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A boot sector (sometimes called a bootblock) is a sector of a hard disk, floppy disk, or similar data storage device that contains code for booting programs (usually, but not necessarily, operating systems) stored in other parts of the disk.

The term boot sector is used for IBM-compatible PCs, whereas bootblock is used when referring to certain other types of computers, such as Sun systems.

The BIOS selects a boot device, then it copies the first sector from the device (which may be an MBR, VBR or any executable code), to address location 0x7C00.

There are two major kinds of boot sectors:

• A Volume Boot Record is the first sector of a data storage device that has not been partitioned, or the first sector of an individual partition on a data storage device that has been partitioned. It may contain code to load and invoke an operating system (or other standalone program) installed on that device or within that partition. To be a valid boot sector, the two-byte hex word 0xAA55, 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.

• A Master Boot Record is the first sector of a data storage device that has been partitioned. Most BIOS code also requires the MBR sector to have a valid signature of 0xAA55 (the byte 55h followed by AAh) either before it will load the MBR sector into memory or before attempting to execute any code it might contain. The MBR sector may contain code to locate the active partition and invoke its Volume Boot Record.

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. If the device is a floppy disk, 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 0xAA55 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. But with such freedom, there's also the possibility for abuse as addressed in the section on boot viruses.

For more details on this topic, see Booting.

Boot sectors are one mechanism by which computer viruses gain control of a system. Boot sector infector viruses replace the bootstrap code in the boot sectors (of floppy disks, hard disks, or both) with viral code.

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• Sample to build a boot program on x86 real mode.