The first thing that the BIOS does when it boots the PC is to perform what is called the Power-On Self-Test, or POST for short. The POST is a built-in diagnostic program that checks your hardware to ensure that everything is present and functioning properly, before the BIOS begins the actual boot. It later continues with additional tests (such as the memory test that you see printed on the screen) as the boot process is proceeding.
The POST runs very quickly, and you will normally not even noticed that it is happening--unless it finds a problem (amazing how many things are like that, isn't it?) You may have encountered a PC that, when turned on, made beeping sounds and then stopped without booting up. That is the POST telling you something is wrong with the machine. The speaker is used because this test happens so early on, that the video isn't even activated yet! These beep patterns can be used to diagnose many hardware problems with your PC. The exact patterns depend on the maker of the BIOS; the most common are Award and AMI BIOSes.
Each time the computer boots up the computer must past the POST. Below is the common steps a POST performs each time your computer starts.
1-Test the power supply to ensure that it is turned on and that it releases its reset signal.
2-CPU must exit the reset status mode and thereafter be able to execute instructions.
3-BIOS checksum must be valid, meaning that it must be readable.
4-CMOS checksum must be valid, meaning that it must be readable.
5-CPU must be able to read all forms of memory such as the memory controller, memory bus, and memory module.
6-The first 64KB of memory must be operational and have the capability to be read and written to and from, and capable of containing the POST code.
7-I/O bus / controller must be accessible.
8-I/O bus must be able to write / read from the video subsystem and be able to read all video RAM.
If the computer does not pass any of the above tests, your computer will receive an irregular POST. An irregular POST is a beep code that is different from the standard one or two beeps. This could be either no beeps at all or a combination of different beeps indicating what is causing the computer not to past the POST.
