DOS Boot Record DBR DOS Boot Sector FORMAT command PC DOS FDISK physical sector active partition table entry Master Boot Record MBR booting Invalid system disk I/O error bootable disk Root Directory Data Area file system sectors track

 
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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. Most commercial applications for disc-recovery are capable of regenerating destroyed boot-records

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).

The DBR is created by the FORMAT command of DOS. This program can be executed from a DOS floppy disk (or directly from another volume, following some OS limits) to create the DBR 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.

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 systems control to the loaded program.

For doing this, 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 as specified as active partition, in the partition table entry.

The Boot program in the DBR looks for the two program files IBMBIO.COM or IO.SYS and IBMDOS.COM or MSDOS.SYS, in the root directory of the partition.  IBMBIO.COM and IBMDOS.COM are two hidden system program files on the PC-DOS systems or original IBM systems. Whereas IO.SYS and MSDOS.SYS are two hidden system program files on a MS-DOS operating system provided with IBM compatible systems.

After that, the IO.SYS (or IBMBIO.COM) program loads the MSDOS.SYS (or IBMDOS.COM) program and the COMMAND.COM program. This complete process is called “booting” of the computer. If these system files are not available in the directory then this MBR program displays error messages soothing like,

“Invalid system disk or Disk I/O error,
 Replace the disk, and then press any key…”

On the screen and waits for the user to put a bootable disk with the above mentioned programs in the floppy drive and press a key.

Since the floppy has no partitions on it therefore it has no MBR or Master Partition Table on its absolute sector 0, instead it contains the DBR on its very first sector.

The following table gives a simple map of a 3½ Inches, 1.44 MB floppy disk’s layout after having been formatted with the FAT12 file system. It shows where the Boot Record, both copies of the FAT, the Root Directory and the beginning of the Data Area are located:

Logical Map of 3½ Inches, 1.44 MB floppy disk, Formatted with the FAT12 File System and having 18 Sectors Per Track, 80 Tracks, 2 Sides and 512 bytes per Sector (using 1 Sector per Cluster).

Absolute Sectors

Contents

0

Boot Record

1 – 9

FAT 1

10 – 18

FAT 2

19 – 32

Root Directory

33 – 2879

Data Area

 

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