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Int 08h                 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 0040h:006Ch (System Timer Counter--long
             integer). After 24 hours of operation, a flag is set at
             memory location 0040h:0070h to signal this condition and
             the System Timer Counter (0040h:006Ch) is reset to 0.

         .   Decrements the Diskette Drive Motor Off Counter at memory
             location 0040h:0040h 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
             0040h:003Fh 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.