TIMER0_ROLLOVER
outb(IO_TIMER1 + TIMER_CNTR0, TIMER0_ROLLOVER % 256);
outb(IO_TIMER1 + TIMER_CNTR0, TIMER0_ROLLOVER / 256);
ticks = timer0last + (TIMER0_ROLLOVER - current);
nticks -= TIMER0_ROLLOVER - (ticks - otick);
return (TIMER0_ROLLOVER - gettick());
.tc_counter_mask = TIMER0_ROLLOVER,