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        Make tone

        Entry:
          AX = 1019h
          BX = frequency in Hertz
          CX = duration in clock ticks (18.2 ticks/sec)

        Return:
          immediately, tone continues to completion

        Notes:  If another tone is already playing, the new tone does not
                start until completion of the previous one.  In DV 2.00,
                it is possible to enqueue about 32 tones before the process
                is blocked until a note completes.
                In DV 2.00, the lowest tone allowed is 20 Hz