As soon as you turn on a computer, its processor begins to carry out the programming instructions contained in the BIOS or EFI. The BIOS and EFI, which are types of firmware, contain the processor-dependent code that starts the computer regardless of the operating system installed. The first set of startup instructions is the POST, which is responsible for the following system and diagnostic functions:

• Performs initial hardware checks, such as determining the amount of memory present
• Verifies that the devices needed to start an operating system, such as a hard disk, are present
• Retrieves system configuration settings from nonvolatile memory, which is located on the motherboard

The contents of the nonvolatile memory remain even after you shut down the computer. Examples of hardware settings stored in the nonvolatile memory include device boot order and Plug and Play (PnP) information.

After the motherboard POST completes, add-on adapters that have their own firmware (for example, video and hard drive controllers) carry out internal diagnostic tests.

If startup fails before or during POST, your computer is experiencing a hardware failure. Generally, the BIOS or EFI displays an error message that indicates the nature of the problem. If video is not functioning correctly, the BIOS or EFI usually indicates the nature of the failure with a series of beeps.