Computerhope.com has very detailed guide on beep codes ans POST.

Well all know how important the beep codes can be for any computer expert

to zero in on any hardware problem when facing issues starting the PC.

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POST ABCs

The computer POST (Power On Self Test) tests the computer, insuring that it meets the necessary system requirements and that all hardware is working properly before starting the remainder of the boot process. If the computer passes the POST the computer will have a single beep (with some computer BIOS manufacturers it may beep twice) as the computer starts and the computer will continue to start normally. However, if the computer fails the POST, the computer will either not beep at all or will generate a beep code, which tells the user the source of the problem.

The steps of a POST

Each time the computer boots up the computer must past the POST. Below is the common steps a POST performs each time your computer starts.

  1. Test 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 checksum must be valid, meaning that it must be readable.
  4. CMOS checksum must be valid, meaning that it must be readable.
  5. CPU must be able to read all forms of memory such as the memory controller, memory bus, and memory module.
  6. 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.
  7. I/O bus / controller must be accessible.
  8. I/O bus must be able to write / read from the video subsystem and be able to read all video RAM.

If the computer does not pass any of the above tests, your computer will receive an irregular POST. An irregular POST is a beep code that is different from the standard one or two beeps. This could be either no beeps at all or a combination of different beeps indicating what is causing the computer not to past the POST.

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Read Complete Article Here: ComputerHope

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