BPB STRUC
bpbBytesPerSec dw ? ;bytes per sector
bpbSecPerClust db ? ;sectors per cluster
bpbResSectors dw ? ;number of reserved sectors
bpbFATs db ? ;number of file allocation tables
bpbRootDirEnts dw ? ;number of root-directory entries
bpbSectors dw ? ;total number of sectors
bpbMedia db ? ;media descriptor
bpbFATsecs dw ? ;number of sectors per FAT
bpbSecPerTrack dw ? ;sectors per track
bpbHeads dw ? ;number of heads
bpbHiddenSecs dd ? ;number of hidden sectors
bpbHugeSectors dd ? ;number of sectors if bpbSectors = 0
BPB ENDS
The BPB structure contains information that defines the format of a disk or other storage medium.
bpbBytesPerSec
Specifies the number of bytes per sector.
bpbSecPerClust
Specifies the number of sectors per cluster. The sectors must be consecutive and a power of 2.
bpbResSectors
Specifies the number of reserved sectors on the drive, beginning with sector 0. Typically, this value is 1 (for the startup sector), unless the disk-drive manufacturer's software reserves additional sectors.
bpbFATs
Specifies the number of file allocation tables (FATs) following the reserved sectors. Most versions of MS-DOS maintain one or more additional copies of the FAT and use the extra copies to recover data on the disk if the first FAT is corrupted.
bpbRootDirEnts
Specifies the maximum number of entries in the root directory.
bpbSectors
Specifies the total number of sectors on the drive. If the size of the drive is greater than 32 MB, this field is zero and the number of sectors is specified by the bpbHugeSectors field.
bpbMedia
Specifies the media descriptor, a value in the range 00h through 0FFh that identifies the type of medium or disk in a drive. Some device drivers use the media descriptor to determine quickly whether the removable medium in a drive has changed. MS-DOS passes the media descriptor to the device driver so that programs can check the type of medium. Also, the first byte in the FAT is often (but not always) identical to the media descriptor.
Following are the most commonly used media descriptors and their corresponding media:
| Value | Type of medium |
| 0F0h | 3.5-inch, 2 sides, 18 sectors/track (1.44 MB); 3.5-inch, 2 sides, 36 sectors/track (2.88 MB); 5.25-inch, 2 sides, 15 sectors/track (1.2 MB). This value is also used to describe other media types. |
| 0F8h | Hard disk, any capacity. |
| 0F9h | 3.5-inch, 2 sides, 9 sectors/track, 80 tracks/side (720K); 5.25-inch, 2 sides, 15 sectors/track, 40 tracks/side (1.2 MB). |
| 0FAh | 5.25-inch, 1 side, 8 sectors/track, (320K). |
| 0FBh | 3.5-inch, 2 sides, 8 sectors/track (640K). |
| 0FCh | 5.25-inch, 1 side, 9 sectors/track, 40 tracks/side (180K). |
| 0FDh | 5.25-inch, 2 sides, 9 sectors/track, 40 tracks/side (360K). This value is also used for 8-inch disks. |
| 0FEh | 5.25-inch, 1 side, 8 sectors/track, 40 tracks/side (160K). This value is also used for 8-inch disks. |
| 0FFh | 5.25-inch, 2 sides, 8 sectors/track, 40 tracks/side (320K). |
bpbFATsecs
Specifies the number of sectors occupied by each FAT.
bpbSecPerTrack
Specifies the number of sectors per track.
bpbHeads
Specifies the number of read/write heads on the drive.
bpbHiddenSecs
Specifies the number of hidden sectors on the drive.
bpbHugeSectors
Specifies the number of sectors if the bpbSectors field is zero. This value supports drives larger than 32 MB.