Media Descriptors FAT booting hard disk drive cluster Root Directories File system Volume size sectors extended boot record information hidden partitions cylinder table disk type DOS

 
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This information helps us to find the location of FAT and some other important values. Putting wrong information here or corrupting these values makes booting from the hard disk drive impossible. Sometimes having wrong information in the disk parameter block will prevents the booting from hard disk drive as well as from the floppy disk drive.

Bytes per sector are almost always 512. If not so even then it must be an integer power of 2 (e.g. 64, 128, and 256).

The number of Sectors per cluster depends on the size of cluster. (See the Clusters section given in this chapter earlier). Number of Copies of FAT is almost always 2.

Number of Root Directories: It depends on the File system and Volume size. (See File System Limits given before and description of Root Directory given next).

Total number of sectors: Excludes hidden sectors. If it is 0 in the BPB, the field in the extended boot record information is used and vice versa. Note that it is possible to determine if the extended information (DOS 4.0 and later) is available by examining the signature byte at offset 26H.

Any sectors before the boot sector of a logical DOS drive are considered to be 'hidden' sectors. The DOS does not interpret hidden sectors. Normal floppy drives have 0 hidden sectors. Hard drive partitions will have a number reflecting their location on the drive. Note that the whole first head of the first cylinder is usually reserved for the partition table, even though it is only the first sector which is actually used.

Media descriptor: Used to give an indication of the media or disk type. Normal values are 0 for an extended DOS partition and F8H for a hard drive. The values of Media Descriptor Bytes have been given in the following table.

Media Descriptors

Type

Capacity

Size and type

F0H

2.88 MB

3.5", 2-Sided, 36 Sectors per Track

F0H

1.44 MB

3.5", 2-Sided, 18 Sectors per Track

F9H

720 KB

3.5", 2-Sided, 9 Sectors per Track

F9H

1.2 MB

5.25", 2-Sided, 15 Sectors per Track

FDH

360 KB

5.25", 2-Sided, 9 Sectors per Track

FFH

320 KB

5.25", 2-Sided, 8 Sectors per Track

FCH

180 KB

5.25", 1-Sided, 9 Sectors per Track

FEH

160 KB

5.25", 1-Sided, 8 Sectors per Track

F8H

---------

Fixed Disk


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