The Booting Process

It is also called “booting up”, is a process that starts operating systems when the user turns on a PC. A boot sequence is the initial set of operations that the computer performs when power on.

The boot loader loads the operating system for the computer.

There are two types of boot or reboots

Hard boot, or cold boot or cold start, involves turning on the power with the on/off switch.

Soft boot, or warm boot, involves using the operating system to reboot.

Chassis has two power buttons for shutdown and reboot as soft power button does a normal operating system shutdown and hard power button abruptly reboots.

Steps of Boot Process

  • Upon starting, CPU runs the instruction at memory address 0xFFFF0 about execution of the BIOS start-up program.
  • BIOS program runs a power-on self test (POST) to check and initialize required devices.
  • The BIOS goes through a pre-configured list of non-volatile storage devices (or “boot device sequence”) to find bootable device (Hard disk, CD/DVD or USB or network).

A bootable device is defined as one that can be read from, and the last two bytes of the first sector contain the word 0xAA55 (also known as the boot signature).

  • Once the BIOS have found a bootable device it loads the boot sector to and transfers execution to the boot code.
  • In the case of a hard disk, this is referred to as the master boot record (MBR) which is not operating system specific.
  • The MBR code checks for partition to be bootable and active after which it loads the boot sector code from that partition and executes it.
  • The boot sector is often operating-system-specific; as it loads and executes the operating system kernel, which continues startup.
  • If there is no active partition, or the active partition’s boot sector is invalid, the system displays “No boot device” message.

Most PCs, if a BIOS chip is present, will show a screen detailing the BIOS chip manufacturer then the amount of computer memory available and other pieces of information about the computer are shown.

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