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Boot Process In Operating System

BOOT BLOCK :-
  Basically for a computer to start running to get an instance when it is powered up or rebooted it needs to have an initial program which is known as Bootstrap need to be simple. It must initialize all aspects of the system, from CPU registers to device controllers and the contents of the main memory and then starts the operating system.
  
  To do this job the bootstrap program basically finds the operating system kernel on disk and then loads the kernal into memory and after this, it jumps to the initial address to begin the operating system execution.
 
USE OF ROM :-
  For most of today's computer bootstrap os stored in ROM. Behind this there are following reasons :-
  
i > This location is good for storage because this place does not require any initialization and moreover location here it is fixed so that processor can start execution when powered up or reset.

ii > ROM is basically read only memory, hence it cannot be the computer virus. 
         The problem is that changing the bootstrap code basically requires changes in the ROM hardware chips. Because of this reason, most system nowdays has the tiny bootstrap loader program in the boot whose only job is to bring the full bootstrap program easily and the new version can be easily written onto the disk.

  Full bootstrap program is stored in the "boot block" at a fixed location on the disk.  A disk which has a boot partition is called 'boot disk". The code in the boot ROM basically instruct the read controller to read boot blocks into the memory. And then starts the execution of code. 


  The full bootstrap program is more complex than bootstrap loader in the boot ROM, it is basically able to load the complete operating system from a non-fixed location on disk to start the operating system running, even though the complete bootstrao program is very small.
 
  So we discussed above that the boot code present in ROM, is responsible for read the full bootstrap program, which is present at disk on boot blocks. The window system places its boot code in the first sector of its harddisk, which it terms master boot recoed (MBR\).

  Now, when system is terned on there is a special programs BIOS stored in ROM contains the boot code, by executing which, the CPU access the very first partition of harddisk i.e MBR. It contains partition table for all partition on harddisk.

  MBR contains the information about where the operating system is being stored and it also contain a program which can read the boot sector record of the partition. Hence  th CPU fetches all this informations and load the operating system into the main memory.

BAD BLOCKS :-
   A bad block is an area o storage media that is no longer reliable for storing and retriving data because it has been physically damaged or corrupted. Bad blocks are also refered to as bad sector. A soft, or logical, bad block occurs when the operating system is unable to read data from sector.

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