IO_TIMER1          19 timer.c        #define TIMER_MODE      (IO_TIMER1 + 3) // timer mode port
IO_TIMER1          29 timer.c          outb(IO_TIMER1, TIMER_DIV(100) % 256);
IO_TIMER1          30 timer.c          outb(IO_TIMER1, TIMER_DIV(100) / 256);