AH = 36h
        DL = drive code (0 = default, 1 = A, 2 = B, etc.)
Return: AX = number of sectors per cluster
             or 0FFFFh if invalid drive
        BX = number of available clusters
        CX = bytes per sector
        DX = total clusters
Note: multiply AX x CX x BX for free space on disk
      multiply AX x CX x DX for total disk space
-----------------------------------------------------------