Boot sector
Encyclopedia
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.
hard disk
s, floppy disk
s 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.
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 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.
es. To combat this behavior, the BIOS often includes an option to prevent writing to the boot sectors of attached hard drives.
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
, floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
, optical disc
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...
, or other data storage device
Data storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
that contains machine code
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
to be loaded into random-access memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
(RAM) by a computer system's built-in firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
. The purpose of a boot sector is to allow the boot
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
process of a computer to load a program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
(usually, but not necessarily, an operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
) stored on the same storage device. The location and size of the boot sector (perhaps corresponding to a logical disk sector
Disk sector
In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user data. Traditional formatting of these storage media provides space for 512 bytes or 2048 bytes of user-accessible data per sector...
) is specified by the design of the computing platform.
On an IBM PC compatible
IBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
machine, the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
selects a boot device, then copies the first sector from the device (which may be a MBR
Master boot record
A master boot record is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk...
, VBR
Volume Boot Record
A volume boot record is a type of boot sector introduced by the IBM Personal Computer...
or any executable code), into physical memory at memory address
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
0x7C00. On other systems, the process may be quite different.
- NOTE: This article is IBM PC-centric and does not explain the use of a boot sector on other types of computer systems.
Kinds of boot sectors
Several major kinds of boot sectors could be encountered on IBM PC compatibleIBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
s, floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
s and similar storage devices:
- A master boot recordMaster boot recordA master boot record is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk...
(MBR) is the first sector of a data storage device that has been partitioned. The MBR sector may contain code to locate the active partition and invoke its Volume Boot Record.
- A volume boot recordVolume Boot RecordA volume boot record is a type of boot sector introduced by the IBM Personal Computer...
(VBR) 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 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.
- CD-ROMs usually have their own structure of boot sectors, for IBM PC compatibleIBM PC compatibleIBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
systems this is subject to El Torito specificationsEl Torito (CD-ROM standard)The El Torito Bootable CD Specification is an extension to the ISO 9660 CD-ROM specification. It is designed to allow a computer to boot from a CD-ROM...
.
- Non IBM PC compatible systems may have different boot sector formats on their disk devices.
Operation
On IBM PC compatibleIBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
machines, the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
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 or USB flash drive
USB flash drive
A flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus interface. flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g...
, 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
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
(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.
Boot sector viruses
Since code in the boot sector is executed automatically, boot sectors have historically been a common attack vector for computer virusComputer virus
A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...
es. To combat this behavior, the BIOS often includes an option to prevent writing to the boot sectors of attached hard drives.