Get disk space (INT 21h, 36h)

        Entry:
          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