Int 21 Fn 1C  - Dos 1+ - Get Allocation Information For Specific Drive     [D]

   AH = 1Ch
   DL = drive (00h = default, 01h = A:, etc)

Return: AL = sectors per cluster (allocation unit), or FFh if invalid drive
   CX = bytes per sector
   DX = total number of clusters
   DS:BX -> media ID byte (see #1044)

Notes: 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
   on a DBLSPACE drive, the total number of clusters is based on the
     estimated compression ratio
   this function may not be properly supported on CD-ROMs and other
     installable file systems (use AX=4402h"CD-ROM" for CD-ROMs
     instead)

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