Casio IT-2000W - Manual (Page 97)

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7.3 Clock Control Driver
7.3.1 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 ms 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(2Ch).
The relationship between the clock control driver and application programs is shown in the
following diagram.
Fig. 7.1
Application Program
Int21h
Int1Ah
MS-DOS
Real Time Clock (RTC)
POWER.EXE Clock control driver