Boot flag
Encyclopedia
A boot flag is a 1-byte
value in a non-extended partition record, within a master boot record
. Its primary function is to indicate to a MS-DOS
/MS Windows-type boot loader which partition to boot. In some cases it is used by Windows XP/2000 to assign
the active partition the letter "C:". The active partition is the partition where the boot flag is set.
Other boot loaders used by third-party boot managers (such as GRUB or XOSL
), can be installed to a master boot record
and can boot primary or extended partitions, which do not have the boot flag set.
The boot flag appears in a disk editor
at the beginning of a partition record, as the value 0x80. A value of 0x00 indicates the partition does not have the boot flag set. Any other value is invalid.
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
value in a non-extended partition record, within a master boot record
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...
. Its primary function is to indicate to a MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
/MS Windows-type boot loader which partition to boot. In some cases it is used by Windows XP/2000 to assign
Drive letter assignment
Drive letter assignment is the process of assigning alphabetical identifiers to physical or logical disk drives or partitions in the root filesystem namespace; this usage is now mostly found in Microsoft operating systems...
the active partition the letter "C:". The active partition is the partition where the boot flag is set.
Other boot loaders used by third-party boot managers (such as GRUB or XOSL
XOSL
xOSL is the name of a bootloader, which is a program product class that launches operating systems from a bootable device such as a hard disk or floppy drive. The letters are an acronym that stand for eXtended Operating System Loader...
), can be installed to a master boot record
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...
and can boot primary or extended partitions, which do not have the boot flag set.
The boot flag appears in a disk editor
Disk editor
A disk editor is a computer program that allows its user to read, edit, and write raw data on disk drives ; as such, they are sometimes called sector editors, since the read/write routines built into the electronics of most disk drives require to read/write data in...
at the beginning of a partition record, as the value 0x80. A value of 0x00 indicates the partition does not have the boot flag set. Any other value is invalid.