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A master boot record (MBR) is a type of boot sector in the first block of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBRs was publicly introduced in 1983 with PC DOS 2.0.
On IBM PC compatible machines, the BIOS is ignorant of the distinction between VBRs and MBRs, and of partitioning. The firmware simply loads and runs the first sector of the storage device. [3] If the device is a floppy or USB flash drive, that will be a VBR. If the device is a hard disk, that will be an MBR.
The EFI (Extensible Firmware Interface) system partition or ESP is a partition on a data storage device (usually a hard disk drive or solid-state drive) that is used by computers that have the Unified Extensible Firmware Interface (UEFI). When a computer is booted, UEFI firmware loads files stored on the ESP to start operating systems and ...
A modern PC is configured to attempt to boot from various devices in a certain order. If a computer is not booting from the device desired, such as the floppy drive, the user may have to enter the BIOS Setup function by pressing a special key when the computer is first turned on (such as Delete, F1, F2, F10 or F12), and then changing the boot order. [6]
On system with BIOS firmware, the BIOS invokes MBR boot code from a hard disk drive at startup. The MBR boot code and the VBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find an active partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition.
The boot code in the VBR can assume that the BIOS has set up its data structures and interrupts and initialized the hardware. The code should not assume more than 32 KB of memory to be present for fail-safe operation; [1] if it needs more memory it should query INT 12h for it, since other pre-boot code (such as f.e. BIOS extension overlays, encryption systems, or remote bootstrap loaders) may ...
In the example 2 above, GRUB 2 stores its core.img in a BIOS boot partition. When used, the BIOS boot partition contains the second stage of the boot loader program, such as the GRUB 2; the first stage is the code that is contained within the Master Boot Record (MBR).
In addition to the standard PC disk partition scheme that uses a master boot record (MBR), UEFI also works with the GUID Partition Table (GPT) partitioning scheme, which is free from many of the limitations of MBR.