Casio IT-2000W manual Clock Control Driver Function, Ms-Dos

7.3Clock Control Driver

7.3.1Function

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.

Application Program

Int21h

MS-DOS

Clock control driver POWER.EXE

Int1Ah

Real Time Clock (RTC)

Fig. 7.1

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