BOOTSECTOR STRUC
bsJump db 3 dup(?) ;E9 XX XX or EB XX 90
bsOemName db '????????' ;OEM name and version
;Start of BIOS parameter block
bsBytesPerSec dw ? ;bytes per sector
bsSecPerClust db ? ;sectors per cluster
bsResSectors dw ? ;number of reserved sectors
bsFATs db ? ;number of file allocation tables
bsRootDirEnts dw ? ;number of root-directory entries
bsSectors dw ? ;total number of sectors
bsMedia db ? ;media descriptor
bsFATsecs dw ? ;number of sectors per FAT
bsSecPerTrack dw ? ;sectors per track
bsHeads dw ? ;number of heads
bsHiddenSecs dd ? ;number of hidden sectors
bsHugeSectors dd ? ;number of sectors if bsSectors = 0
;End of BIOS parameter block
bsDriveNumber db ? ;drive number (80h)
bsReserved1 db ? ;reserved
bsBootSignature db ? ;extended boot signature (29h)
bsVolumeID dd ? ;volume ID number
bsVolumeLabel db 11 dup(?) ;volume label
bsFileSysType db 8 dup(?) ;file-system type
BOOTSECTOR ENDS
The BOOTSECTOR structure contains information about the disk (or other storage medium) for a particular drive. The structure appears at the beginning of the first sector (the boot, or startup, sector) of the disk.
bsJump
Contains a jump instruction to the bootstrap routine, which loads the operating system from the drive.
bsOemName
Specifies the name of the original equipment manufacturer (OEM) and the manufacturer's version of MS-DOS.
bsBytesPerSec
Specifies the number of bytes per sector.
bsSecPerClust
Specifies the number of sectors in a cluster. The sectors must be consecutive, and the number must be a power of 2.
bsResSectors
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.
bsFATs
Specifies the number of file allocation tables (FATs) following the reserved sectors. Most versions of MS-DOS maintain one or more copies of the primary FAT and use the extra copies to recover data on the disk if the first FAT is corrupted.
bsRootDirEnts
Specifies the maximum number of entries in the root directory.
bsSectors
Specifies the 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 bsHugeSectors field.
bsMedia
Specifies the media descriptor, a value that identifies the type of media 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 media type. Also, the first byte in the FAT is often (but not always) identical to the media descriptor.
Following is a list of 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). |
bsFATsecs
Specifies the number of sectors occupied by each FAT.
bsSecPerTrack
Specifies the number of sectors on a single track.
bsHeads
Specifies the number of read/write heads on the drive.
bsHiddenSecs
Specifies the number of hidden sectors on the drive.
bsHugeSectors
Specifies the number of sectors if the bsSectors field is zero. This value supports drives larger than 32 MB.
bsDriveNumber
Specifies whether the drive is the first hard disk drive, in which case the value is 80h; otherwise, the value is 00h. This field is used internally by MS-DOS.
bsReserved1
Reserved; do not use.
bsBootSignature
Specifies the extended boot-signature record. This value is 29h.
bsVolumeID
Specifies the volume serial number.
bsVolumeLabel
Specifies the volume label.
bsFileSysType
Specifies the type of file system, given as an 8-byte ASCII string. This field can be one of the following values:
| Name | Meaning |
| FAT12 | 12-bit FAT |
| FAT16 | 16-bit FAT |
If the name has fewer than eight characters, space characters (ASCII 20h) fill the remaining bytes in the field.
The BOOTSECTOR structure shares the first sector with the bootstrap routine and the boot-sector signature. The boot-sector signature, stored in the last two bytes of the sector, must be 0AA55h.