Once the computer is powered up, the first job is to check whether all the components of the computer are properly working or not. This job is done by BIOS through a series of tests known as Power On Self Test (POST in short form). POST is carried out by BIOS which resides on the computer motherboard. The primary objective of POST is to provide diagnostic information about the computer to the user.
Sequence Of POST
When the computer is switched on, BIOS is activated first. BIOS is a hard coded program,embedded on the motherboard,which boots up the computer. It also store the hardware configuration data. Every-time the computer is powered up, BIOS loads the configuration data in the main memory first and then executes POST.
Computer components checked by POST
POST checks the health of all computer components. Some of very crucial components are checked first. Generally power supply, BIOS, memory, heating and cooling,hardware tests are executed in the mentioned order. Hardware tests include keyboard,monitor,mouse,printer and scanner testing.
Detailed operation of POST
When the CPU is reset, BIOS starts POST execution. Code execution starts from reset vector. In the case of cold boot, the POST is executed from flash memory,while in the case of warm boot, POST is executed from RAM. The first job of POST is to confirm the integrity of BIOS code itself. Once the BIOS code is verified, it finds the size of system memory and runs a memory test on system memory. After that, system buses are detected,tested and then initialized. If any other system BIOS is present then control is passed to it. POST provides an user interface to system configuration. It detects,organizes and initializes all devices which are present during booting. It also constructs system environment which is required by the target OS.
Once the POST execution is over, the result is produced to the user. Basically there are two types of diagnostic information, namely- POST Codes, Beep Codes. POST codes generate a visual symbol of two characters which indicates to what extent POST is carried out. Beep codes occur if the motherboard is working well or if there are issues. In most cases only one beep is heard, if the result of POST is OK and the mother board is functioning properly. In case, POST execution throws a result showing a failure, PC support will be required to fix the issue.
The initial value of most of the POST codes is 00, FF or 88 by default. After the POST execution is over and error code,if any, is displayed, the value of POST codes will return back to 00,FF or 88. Once the BIOS releases control of the motherboard to the operating system there are no more POST codes.
After the diagnostic tests are executed, a series of beeps is generated by the BIOS from from the loudspeaker attached to motherboard to indicate error codes. Beep codes differ from manufacturer to manufacturer. Generally 1 short beep indicates that the computer system is OK while 2 short beeps signal some system problems. In case, if any problem occurs, it should be resolved directly or through remote support.