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:".[1] 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.