The Master Boot Record (MBR)
Article reprinted with permission of NTFS.com
On a hard disk, data is stored in thin, concentric bands. A drive head,
while in one position can read or write a circular ring, or band called a
track. Sections within each track are called sectors. A sector is the smallest
physical storage unit on a disk, and is almost always 512 bytes (0.5 kB) in size.
The Master Boot Record, created when you create the first partition on the hard
disk, is probably the most important data structure on the disk. It 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
system partition's starting location on the disk, and loads a copy of its Partition
Boot Sector into memory. The Master Boot Record then transfers execution to executable
code in the Partition 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 the system partition.
The example 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 80 01 .........N......
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 Disk Probe 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.
Download a Free program at www.NTFS.com
that creates a NTFS Boot Disk that lets you access NTFS partitions in Windows XP, 2000, or NT.
More Computer Anatomy Articles:
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• Hardware Resources Explained
• SDRAM Types: DDR, DDR2, DDR3, DDR4
• How to Build a Computer
• PC Motherboard Expansion Cards
• PC Technician Street Smarts: A Real World Guide to CompTIA A+ Skills
• How Does Wireless Internet Work?
• Understanding Your Motherboard
• CompTIA Strata Study Guide
• Basic Computer Thermodynamics
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