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When a computer boots up, it starts by executing hard-wired instructions stored in the BIOS. These instructions do a check of the various essential machine functions, and then turn control of the system over to the boot loader routine stored on the hard drive. The series of instructions the BIOS does are called the Power-On Self Test, abbreviated POST.
At each step of the POST, the BIOS puts a code out on the bus. The original purpose of these codes was to allow the programmers to debug the BIOS program, but the codes are still present in the completed computer.
A POST-code reader monitors these codes and displays them on a 2-digit readout that is called a hex display because it changes an electronic hexadecimal signal into a visual decimal number. Normally the codes change so fast they are just a blur, but if the BIOS fails to get past a particular point in the POST, the boot process will stop and the reader will display the code for the last step that was being attempted. That code will tell us what step the BIOS was unable to do.
If all BIOS manufacturers had used the same codes, it might be possible to memorize them. Unfortunately, not only do the codes differ from one maker to the next, but even from one version to the next for the same manufacturer. Therefore it is necessary to refer to a table listing the codes and their meaning for each BIOS being used.
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