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.
- "Fixing Windows 2000/XP Drive Letters". Understanding MultiBooting and Booting Windows from an Extended Partition.