INT 1aH services provide access to the system clock.  Also INT 1aH may be
intercepted by a hardware-based MRCI compression system.
AX    Service
b001H Query MRCI hardware support

AH  BIOS Service              Supported by
00H read the clock (ticks)    [PC] [XT] [AT]
01H set the clock (ticks)     [PC] [XT] [AT]

02H read time from CMOS RTC             [AT]
03H set the time on CMOS RTC            [AT]

04H read date from CMOS RTC             [AT]
05H set the date on CMOS RTC            [AT]

06H set alarm on CMOS RTC               [AT]
07H cancel alarm on CMOS RTC            [AT]

The PC BIOS works with a "tick count" of the number of 55ms intervals
which have occurred since the PC was turned on or reset.  The AT BIOS also
provides access to the real-time clock values which are updated constantly
without CPU action, and are stored in the AT's CMOS Memory.

On AT-compatible machines, most short-interval timer/alarm operations are
perfomed via INT 15H.

BIOS Data Area (0040:006c)
- -

INT 1aH: Timer I/O (and MRCI POST Hook)