When the program is executed, it determines which partition contains the active operating system and branches to the boot program in that partition's boot sector to load it. See boot sector.

BIOS uses the Master Boot Record (MBR) and UEFI uses the GUID Partition Table (GPT). As such, BIOS saves all its information on a BIOS chip on the motherboard and UEFI saves its data to an EFI ...