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  Boot process std.
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Standard boot process

The standard boot process (without boot manager) proceeds as follows: After starting the computer the BIOS automatically tries to read the first sector of the boot device (C: or A:). When this succeeds the code in that sector is executed. In case of an read error (eg. booting from A: without diskette in drive) the BIOS tries to boot from the next boot device.

Hence when booting from C: the BIOS reads the first sector --- the Master Boot Record (MBR) --- from the first hard disk and starts the boot loader code found on this sector. This small program (size 200 - 300 bytes) searches the partition table for the active partition. The partition entry contains the position of the corresponding boot sector. When an active partition is found then the boot loader code reads the boot sector (from the first hard disk) and executes the code contained on the boot sector. This code finally boots the corresponding operating system.

Since the standard boot loader searches only in the partition table on the first hard disk, it allows booting an operating system only from the first hard disk. Also booting from a logical drive is not possible with the standard boot loader.

On the diskette there is no MBR. Rather the first sector on a diskette already contains the boot sector of an operating system in case the diskette is bootable, or the well-known little program which asks the user to remove the diskette and press any key.

General principle:
The load process outlined above is an example of a widely used general principle. In the first step only a single sector is loaded and executed. This sector contains the necessary code to load additional sectors. Only these sectors loaded in the second step contain the actual program to be executed. In this way a big program (e.g. an operating system) can be started simply be loading a single sector (e.g. a boot sector).