Bootflag

BootFlag is a term used to describe a flag in disk partition tables that marks a partition as bootable in the context of legacy BIOS booting. It is most commonly associated with the Master Boot Record (MBR) partitioning scheme, where each partition entry includes a boot indicator byte. When this byte is set (typically 0x80), the partition is designated as bootable and a BIOS bootloader may attempt to load code from that partition’s boot sector during startup.

In practice, the BootFlag serves as a hint to the firmware or bootloader about which partition to boot from when multiple partitions could potentially host bootloaders. However, the flag is not a guarantee of a successful boot. A valid boot sector, a correctly installed bootloader, and a configured boot order are also required. Consequently, some systems ignore the flag entirely, especially in configurations that use modern boot methods or complex multi-boot setups.

GPT-based disks and UEFI systems largely render the traditional boot flag obsolete. UEFI firmware boots from an EFI System Partition and uses boot entries rather than a partition flag to select a boot source. Some legacy BIOS environments on GPT disks may still display a boot flag in partitioning tools, but it has limited or no functional impact in standard UEFI workflows.

Common tooling and procedures exist to view and set BootFlag. In Linux, fdisk can display the bootable partition and toggle the flag with the a command; parted and other partition editors offer equivalent controls (for example, set a partition boot on).