The Booting Process

Intoduction

When a computer  starts up ( obviously by pressing the power button), the first thing that occurs is it  send a signal to motherboard which in turn starts the power supply. After supplying the correct amount of power to each device, it send a signal called "Power OK" to BIOS which resides on motherboard.

Once the BIOS receive the "Power OK" signal, it starts the booting process by first initializing a process called POST (Power On Self Test) . POST first check that every device has right amount of power and then it check whether the memory is not corrupted. Then it initialize each devices and finally it gives control to BIOS for further booting.

Now the final process of booting begins. For this the BIOS first find 512 bytes of image called MBR (Master Boot Record)  or Bootsector  from the floppy disk or hard disk  which is used for booting. The priority of boot devices is set by the user in BIOS setting. The normal priority is floppy disk first, then hard disk.

Once BIOS finds the bootsector it loads the image in memory and execute it. If a valid bootsector is not found, BIOS check for next drive in boot sequence until it find valid bootsector. If BIOS fails to get valid bootsector, generally it stops the execution and gives an error message "Disk boot failure".

It is bootsectors responsibility to load the operating system in memory and execute it.