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Int 15h, 83h            Event Wait                                       many

    Modifies a byte in the user's memory after a specified time
    interval. Control is transferred to the caller immediately after
    this service is called.

        Entry   AH    = 83h
                AL    = Subservice
                        | 00h  Set interval
                        | 01h  Cancel set interval
                ES:BX = Pointer to byte that will be posted
                CX:DX = Microseconds until posting (long integer)

        Return  CF Set if service not supported, else cleared

    ------------------------------------------------------------------

    After this service is called, control is returned immediately to
    the caller. This service does not work by delaying and then
    returning control to the user, but returns control immediately;
    the user must periodically look at the byte specified (pointed to
    by ES:BX) to determine when the interval has elapsed.


    The PC, PCjr, and the XT dated 11/08/82 do not support this
    service. The AT dated 1/10/84 is only able to set the interval;
    thus the value in AL is not used.

    The byte is posted by setting the high bit (bit 7). To test the
    end of the interval, make sure the high bit of the byte is clear
    before calling Subservice 0 (Set Interval).

    The real-time clock is used to count the number of microseconds.

    The granularity of the time is 976 microseconds.

    CX is the high-order word of the count. For example, if CX is 98h
    and DX is 9680h, then a 10-second delay would be specified.