POST (Power-On Self Test)

POST—Power-On Self Test

The system BIOS program starts by invoking a special program (stored in a ROM chip) called POST.ROM BIOS run the POST program that automatically performs a series of tests that check the primary component in the system, such as the CPU, RAM, motherboard support circuitry, memory and major peripherals such as expansion chassis etc. POST program is executed before the loading of the operating system.

The POST has two stages.
 
§ Test 1 occurs before and during the test of the video.
§ Test 2 occurs after the video has been tested.

Beep Codes Before and During the Video Test

Beep Codes Before and During the Video Test

The purpose of POST to first test and check the most basic components of the system, then it transfers control to the boot drive, and the operating system is loaded. In a healthy system, the POST reports by using a series of beep codes and screen messages to convey that all components are working properly or not.

If the POST detects a problem, that enough to keep the system from operating properly, and halts the system boot process. It sometime called fatal errors because they prevent the system from booting. The following table lists the basic beep codes for AMI and phoenix BIOS. (The following table is an example only. Each ROM BIOS encompasses its own beep codes & their respective problems).

Detecting System Errors Using Beep Codes

Beep Codes:-

Number of Beeps	Possible Problem
1	DRAM refresh failure
2	Parity circuit failure
3	Base 64 KB or CMOS RAM failure
4	System timer
5	Processor failure
6	Keyboard controller or Gate A20 error
7	Virtual mode exception error
8	Display monitor write/read test failure
9	ROM BIOS check-sum error
10	CMOS RAM shutdown register failure
1 long, 3 short	Conventional/extended memory test failure
1 long, 8 short	Display test and display vertical and horizontal retrace test failure.

You may use this table to identify your system errors even if you have no Display and have only some beeps.