A boot sector is the part of a hard disk where the code is stored that can be used to boot the operating system or a specific standalone program. In addition, the boot sector contains other key features for the operation of the corresponding storage medium and operating system.
With the boot records one differentiates between the Master Boot Record( MBR) in partitionable hard disks, and the Volume Boot Records( VBR) in non-partitionable disks. Boot records are terminated at the end with a hexadecimal sequence consisting of two bytes (0x55, 0xAA). This byte combination is called the boot sector signature and indicates the validity of the boot sector. If the Boot Sector Signature is not present, then either the Basic Input/Output System( BIOS) or the Master Boot Record generates an error message and booting is interrupted.
The Basic Input/Output System (BIOS) of the computer, which works first after switching on, checks whether the boot sector is a Master Boot Record or a Volume Boot Record (VBR) and then decides on the further procedure.