Power-on Self Test (POST)

Techopedia explains Power-on Self Test (POST)

The initial tests are executed by the ROM BIOS startup program. The ROM BIOS tests include reading configuration information stored in the complementary metal-oxide-semiconductor (CMOS) chip, dual inline package (DIP) switches and jumpers. This information is then compared to hardware devices such as the CPU, hard drive, disc drives and video card. ROM BIOS then assigns system resources as needed. These set up the environment needed for the OS. Once these tests are completed, POST generally alerts the OS with one or more beeps depending on the system.

The BIOS issues an error message if hardware is not working properly or if it is not identified. The error message consists of either text on the display screen or a series of coded beep sounds. Since POST is initiated prior to activating the video card, a display screen message is not typical. There are a variety of beep codes, which are properly descriptive for troubleshooting the error. A beep code can indicate a parity error, base memory read/write error, memory refresh timer error, display memory error, motherboard timer not functioning, cache memory failed or numerous other errors.

Sometimes an error can stop the boot process until the error is corrected. The device that has an error is not allowed to run. This is to ensure safety. An error message can be basic; for example, error 161 means the system board has a dead battery. Sometimes a POST error can be drastic, such as the motherboard not detecting a RAM component.

POST is part of a devices pre-boot sequence. When POST is successfully finalized, bootstrapping is enabled. Bootstrapping starts the initialization of the OS. Examples of a bootstrapping program are the Linux Loader, Windows NT Loader and Linux’s Grand Unified Bootloader.