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The Master Boot Record (MBR)

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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• External Serial Advanced Technology Attachment (eSATA)
• Mach Speed P4MSD-800 D2 Via Socket 775 MicroATX Motherboard and Intel Pentium D 940 3.20GHz Processor Bundle
• PC Sound Circuitry
• SD (Secure Digital) Memory Card Basics
• Windows Registry Basics
• Understanding Your Motherboard
• A+ Certification Study Guide, Sixth Edition
• How to Build Your Own PC - The Smart Way

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