The POST test is a two stage process, first it will check all of the basic components including amongst other things, the system clock, the processor
, the keyboard controller and the video card.
When a device fails the first part of the POST the BIOS
will send a series of beeps from the PC's speaker (internal) to inform the PC user that there is a problem. These beeps are coded allowing the user to diagnose the troublesome component.
After the video has been tested successfully the BIOS will then display any errors encountered on the screen. The error message will be either a numeric code or a text message.
When the POST has been completed successfully the PC will make a short beep to let the user know everything is as it should be.
Below are some common beep codes for AMI and Phoenix BIOS.