Int 21 Fn 1B  - DOS 1+ - Get Allocation Information For Default Drive      [D]

   AH = 1Bh

Return: AL = sectors per cluster (allocation unit)
   CX = bytes per sector
   DX = total number of clusters
   DS:BX -> media ID byte (see below)

Note:  under DOS 1.x, DS:BX points at an actual copy of the FAT; later
     versions return a pointer to a copy of the FAT's ID byte

See Also: AH=1Ch,AH=36h

Values for media ID byte:
 FFh   floppy, double-sided, 8 sectors per track (320K)
 FEh   floppy, single-sided, 8 sectors per track (160K)
 FDh   floppy, double-sided, 9 sectors per track (360K)
 FCh   floppy, single-sided, 9 sectors per track (180K)
 F9h   floppy, double-sided, 15 sectors per track (1.2M)
 F8h   hard disk
 F0h   other