[AT]
Expects: AH    06H
CH    hours, in BCD 
CL    minutes, in BCD
DH    seconds, in BCD
                                                                  
Returns: AH    0
CF    NC (0) no error
CY (1) alarm already operating
                                                                  
Info: Sets the BIOS to invoke INT 4aH at the specified time.

You must supply an INT 4aH handler if you want to get control at
the specified time (and avoid a system crash!).  The vector for
INT 4aH is at 0000:0128H.

Only one alarm may be set at a time.  If an alarm is already
active, this returns with the Carry Flag set and take no action.

Use INT 1aH 07H to cancel a previously-set alarm.

Note: Using this as described is quite unusual.  A more common
technique is to patch a TSR into INT 08H and keep an eye on the
system time.  That way you can set up multiple timers, etc.

See Also: INT 1aH (BIOS Time I/O)
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INT 1aH 06H: Set Real-Time Clock Alarm