On IBM PC-compatible computers, the bootstrapping firmware contained within the ROM BIOS loads and executes the master boot record. The PC/XT (type 5160) used an Intel 8088 microprocessor. In order to remain compatible, all x86 architecture systems start with the microprocessor in an operating mode referred to as real mode. The BIOS reads the MBR from the storage device into physical memory, and then it directs the microprocessor to the start of the boot code. Since the BIOS runs in real mode, the processor is in real mode when the MBR program begins to execute, and so the beginning of the MBR is expected to contain real mode machine language instructions.
Due to the restricted size of the MBR's code section, it typically contains only a small program that copies additional code (such as a boot loader) from the storage device into memory. Control is then passed to this code, which is responsible for loading the actual operating system. This process is known as chain loading.
Popular MBR code programs were created for booting PC DOS and MS-DOS, and similar boot code remains in wide use. These boot sectors expect the
fdisk partition table scheme to be in use, and scans the list of partitions in the MBR's embedded partition table to find the only one that is marked with the active flag. It then loads and runs the Volume Boot Record (VBR) of the active partition.
There are alternative boot code implementations, some of which are installed by boot managers, which operate in a variety of ways. Some MBR code loads additional code for a boot manager from the first track of the disk, which it assumes to be "free" space that is not allocated to any disk partition, and executes it. A MBR program may interact with the user to determine which partition on which drive should boot, and may transfer control to the MBR of a different drive. Other MBR code contains a list of disk locations (often corresponding to the contents of files in a filesystem) of the remainder of the boot manager code to load and to execute. (The first relies on behavior that is not universal across all disk partitioning utilities, most notably those which read and write GPTs. The last requires that the embedded list of disk locations be updated when changes are made that would relocate the remainder of the code.)
On machines that do not use x86 processors, or on x86 machines with non-BIOS firmware such as Open Firmware or Extensible Firmware Interface (EFI) firmware, this design is unsuitable, and the MBR is not used as part of the system bootstrap. EFI firmware is instead capable of directly understanding the GPT partitioning scheme and the FAT filesystem format, and loads and runs programs held as files in the EFI System partition. The MBR will be involved only insofar as it might contain a partition table for compatibility purposes if the GPT partition table scheme has been used.
There is some MBR replacement code that emulates EFI firmware's bootstrap, which makes non-EFI machines capable of booting from disks using the GPT partitioning scheme. It detects a GPT, places the processor in the correct operating mode, and loads the EFI compatible code from disk to complete this task.
Read more about this topic: Master Boot Record
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