What is a BIOS?
"BIOS" stands for Basic Input Output Software. This software resides on the PC motherboard either in ROM or Flash memory and gets control of the computer even before the operating system loads.
What are POST Codes?
"POST" stands for Power On Self Test. When the BIOS starts to run, it does a Self Test of the components on the motherboard, and in some cases, boards that are plugged into the various slots. It goes through several steps of checking, testing and initializing the hardware components. Before entering each step, the BIOS writes a 2 digit identifying code to an external address. This code is commonly referred to as a POST code. The meaning of the codes varies widely. Most computer manufacturers use a BIOS supplied by a third party, the most common are Phoenix and AMI, but there are many others. The codes also vary depending on the motherboard and the manufacturer's requirements. These codes are useful during the manufacturing process to help identify problems. To the consumer, they are useful in cases where the system won't boot up and the video screen does not work, or if you are attempting to make modifications. If the BIOS detects a problem, it will stop on the problem, and the last code that was output will indicate what the failure is.
In order to see the codes being generated by your BIOS, you will need a Post Code Master Display card, which plugs into your PC and displays these codes as the BIOS performs its tests. There are versions available for both ISA and PCI motherboard slots. The card displays HEX digits which consist of the numbers 0-9 and the letters A-F. The letters B and D are displayed as lower case (b & d).