INT 1Ah,  06h (6)        Set Real-Time Clock Alarm                       many

    Sets the time at which the computer will issue an alarm by calling INT
    4Ah.

       On entry:      AH         06h
                      CH         Hours (BCD)
                      CL         Minutes (BCD)
                      DH         Seconds (BCD)

       Returns:       CF         Set if clock not operating or alarm already
                                 set; else cleared

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       Notes:         All dates are in Binary Coded Decimal (BCD). For
                      example, June 1, 1985 will be set as CX = 1985; DX =
                      0601.

                      The real-time clock is the clock that runs even when
                      the computer is turned off. A CMOS battery is used
                      so that, even while the computer is off, the date,
                      time, and alarm time are maintained.

                      This service is available only for ATs, XT-286s, and
                      PC Convertibles. Previous machines will report
                      unpredictable results (including the state of the
                      Carry Flag).

                      After this service is executed, an INT 4Ah will be
                      generated as soon as the hour, minute, and second
                      specified match the time on the real-time clock
                      date. It is up to the programmer to install an
                      interrupt handler for INT 4Ah before calling this
                      service

                      The alarm interrupt (INT 4Ah) will continue to occur
                      every 24 hours at the specified time until it is
                      reset via Service 07h (Reset Real-Time Clock Alarm).
                      Only one alarm time can be active at a time.

                      Because the real-time clock is updated even if the
                      computer is turned off, the alarm time set via this
                      service will remain active even after the computer
                      is turned off (of course, the alarm won't be
                      generated while the computer is off).

                      Setting the real-time clock date will not affect the
                      system date. This is because the real-time clock is
                      read once upon system startup, updating the system
                      timer. All other references to date use the system
                      timer. If either the real-time clock date is changed
                      (via Service 05h) or the system timer is changed
                      (via Service 01h), the other date is not changed
                      accordingly.

                      Even though the system is using the system timer for
                      date information, both the real-time clock and the
                      system timer are updated continuously.