Int 21 Fn 36  - DOS 2+ - Get Free Disk Space                               [D]

   AH = 36h
   DL = drive number (00h = default, 01h = A:, etc)

Return: AX = FFFFh if invalid drive
   else
       AX = sectors per cluster
       BX = number of free clusters
       CX = bytes per sector
       DX = total clusters on drive

Notes: free space on drive in bytes is AX * BX * CX
   total space on drive in bytes is AX * CX * DX
   "lost clusters" are considered to be in use
   according to Dave Williams' MS-DOS reference, the value in DX is
     incorrect for non-default drives after ASSIGN is run

See Also: AH=1Bh,AH=1Ch