Sunday, September 11, 2011


Today let's discuss about booting and what actually happens when a system boots.

During the early days of computers, the boot program is made read only and is put in hardware chips. This boot program is called a bootstrap in computer terminology. The bootstrap initiates all the CPU registers, device controllers and finally loads the operating system into memory. The advantage of this scheme is that, since the bootstrap is read only, no virus can effect it. But the disadvantages outweigh this. First, if we want to add any external hardware,  it's device driver needs to be loaded when the system starts. But, since the bootstrap is read only, we need to change the hardware chips. Also, the operating system must be in a fixed location as specified in the boot strap, if we install the operating system in a different area, then also there is a need to change the chip.

So our computer scientists thought over this and came up with a simple solution. Instead of making ROM's with complete bootstrap program in them, they started making ROM's with tiny bootstrap program's called bootstrap loader. And the main bootstrap program is stored in the disk. Since changing the bootstrap program simply requires a write to the disk, our problem is solved. The bootstrap program on the disk is generally stored in first sector of the hard disk called boot sector.

But... a new problem arrived -- Boot sector virus

Enough of computer terminology, now I will explain in simple terms. BIOS is generally present in non-volatile ROM chips as you know, and this generally initiates the main memory, CPU registers, device controllers and starts the tiny bootstrap loader that is present in it. The BIOS is built at the time of manufacture of a PC and when you start the PC, it is the first software that normally executes, even before the main memory or disk gets initialised. It looks as below

The full bootstrap program as I told earlier, is stored in first sector of harddisk called boot sector. After initialisation of all aspects, the BIOS loads this program to main memory and hands the baton to it for execution. This boot sector in windows computers is called Master Boot Record or simply MBR. MBR contains the full boot code, along with a partition table. This partition table contains the information of partitions along with information whether an Operating system is present in each one or not. If only one OS is present, it is shown or by default it's loaded since there is no need for user to make choice. If more than one OS is present, the user is presented with options to load what ever operating system he wants to be loaded.

Last, about boot sector virus. To prevent boot sector virus, there is an option in BIOS, which makes the bootstrap program on the disk read-only. When ever you think you are not going to install any more OS, check it on and when you want to change your OS, check it off. Happy now...virus problem also solved


  1. Hi, nice description about Booting in System.Thanks for your help..


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