Chapter – 13
Reading and Modifying DBR with Programming
DOS Boot Record (DBR) / DOS Boot Sector
After the partition table, the DOS Boot Record (DBR) or sometimes called DOS Boot Sector is the second most important information on your hard drive.
For a Detailed study about DBR, refer the chapter, “Logical Approach to Disks and OS”, Discussed earlier in this book.
First logical sector of each DOS partition will contain a DOS Boot Record (DBR) or DOS Boot Sector. The job of the DBR is to load the operating system from the hard disk drive into the main memory of computer and give the system’s control to the loaded program.
The DOS Boot Record (DBR) for the first partition on a hard disk is usually found at Absolute Sector 63 (the 64th sector on the disk drive) or in CHS form we can say C–H–S = 0–1–1 for most drives.
However this location may vary depending upon the SPT (Sectors per Track) of the Drive. For example, on an old 245MB drive having only 31 SPT, the Boot Record was located on the 32nd sector (Absolute Sector 31).
Since the floppy has no partitions on it therefore it has no MBR or Master Partition Table on its first sector, instead it contains the DBR on its very first sector.
The DBR is created by the FORMAT command of DOS, after partitioning is done using the FDISK command. The sector on which DBR resides becomes logical sector 1 of that particular partition for the DOS. The sector number used by DOS starts from the physical sector on which DBR is located.
The DBR contains a small program which is executed by the Master Boot Record (MBR) Executable program. All DOS partitions contain the program code to boot the machine i.e. load the operating system, but only that partition is given control by the Master Boot Record which is specified as active partition, in the partition table entry.
If the DBR is corrupted any how, the drive should be accessible if you boot the system from the bootable floppy or CD. Although the hard disk is not bootable (if the DBR of Active partition is corrupted), yet generally that should not affect access to the data of the disk drive. After booting the system with the bootable disk you can access the data.
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