Each time a PC initializes, the BIOS executes a series of tests collectively known as the POST. The test checks each of the primary areas of the system, including the motherboard, video system, drive system, and keyboard, and ensures that all components can be used safely. If a fault is detected, the POST reports it as an audible series of beeps or a hexadecimal code written to an I/O port.
Procedure of the POST:
- 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.
- CPU must exit the reset status mode and thereafter be able to execute instructions.
- BIOS must be have readable.
- BIOS checksum must be valid, meaning that it needs to be readable.
- CMOS be accessible for reading.
- CMOS checksum must be valid, meaning that it needs to be readable.
- CPU must be able to read all forms of memory such as the memory controller, memory bus, and memory module.
- 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.
- I/O bus / controller must be accessible.
- I/O bus must be able to write / read from the video subsystem and be able to read all video RAM.
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