INT 15h,  83h (131)      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.

       On 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)

       Returns:       Carry      Set if service not supported, else cleared

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    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.

       Notes:         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.