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INT 08h (8)  (IRQ 0)     System Timer

    This interrupt is a hardware interrupt (IRQ 0) activated by the system
    timer 18.2 times per second (every 55 ms). The default handler does
    the following:

         .   Keeps a count of the number of timer ticks at memory location
             0:46Ch (System Timer Counter--long integer). After 24 hours of
             operation, a flag is set at memory location 0:470h to signal
             this condition and the System Timer Counter (0:46Ch) is reset to
             0.

         .   Decrements the Diskette Drive Motor Off Counter at memory
             location 0:440h if it is not 0. When this location reaches 0,
             the diskette drive motor is turned off and the Diskette Drive
             Motor Status byte at memory location 0:43Fh is updated to
             reflect that the motor has been turned off.

         .   Generates INT 1Ch.

         .   For the PC Convertible, generates INT 4Ah if an alarm interrupt
             occurs.