POWER ON SELF TEST (POST)

Definition: A POST error message is an error message displayed on the monitor during the Power On Self Test if the BIOS encounters some kind of problem while starting the PC.

A POST error message will only display on screen if the computer is capable of booting this far.

If the POST detects an error before this point, a beep code or POST code will be generated instead.

POST error messages are usually fairly descriptive and should give you enough information to begin troubleshooting whatever problem the POST found.

POST errors can be display in the following three ways:◦Beep codes – heard through the speaker or

beeper attached to the motherboard. Note that some systems do not include a speaker.

◦POST checkpoint codes – Hexadecimal checkpoint codes sent to an I/O port address. A special card plugged into an available expansion slot is required to view these codes.

◦Onscreen messages – Error messages displayed onscreen after the video adapter is initialized.

BIOS POST Beep Codes

Beep codes are used for fatal errors only, which are errors that occur so early in the process that the video card and other devices are not yet functional.

Because no display is available, these codes take the form of a series of beeps that identify the faulty component.

When your computer is functioning normally, you should hear one short beep when the system starts up at the completion of the POST, although some systems (such as Compaq’s) beep twice at the end of a normal POST.

If a problem is detected, a different number of beeps sound, sometimes in a combination of short and long tones.

Some implementations of UEFI firmware also use POST codes to report errors.

BIOS POST Checkpoint Codes

POST checkpoint codes are hexadecimal numeric codes written by POST routines to I/O port address 80h as each major step is begun.

These are often simply called POST codes. These POST codes can only be read by a

special adapter card plugged into one of the system slots.

These cards originally were designed for system manufacturers to use for burn-in testing of the motherboard.

POST checkpoint codes can track the system’s progress through the boot process from power-on right up to the point at which the bootstrap loader runs (when the operating system load begins).

When you plug a POST code reader card into a slot, during the POST you see two-digit hexadecimal numbers flash on the card’s display.

If the system stops unexpectedly or hangs, you can identify the test that was in progress during the hang from the two-digit code.

This step usually helps to identify the malfunctioning component.

most POST reader cards plugged into the 8-bit connector that is a part of the ISA or Enhanced Industry Standard Architecture (EISA) bus.

Some older PCI-based systems do still have ISA connectors that can use these cards.

BIOS POST Onscreen Messages

Onscreen messages are brief messages that attempt to indicate a specific failure. These messages can be displayed only after the point at which the video adapter card and display have been initialized.

These different types of error messages are BIOS dependent and vary among BIOS manufacturers, and even in some cases among different BIOSs from the same manufacturer.

The following sections list the codes used by the most popular ROM BIOS versions (AMI, Award, Phoenix, and IBM BIOS), but you should consult your motherboard or ROM BIOS manufacturer for the codes specific to your board and BIOS.

Most POST code cards come with documentation listing the POST checkpoint codes for various BIOS versions