Guide >> Concepts >> Understanding Hardware... >> Master Boot Record

Master Boot Record (MBR)

The Master Boot Record is made when the first partition is created the hard disk.  It is probably the most important data structure on the disk and is the first sector on every disk. The location is always track (cylinder) 0, side (head) 0, and sector 1.

The Master Boot Record contains the partition table for the disk and a small amount of executable code. On x86-based computers, the executable code examines the partition table, and identifies the system partition.  The Master Boot Record then finds the location of the system’s starting partition on the disk, and loads a copy of the partition’s boot sector into memory. The Master Boot Record then transfers execution to executable code in the partition’s boot sector.

Note

Although there is a Master Boot Record on every hard disk, the executable code in the sector is used only if the disk is connected to an x86-based computer and the disk contains a system partition.

The figure below shows a hex dump of the sector containing the Master Boot Record. The figure shows the sector in two parts. The first part is the Master Boot Record, which occupies the first 446 bytes of the sector. The disk signature (FD 4E F2 14) is at the end of the Master Boot Record code. The second part is the Partition Table.

Physical Sector: Cyl 0, Side 0, Sector 1     
00000000: 00 33 C0 8E D0 BC 00 7C  - 8B F4 50 07 50 1F FB FC   .3.....|..P.P..
00000010: BF 00 06 B9 00 01 F2 A5  - EA 1D 06 00 00 BE BE 07   ................
00000020: B3 04 80 3C 80 74 0E 80  - 3C 00 75 1C 83 C6 10 FE   ...<.t..<.u.....
00000030: CB 75 EF CD 18 8B 14 8B  - 4C 02 8B EE 83 C6 10 FE   .u......L.......
00000040: CB 74 1A 80 3C 00 74 F4  - BE 8B 06 AC 3C 00 74 0B   .t..<.t.....<.t.
00000050: 56 BB 07 00 B4 0E CD 10  - 5E EB F0 EB FE BF 05 00   V.......^.......
00000060: BB 00 7C B8 01 02 57 CD  - 13 5F 73 0C 33 C0 CD 13   ..|...W.._s.3...
00000070: 4F 75 ED BE A3 06 EB D3  - BE C2 06 BF FE 7D 81 3D   Ou...........}.=
00000080: 55 AA 75 C7 8B F5 EA 00  - 7C 00 00 49 6E 76 61 6C   U.u.....|..Inval
00000090: 69 64 20 70 61 72 74 69  - 74 69 6F 6E 20 74 61 62   id partition tab
000000A0: 6C 65 00 45 72 72 6F 72  - 20 6C 6F 61 64 69 6E 67   le.Error loading
000000B0: 20 6F 70 65 72 61 74 69  - 6E 67 20 73 79 73 74 65    operating syste
000000C0: 6D 00 4D 69 73 73 69 6E  - 67 20 6F 70 65 72 61 74   m.Missing operat
000000D0: 69 6E 67 20 73 79 73 74  - 65 6D 00 00 80 45 14 15   ing system...E..
000000E0: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
000000F0: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000100: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000110: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000120: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000130: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000140: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000150: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000160: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000170: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000180: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
00000190: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
000001A0: 00 00 00 00 00 00 00 00  - 00 00 00 00 00 00 00 00   ................
000001B0: 00 00 00 00 00 00 00 00  - FD 4E F2 14 00 00         .........N......

                                                       80 01                 ..
000001C0: 01 00 06 0F 7F 96 3F 00  - 00 00 51 42 06 00 00 00   .....?...QB....
000001D0: 41 97 07 0F FF 2C 90 42  - 06 00 A0 3E 06 00 00 00   A....,.B...>....
000001E0: C1 2D 05 0F FF 92 30 81  - 0C 00 A0 91 01 00 00 00   .-....0.........
000001F0: C1 93 01 0F FF A6 D0 12  - 0E 00 C0 4E 00 00 55 AA   ...........N..U.

Viruses Can Infect the Master Boot Record

Many destructive viruses damage the Master Boot Record and make it impossible to start the computer from the hard disk. Because the code in the Master Boot Record executes before any operating system is started, no operating system can detect or recover from corruption of the Master Boot Record. You can use, for example, the DiskProbe program on Windows NT Workstation Resource Kit CD to display the Master Boot Record, and compare it to the Master Boot Record shown above. There are also utilities on the Microsoft Windows Resource Kits that enable you to save and restore the Master Boot Record.

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