The POST (Power On Self Test)

During startup the BIOS also invokes The POST (Power On Self Test), also stored in ROM, a program that runs multiple self-diagnostic routines. When the computer does not pass the POST it will display an error code or message, or generate a beep code through the speaker when display is not functioning. Failing the POST does not always mean the computer will not be able to boot, required devices that need to pass the POST are CPU, RAM, display adapter and boot device, but if the floppy drive is missing the computer can still be able to continue the boot process. The meaning of the beep codes vary depending of the manufacturer mostly they indicate memory, CPU, display and keyboard problems. Most BIOSs beep once to check if the speaker is working or to confirm that the post has passed successfully. For the exact meaning of beep codes for AMI BIOSs click here and for Phoenix and Award click here. Although most BIOS manufacturers started using error text messages, some use numeric POST error codes. Some of them are listed below:
POST error code	Probable Cause
1xx	Mainboard related errors
200-299	Memory related errors.
3xx	Keyboard related errors
151	Real-time clock failure
161	BAD CMOS memory
6xx	Floppy Disk related errors
1700-x	Hard drive controller
31xx	Network Adapter related errors