The Role of POST in the Boot Sequence

The boot sequence is the process of starting a computer/system. The boot process is initiated when the power button is pressed, it sends power to the boot-loader in the cache memory. The Boot loader performs POST as a preboot sequence and if everything is working well without any errors the BIOS(Basic Input Output System) is activated which finds and loads the operating system.

Finally the software has to interact with the hardware units to complete the process. To avoid any hardware errors while executing a software program, the pre-boot sequence would test the hardware and initiate the OS if and only if the basic hardware units are functioning as expected. he principal duties of the main BIOS during POST are as follows:

    Find, size, and verify the system main memory.
    Initialize BIOS.
    Identify, organize, and select which devices are available for booting.
    Verify CPU registers.
    Verify the integrity of the BIOS code itself.
    Verify some basic components like DMA, timer, interrupt controller.
    Pass control to other specialized extensions BIOS (if installed).

The checks are performed majorly on:

    Hardware elements like processor, storage devices and memory.
    Basic System Devices like keyboard, and other peripheral devices.
    CPU Registers
    DMA (Direct Memory Access)
    Timer
    Interrupt controller

Types of Errors encountered during POST

If there are any errors in the POST the system would fail to proceed towards the next steps(boot process) as it would throw a fatal error. The POST sequence is executed irrespective of the Operating System. However, how the POST is handled and the types of errors thrown can be modified by the Vendor as per the system requirements.


The error message could consist of text messages on the console or audio in the form of beeps. Irrespective of the vendor there would be a manual describing the types of errors and the error message mappings to help us troubleshoot. The error messages can vary from a parity error to a faulty motherboard.