DOS defines a 1-byte code that (sometimes) can be used to identify the
type of media that is being used by a disk device.  The Media Descriptor
(also called the FAT ID byte) will be seen:

  as a value obtained from DOS Fns 1bH, 1cH, and 32H
  as the first byte of the FAT (DOS logical sector 1)
  at offset 0aH of a BPBRec
  at offset 15H of a BootSectorRec (in DOS logical sector 0)
  in the data block used by DOS Fn 440dH (Generic IOCTL)

Value  Media Types Defined as of DOS 5.0
f0H   3л" (1.44 MB) 2 sides, 18 sectors
3л" (2.88 MB  2 sides, 36 sectors
5м" (1.2 MB)  2 sides, 15 sectors
(also used for other media types)

f8H   fixed disk, any capacity

f9H   3л" (720K)    2 sides,  9 sectors, 80 tracks
5м" (1.2 MB)  2 sides, 15 sectors, 80 tracks
faH   5м" (320K)    1 side,   8 sectors, 80 tracks
fbH   3л" (640K)    2 sides,  8 sectors, 80 tracks

fcH   5м" (180K)    1 side,   9 sectors, 40 tracks
fdH   5м" (360K)    2 sides,  9 sectors, 40 tracks; also used on 8" disks
feH   5м" (160K)    1 side,   8 sectors, 40 tracks; also used on 8" disks
ffH   5м" (320K)    2 sides,  8 sectors, 40 tracks

The media descriptor byte is not a particularly reliable way to ascertain
media type.  Refer to other fields of the BPBRec or BootSectorRec to
obtain track, sector, and head information.

For instance, the boot sector of a RAM disk may show it to be a hard disk
(f8H) or even some other value.

See Also: fn 1bH and fn 1cH (get FAT info)
fn 32H (get drive parameter block)
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Media Descriptor