- The BIOS loads the beginning 512 bytes Master Boot Record) of the first valid disk in the BIOS disk order.
- Of these 512 bytes, the first 440 contains the first stage of a boot loader like GRUB, Syslinux or LILO.
- Since very little can be achieved by a program of this size, the second stage (residing on the next disk sectors) is loaded from here and looks up a file stored on the partition itself (the actual bootloader). This then loads an operating system by either chain-loading or directly loading the operating system kernel.
- UEFI has support for reading both the partition table as well as understanding filesystems.
- UEFI does not launch any boot code in the MBR whether it exists or not. Instead it uses a special partition in the partition table called EFI System Partition in which files required to be launched by the firmware are stored.
- Each vendor can store its files under
"EFI SYSTEM PARTITION"/EFI/"VENDOR NAME"/folder and can use the firmware or its shell (UEFI shell) to launch the boot program.
- An EFI System Partition is usually formatted as FAT32 or (less commonly) FAT16.