On Self Test POST is s hardware diagnostic routine built into the BIOS that checks the hardware to make sure everything is working. The POST will continue past non-fatal errors. Usually if the POST detects an error early in the process it will use a beep code to indicate the problem.
Beep codes are all different according to motherboard manufacturers.
Almost all of the BIOS programs will sound a single beep just before the startup screen. There are three major manufacturers of BIOS, American Megatrends International, Award and Phoenix. Each has it s own beep codes. Phoenix had as many as 100 beep codes. These beeps are usually in groups.