The computer POST (Power On Self Test) tests the computer, insuring that it meets the necessary system requirements before booting up. If the computer fails the POST, the computer then returns a beep code indicating what is causing the computer not to pass the POST. Generally, if the computer POSTs properly, you will receive a single beep (with some computer BIOS manufacturers it may beep twice).
Generally what happens in the POST (in order) is:
1. The first step of POST is the testing of 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 must be readable.
4. BIOS checksum must be valid, meaning that it must be readable.
5. CMOS be accessible for reading.
6. CMOS checksum must be valid, meaning that it must be readable.
7. CPU must be able to read all forms of memory such as the memory controller, memory bus, and memory module.
8. 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.
9. I/O bus / controller must be accessible.
10. I/O bus must be able to write / read from the video subsystem and be able to read all video RAM..
If any of the above conditions do not pass, the computer will not boot, and hopefully the number of beeps will give an indication of where the problem is. Following is a list of beep codes for the two most common on BIOS manufacturers, AMI and Award.
Note: because of the wide variety of different computer manufacturers, the beep codes may vary.
AMI POST CODES:
Beep Code Descriptions
1 short DRAM refresh failure
2 short Parity circuit failure
3 short Base 64K RAM failure
4 short System timer failure
5 short Process failure
6 short Keyboard controller Gate A20 error
7 short Virtual mode exception error
8 short Display memory Read/Write test failure
9 short ROM BIOS checksum failure
10 short CMOS shutdown Read/Write error
11 short Cache Memory error
1 long, 3 short Conventional/Extended memory failure
1 long, 8 short Display/Retrace test failed
AWARD POST CODES:
Beep Code Description
1 long, 2 short Indicates a video error has occurred and the BIOS cannot initialize the video screen to display any additional information
Any other beep(s) RAM problem.