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Chapter – 12
Reading and Modifying MBR with Programming
Master Boot Record (MBR) or Master Partition Table (MPT)
The Master Boot Record (MBR) or sometimes referred as The master partition table (MPT), is created on the hard disk drive by executing FDISK.EXE command of DOS.
The MBR contains a small program to load and start the active (or bootable) partition from the hard disk drive. The Master boot Record contains information about all four primary partitions on the hard disk drive such as the starting sector, ending sector, size of the partition etc.
The MBR is located at Absolute Sector 0 or we can say at cylinder 0, head 0, and sector1 and if there is more than one partition are present in the disk there are Extended Master Boot Records, located at the beginning of each extended partition volume.
For detailed description refer the chapter “Logical Approach to Disks and OS”, discussed earlier in this book.
Master boot record format
We may partition the Hard Disk Drive into several logical drives which are generally assigned their own drive letter by DOS). Only one partition at a time can be marked as the active (or bootable) Partition.
The Master Boot Record has the limit of four entries in the Master Partition Table. However the location of Extended Master Boot Record can be obtained with the help of Master Boot Record that contains Extended Partition Tables, whose format is exactly the same as of the main Partition Table except there is no boot code and this space of 446 Bytes is normally reserved for the boot code and remains empty.
All the 512Bytes of The Master Boot Record are Broken as follows, given in the Table:
Offset |
Description |
Size |
000H |
Initial Program Loader (IPL), Executable Code (Provides very first booting to the Computer) |
446 Bytes |
1BEH |
First Partition Entry (See Next Table) |
16 Bytes |
1CEH |
Second Partition Entry |
16 Bytes |
1DEH |
Third Partition Entry |
16 Bytes |
1EEH |
Fourth Partition Entry |
16 Bytes |
1FEH |
Executable Marker or Bootable Sector Signature or Magic Number (AAH 55H) |
2 Bytes |
Total = 512 Bytes |
All the extended partitions should exist within the space reserved by the extended partition entry. Only two of the extended partitions are meant to be used, the first as a normal partition and the second as another extended partition if exists.
Thus with the help of one Master Partition Table We can get the location of another Extended Master Partition Table next to it, if present.
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