A boot sector or boot block is a region of a hard disk, floppy disk, optical disc, or other data storage device that contains machine code to be loaded into random-access memory (RAM) by a computer system's built-in firmware. The purpose of a boot sector is to allow the boot process of a computer to load a program (usually, but not necessarily, an operating system) stored on the same storage device. The location and size of the boot sector (perhaps corresponding to a logical disk sector) is specified by the design of the computing platform.
On an IBM PC compatible machine, the BIOS selects a boot device, then copies the first sector from the device (which may be a MBR, VBR or any executable code), into physical memory at memory address 0x7C00. On other systems, the process may be quite different.
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Several major kinds of boot sectors could be encountered on IBM PC compatible hard disks, floppy disks and similar storage devices:
To be a valid boot sector, the two-byte hexadecimal sequence 0x55, 0xAA (called the boot sector signature) must exist at the end of the sector; otherwise, either the BIOS or MBR code will report an error message and halt any OS bootstrapping process.
Systems not following the above described design are:
On IBM PC compatible machines, the BIOS is ignorant of the distinction between VBRs and MBRs, and of partitioning. The firmware simply loads and runs the first sector of the storage device.[1] If the device is a floppy or USB flash drive, that will be a VBR. If the device is a hard disk, that will be an MBR. It is the code in the MBR which generally understands disk partitioning, and in turn, is responsible for loading and running the VBR of whichever primary partition is set to boot (the active partition). The VBR then loads a second-stage bootloader from another location on the disk.
Furthermore, whatever is stored in the first sector of a floppy diskette, USB device, hard disk or any other bootable storage device, is not required to immediately load any bootstrap code for an OS, if ever. The BIOS merely passes control to whatever exists there, as long as the sector meets the very simple qualification of having the boot record signature of 0x55, 0xAA in its last two bytes. This is why it's easy to replace the usual bootstrap code found in an MBR with more complex loaders, even large multi-functional boot managers (programs stored elsewhere on the device which can run without an operating system), allowing users a number of choices in what occurs next. With this kind of freedom, abuse often occurs in the form of boot sector viruses.
Since code in the boot sector is executed automatically, boot sectors have historically been a common attack vector for computer viruses. To combat this behavior, the BIOS often includes an option to prevent writing to the boot sectors of attached hard drives.