A boot sector is a sector of a hard disc, floppy disc, or similar data storage device that contains code for bootstrapping programs (usually, but not necessarily, operating systems) stored in other parts of the disc. A sector is a part of a whole. ... Typical hard drives of the mid-1990s. ... A floppy disk is a data storage device that comprises a circular piece of thin, flexible (hence floppy) magnetic storage medium encased in a square or rectangular plastic wallet. ... In computing, a data storage device—as the name implies—is a device for storing data. ... Bootstrapping alludes to a German legend about a Baron Münchhausen, who was able to lift himself out of a swamp by pulling himself up by his bootstraps. ... The terms computer program, software program, applications program, system software, or just program are used to refer to either an executable program by both lay people and computer programmers or the collection of source code from which an executable program is created (eg, compiled). ... An operating system (OS) is a software program that manages the hardware and software resources of a computer. ...

Kinds of boot sectors

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 contains code to load and invoke the operating system (or other standalone program) installed on that device or within that partition.
• A Master Boot Record is the first sector of a data storage device that has been partitioned. It contains code to locate the active partition and to 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 disc, that will be a VBR. If the device is a hard disc, that will be an MBR. It is the code in the MBR that understands disc partitioning, and that is responsible for in turn loading and running the VBR of the active (primary) partition. A Volume Boot Record (also known as a volume boot sector or a partition boot sector, although the latter is not strictly correct) is a type of boot sector, stored in a disc volume on a hard disc, floppy disc, or similar data storage device, that contains code for bootstrapping... In computer engineering, hard disk drive partitioning is the creation of logical divisions upon a hard disk that allows one to apply operating system-specific logical formatting. ... In the IBM PC architecture the Master Boot Record (MBR), or partition sector, is the 512-byte boot sector, i. ... One of the first PCs from IBM - the IBM PC model 5150. ... Phoenix AwardBIOS on a standard PC BIOS, in computing, stands for Basic Input/Output System or Basic Integrated Operating System. ...

Boot sectors and computer viruses

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 discs, hard discs, or both) with viral code. In computer security, a computer virus is a self-replicating computer program that spreads by inserting copies of itself into other executable code or documents. ...

As well as being ignorant of whether a disc has been partitioned, the BIOS on IBM PC compatible machines is also ignorant of whether a disc has in fact been high-level formatted and had an operating system installed in it. The error message displayed when a machine is bootstrapped from a disc without an operating system installed on it (asking the user to insert a bootable disc and press a key) is in fact displayed by code in the boot sector itself, not by the machine firmware. Formatting a hard drive using MS-DOS Disk formatting is the only process of preparing a hard disk or other storage medium for use with the file system (FAT, NTFS, UFS, etc. ...

This results in a security vulnerability. A user who sees the error message may not be aware that the code in the boot sector of the disc has already been run by that point, and that if the disc was infected by a boot-sector computer virus, the virus will have already gained control of the machine. Because of this vulnerability, computer security experts tend to recommend that booting from devices other than the one containing the installed operating system, such as removable media devices (e.g. floppy disc devices, CD-ROMs, and USB flash drives), be disabled in normal operation via the BIOS setup utility, and only re-enabled on those specific occasions when booting from such devices is actually required. The CD-ROM (an abbreviation for Compact Disc Read-Only Memory (ROM)) is a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

References

Further reading

• Mary Landesman. Boot sector viruses.
• Microsoft. How to Protect Boot Sector from Viruses in Windows. KnowledgeBase.
• Denny Lin. Inexpensive boot sector virus detection and prevention techniques.
• Kaspersky Lab. Boot sector viruses. Virus Encyclopedia / Malware Descriptions / Classic Viruses.