The INT 4aH vector (at 0:0128) is initialized by BIOS to point to an IRET.

It must be revectored to a user-supplied handler whenever using
INT 1aH 06H (set RTC alarm).

If you intercept this interrupt, be sure to save all registers on entry
and restore all registers on exit.

Only AT-class and better computers support this interrupt.

See Also: INT 1aH 06H (set user alarm)
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INT 4aH: User Alarm Handler