When a computer is first turned on or reset it performs a diagnostic test called a POST (Power-On Self Test) that checks all of the on-board and attached components in the computer. The POST program sends out a signal to each device initializing each device's built-in self test procedure.

The POST test is a two stage process, first it will check all of the basic components such as the system clock, the processor, RAM, the keyboard controller, video card, floppy and hard drive controllers.

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 computer user that there is a problem, these beeps are coded allowing the user to diagnose the troublesome component.

After the video has been tested the BIOS will then indicate any errors encountered by displaying either a numeric code or a text message on the screen.

When the POST has been completed successfully the computer will make a short beep to let the user know everything is as it should be.