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Master Boot Record

Part of series osdev |

Master boot record

  • First sector of a physical hard disk.
  • This sector is not a part of any partition (OS or data).
  • Contains two parts:
    • MBR bootstrap program to locates the boot sector of other partitions.
    • Partition table contains a master list of all available partitions on all available disks.
  • Master boot record is detected by boot signature (magic number).

MBR Partition table

  • Apply to a hard disk drive, use to manage partitions in disk
  • 16 bytes, total’s 64 bytes. Limited to 4 entries. To create more, use an extended partition.
  • Maximum size of a single MBR partition is 2TB.
  • Elements:
    • Boot indicator: inactive - active/bootable partition
    • Bits to define Starting head, sector and Cylinder
    • System ID: define filesystem of the partition (ext2, fat32, NTFS, etc.)
    • Relative sector
    • Total sectors in the partition

Master Boot Record format

Extended partitions

  • A solution to add more than 4 partitions to the partition table.
  • Partition can have one and only one entry with SystemID = 0x5 or 0xF to define as an extended partition.
  • Extended partition is a physical partition that’s subpartitioned into “logical” partitions.
  • partition table entry describing it has a StartLBA and NumSectors that describe the space inside which any number of logical partitions may sit.
  • At the start of an extended partition is an extended partition table. Format of extended partition table is same with MBR partition table.

Master Boot Record sequence

  • After BIOS finish their job, they load the first sector (512 bytes) of devices (Hardisk, disc, USB, etc.) to check boot signature.
  • The boot signature is called a magic number. If a device has a boot signature, it’s can bootable.
    • Boot signature is in a boot sector number 0.
    • Contains byte sequence: 0x55, 0xAA at byte offets 510 (0x1FE) and 511 (0x1FF).
    • 0x1FE and 0x1FF is 2 last bytes of 512 bytes of the first sector.
  • When BIOS recognize a boot device, master boot record is loaded into memory at 0x0000:0x7c00 (sector 0, address 0x7c00) from this device. It contains executable code.
  • Execution is transferred to the loaded boot record bootstrap program.

Master Boot Record format

Master boot record bootstrap program:

  • Currently, the DL register is set to “drive number” that MBR was loaded.
  • Master boot record process relocates itself away from 0x7c00: memory copy and far jump.
  • Determine which partition/device to boot OS. If the user selects inactive partition, then set the partition chosen entry to active and clear active state from others.
  • Rewrite MBR if the partition table entries were modified.
  • Load Volume Boot Record (boot sector of bootloader) from selected partition.
  • Set DS:SI pointing to the selected partition table entry.
  • Jump to 0x7c00 (with CS set to 0, and DL set to the “drive number”).

Volume boot record

  • First sector of the first partition containing the OS.
  • Contains boot loader code and some file system info.


  • A program is in the boot sector (master boot record or Volume boot record).
  • Use to loads OS kernel.
Tags: x86 | mbr | boot | kernel |