How Boot process occurs ?
► Reset event on CPU (power up, reboot)
causes instruction register to be loaded with
a predefined memory location. It contains a
jump instruction that transfers execution to
the location of Bootstrap program.
► This program is form of ROM, since RAM is
in unknown state at system startup. ROM is
convenient as it needs no initialization and
can’t be affected by virus.
Why is Booting Required ?
► Hardware doesn’t know where the operating system resides and how to load it.
► Need a special program to do this job – Bootstrap loader. ▪ E.g. BIOS – Boot Input Output System.
► Bootstrap loader locates the kernel, loads it into main memory and starts its execution.
► In some systems, a simple bootstrap loader fetches a more complex boot program from disk, which in turn loads the kernel.
Tasks performed at boot up
► Run diagnostics to determine the state of machine. If diagnostics pass, booting continues.
► Runs a Power-On Self Test (POST) to check the devices that the computer will rely on, are functioning.
► BIOS goes through a preconfigured list of devices until it finds one that is bootable. If it finds no such device, an error is given and the boot process stops.
► Initializes CPU registers, device controllers and contents of the main memory. After this, it loads the OS.
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