--------D-2136-------------------------------
INT 21 - DOS 2+ - GET FREE DISK SPACE
      AH = 36h
      DL = drive number (00h = default, 01h = A:, etc)
      Return: AX = FFFFh if invalid drive
      Valid drive returns:
          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