Casio IT-2000D - Manual (Page 97)

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6.3 Clock Control Driver
6.3.1 Function
This driver adjusts the system time on this terminal. This driver must be installed.
On a general PC a timer interrupt occurs every 55 msec to update the clock tick counter, which is
one of the BIOS system variables, and the clock overflow counter. The clock tick counter is
incremented each time the timer interrupt occurs and read out from the real-time clock (RTC) when
the PC power is turned on, and disappears when the power is off. However, in the case of a handheld
terminal, since the suspend/resume state is frequently cycled, the clock tick counter is initialized
only once, at the initial boot. Therefore, the clock time may be slightly off if the terminal is operated
for a long period of time. To avoid this problem the terminal uses this driver to control the clock in
cooperation with POWER.EXE so that the time can be directly read from the RTC. This ensures that
the correct time can always be obtained, irrespective of the length of operation. However, since the
time is read from the RTC in seconds, the 1/100 of a seconds digit will be ignored if the time is read
using INT21h.
The relationship between the clock control driver and application programs is shown in the
following diagram.
Fig. 6.1
Application Program
Int21h
Int1Ah
MS-DOS
Real Time Clock (RTC)
POWER.EXE Clock control driver