A boot flag is a 1-byte value in a non-extended partition record, within a master boot record. It appears 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.
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. DOS and Windows allow only one boot partition to be set with the boot flag.
Some buggy BIOSes test if the boot flag of at least one partition is set, otherwise they ignore the device in boot-order. Therefore, even if the bootloader does not need the flag, it has to be set to start the boot code from BIOS.
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"Master Boot Record". Microsoft TechNet. Retrieved 2015-04-20.
0x01BE [...] Boot Indicator. Indicates whether the volume is the active partition. Legal values include: 00. Do not use for booting. 80. Active partition.
- Goodell, Dan. "Fixing Windows 2000/XP Drive Letters". Understanding MultiBooting and Booting Windows from an Extended Partition. Retrieved 2014-12-14.
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- Smith, Rod. "Legacy BIOS issues with GPT". GPT fdisk Tutorial. Retrieved 2020-06-14.