Tuesday, 19 February 2013

Power On Self Test POST

Power On Self Test (POST) is a collection of diagnostic programs loaded from the BIOS ROM during the system power on time. The function of these programs is testing various functional units in the system, verifying whether all the major system components present, and are working properly or not. The idea behind it is gaining confidence with the hardware before using it. Whenever the system is power on or reset, the POST programs are automatically executed by the system.  In-order to successful beginning of POST certain minimum hardware should be working properly. This minimum hardware is known as Initial Program Load (IPL) hardware. After power on or reset, the IPL hardware transfers control to the POST. The POST after completion of all tests transfers control to the bootstrap program.

Some of the main duties of the main BIOS during POST are as follows:
·        Verify the BIOS code itself
·        Initialize BIOS
·        Check CPU registers

·        Select the boot device
·        Find and calculate the size of the main memory

The IPL hardware consists of the power supply, clock logic, bus controller, microprocessor, data bus and address bus transceivers, BIOS ROM and ROM address decode logic, etc. If one or more of these hardware components fail, then any one of the following events will happen:
1.     The POST does not start at all
2.     The POST does not begin from the correct starting point
3.     The POST starts but loss control after some time

Any error or abnormal behavior encountered by the POST is indicated to the user in many different ways. Faults are indicated as direct messages or as speaker tones or by displaying error codes. Unless the hardware cooperates, the POST will remain handicapped. This has been given due consideration by the POST designers. There are two critical aspects to be considered here: POST itself resides in the hardware and the POST has to be executed by the hardware.  

POST produces beep codes whenever certain hardware failure at checking time, like long continues beep code implies memory failure, one short beep implies normal start, one long; one short beep indicates system board problem.
POST some times produce error codes to inform a failure. 

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