BIOS POST Code Testing

BIOS POST codes are still relevant and a Virtual Platform Engineer might spend some time as engineers bring up the platform for first time. So its important to know the basics. Recently BIOS is being replaced with EFI.

When Computer boots, pre-boot code executed is called Power-On-Self_test. When a machine is powered on, its tested for basic functions with a special BIOS code called POST.  For a PC, BIOS reports messages on IO port 80. Using post-code, it can be identified what's going on in the machine.

Using a BIOS diagnostics card (e.g. PCI based card etc), Post codes can be displayed on a 2 segment LED display. Codes can be deciphered from BIOS manuals to understand what's going on in the sequence.

Here are some manuals that can be found on Phoenix/ AMI websites:

Phoenix technologies: Medallion BIOS™ Version 1.00

AMI: AMIBIOS8

APTIO: AMI

As an example, while working with my virtual machine, I reached boot sequence post code F4, which meant that it's a checkpoint, where Firmware has been loaded. Some platform teams put special codes for F5-F8 for attaching various tools to virtual platforms.