How Computer Starts?

Powering Up a PC

In simple and short “Booting is a process that loads & starts the operating system. The boot process is controlled by the PC’s basic input-output system (BIOS).The BIOS is software stored on a flash memory chip. When the boot loader finishes its task then a control passed to operating system. Then, the OS is ready for user”.

Sometimes a single PC contains number of different operating systems. It displays the list of OS at the starting screen and you have to choose which OS you are going to use. Once you have selected the OS, the selected OS is loaded into the memory.

List is displayed for few seconds (timing is set in BIOS).


How does a computer work?

  • When you first press the power button, it converts the alternating current (AC) into a direct current (DC) to supply the computer and its components with the proper amount of voltage and electricity.
  • Once the computer receives power, it sends a signal using transistors to the motherboard and CPU.
  • F000is the location of the first instruction that tells the CPU that it’s ready to process the instructions contained in (BIOS).

BIOS and the POST

  • When the computer first looks at the BIOS, it begins the power-on self-test (POST) to make sure the components in the computer are present and functioning properly.
  • Next it looks at the first 64-bytes of memory located in (CMOS) chip, which is kept alive by the CMOS battery even when the computer is turned off. This chip contains info of system time, date and all hardware installed in your computer.
  • Then it loads the basic device drivers which allow communication with h/w devices, and computer continues its boot process.
  • Finally, you’ll get a picture on your display after the POST has loaded the memory contained on the display adapter and has made its part of the overall system BIOS.
  • If all drives pass the test, the POST is complete, and it will instruct the computer to start the process of loading the operating system.

Booting the operating system

  • After the computer has passed the POST, the computer will start the boot process, which loads the operating system
  • The BIOS first hands control over to the bootstrap loader, which looks at the boot sector on the hard drive.
  • If your boot sequence in CMOS setup is not setup to look at the hard drive first
  • In this example, the Microsoft Windows XP NT Loader (NTLDR) is found on the boot sector and tells the computer where to find the remaining code on the hard drive.
  • Next, Windows loads the detect file, which displays the Windows splash screen and loads the Windows registry.
  • After loading the registry, Windows begins to load dozens of low-level programs that make up the operating system into memory.
  • After the registry has loaded the initial basic hardware devices, it begins to load Plug and Play devices, PCI, and ISA devices.
  • After loading all these devices, Windows then moves to loading full support of the hard drive, partitions, and any other disk drives and then moves to all other drivers that have been installed.
  • Finally, after successfully completing all steps Window


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  1. research paper April 22, 2016 Reply

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