3.4 Timer

Nachos provides an instance of a Timer to simulate a real-time clock, generating interrupts at regular intervals. It is implemented using the event driven interrupt mechanism described above. Machine.timer() returns a reference to this timer.

Timer supports only two operations:

• getTime() returns the number of ticks since Nachos started.
• setInterruptHandler() sets the timer interrupt handler, which is invoked by the simulated timer approximately every Stats.TimerTicks ticks.

The timer can be used to provide preemption. Note however that the timer interrupts do not always occur at exactly the same intervals. Do not rely on timer interrupts being equally spaced; instead, use getTime().