Booting Process

booting is the process of powering on a computer and getting into the operating system. During the boot process, the computer will perform a self-diagnostic, also known as a POST and load necessary drivers and programs that help the computer and devices communicate. As your computer is booting, you may see a screen similar to the example picture below.


Computer Boot screen

Booting Process

  • The heart of a modern computer is one or more Central Processing Units.
  • A CPU gets its instructions from memory.
  • The CPU reads instruction from the BIOS and searches for the hard disks, CD drives and other hardware.
  • The BIOS program looks at the first sector for boot code.
  • Devices that feed the initial CPU instructions are known as bootstrap media

  • The Booting is a process involving 2 stages:
    • getting hardware up and running, and
    • getting the OS and other software up and running


 Booting a Computer

  1. Power up; computer runs POST
  2. Boot sequence governed by BIOS ROM
    • BIOS parameters stored in CMOS
    • BIOS ROM may be password protected
  3. Control passes to the MBR of the first bootable device detected
  4. MBR points to boot record of selected operating system
  5. Operating system takes control


  • What is a BIOS?
    • Basic input/output system
    • Built into the PC:
      • BIOS software stored permanently(*) on a ROM chip on the motherboard
    • The first code run when a PC is powered on
    • Identify system devices

    (*) In modern computers BIOS chip can be rewritten, allowing BIOS software to be upgraded.



  • What is a POST?
    • power-on self-test — one of the first processes that a computer undergoes when booting
    • POST tests the computer to ensure that it is working as it is supposed to.
    • POST can detect some errors with the processor, motherboard, RAM and other memory, as well as the video card.
    • Most BIOS chips use a system of beep codes to indicate the POST status to the user and each BIOS chipset uses a different code.
    • The IBM PC BIOS code standard, for example, uses one short beep to indicate a successful POST and two short beeps to indicate a POST error while AMI BIOS uses these same beep codes to indicate a DRAM refresh failure or parity circuit failure, respectively.



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