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Int 1Ah, 06h            Set Real-Time Clock Alarm                        many

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

        Entry   AH = 06h
                CH = Hours (BCD)
                CL = Minutes (BCD)
                DH = Seconds (BCD)

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

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

    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.