loudspeaker (if present and working) to signal error codes. Some vendors developed proprietary variants or enhancements, such as MSI's D-Bracket. POST beep codes vary from manufacturer to manufacturer.
Information on numeric and beep codes is available from manufacturers of BIOSes and motherboards. There are websites which collect codes for many BIOSes[1].
| Beeps | Meaning |
|---|---|
| 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 |
| 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 |
| Beeps | Meaning |
|---|---|
| 1 | Memory refresh timer error |
| 2 | Parity error in base memory (first 64 KiB block) |
| 3 | Base memory read/write test error |
| 4 | Motherboard timer not operational (check all PSU to MB connectors seated) |
| 5 | Processor failure |
| 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 |
| 12 | Motherboard does not detect a RAM module (continuous beeping) |
Reference: AMIBIOS8 Check Point and Beep Code List, version 2.0, last updated 10 June 2008
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. The system will normally beep one short beep. |
| One long, two short beeps | Video card failure |
| Code | Meaning |
|---|---|
| 100–199 | System boards |
| 200–299 | Memory |
| 300–399 | Keyboard |
| 400–499 | Monochrome display |
| 500–599 | Color/graphics display |
| 600–699 | Floppy-disk drive or adapter |
| 700–799 | Math coprocessor |
| 900–999 | Parallel printer port |
| 1000–1099 | Alternate printer adapter |
| 1100–1299 | Asynchronous communication device, adapter, or port |
| 1300–1399 | Game port |
| 1400–1499 | Color/graphics printer |
| 1500–1599 | Synchronous communication device, adapter, or port |
| 1700–1799 | Hard drive and/or adapter |
| 1800–1899 | Expansion unit (XT) |
| 2000–2199 | Bisynchronous communication adapter |
| 2400–2599 | EGA system-board video (MCA) |
| 3000–3199 | LAN adapter |
| 4800–4999 | Internal modem |
| 7000–7099 | Phoenix BIOS chips |
| 7300–7399 | 3.5-inch disk drive |
| 8900–8999 | MIDI adapter |
| 11200–11299 | SCSI adapter |
| 21000–21099 | SCSI fixed disk and controller |
| 21500–21599 | SCSI CD-ROM system |
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.
Macs made after 1987 but 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, if working, will be the Sad Mac icon, along with two hexadecimal strings, which can be used to identify the problem. Macs made prior to 1987 crashed silently with the hexadecimal string and a Sad Mac icon.
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:[2]
| Beeps | Meaning |
|---|---|
| 1 | No RAM installed/detected |
| 2 | Incompatible RAM type installed (for example, EDO) |
| 3 | No RAM banks passed memory testing |
| 4 | Bad checksum for the remainder of the boot ROM |
| 5 | Bad checksum for the ROM boot block |
The beep codes were revised in October 1999,[3] and have been the same since. In addition, on some models, the power LED would flash in cadence.
| Beeps | Meaning |
|---|---|
| 1 | No RAM installed |
| 2 | Incompatible RAM types |
| 3 | No good banks |
| 4 | No good boot images in the boot ROM (and/or bad sys config block) |
| 5 | Processor is not usable |
Amiga historical line of computers, from A1000 to 4000 present an interesting POST sequence that prompts the user with a sequence of flashing screens of different colors (rather than audible beeps as in other systems) to show if various hardware POST tests were correct or else if they failed:
The Amiga system performs the following tests at boot:
If the screen remains a light gray colors and the tests continue, the hardware is OK. If an error occurs, the system halts.
If the system fails the ROM test, the screen display turns red and the system halts.
Almost all Amiga models present the same color sequence when turned on:
Black screen, dark gray, light gray color screens filling all monitor screen in a rapid sequence (Amigas taken up usually 2, or at least max 3 seconds to turn on and boot).
| Color | Meaning | |
|---|---|---|
| Red | Bad ROM | |
| Yellow | CPU Exception Before Bootstrap Code is Loaded | |
| Green | Bad Chip RAM or fail of Agnus Chip (check seating of Agnus) | |
| Black | No CPU | |
| White | Expansion passed test successfully | |
| Grey | Turn on | |
| Constant white | Failure of CPU | |
A4000 presents just a light gray screen during its boot time (it just occurs in 2 or max 3 seconds)
| Color | Meaning | |
|---|---|---|
| Red | ROM Error - Reset or replace | |
| Green | CHIP RAM error (reset AGNUS and re-test) | |
| Blue | Custom Chip(s) Error | |
| Yellow | 68000 detected error before software trapped it (GURU) | |
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:
| Blinks | Meaning |
|---|---|
| 1 | ROM checksum failure |
| 2 | RAM test failed |
| 3 | Watchdog timer failed |
| 4 | A shortcut exists between two row lines or one of the seven special keys (not implemented) |