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

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

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


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

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

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

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

See also: INT 1Ah, 07h
See also: INT 1Ah, 08h
See also: INT 1Ah, 09h

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