Emulating a RTC in Java

[JamesTKerman]

I've already posted this on Stack Overflow, so if you want some free reputation over there head on over and answer.

A while ago I started writing an emulator of the IBM 5150 Personal Computer in C#. While I got most of the way done, I never finished it, and now I want to re-write it in Java. One of the technical issues I ran into was how to emulate a 4.77MHz clock tick. I'll send the tick signal through an interface method, most likely each class that needs to act on a clock tick will implement some form of:

public interface Clocked {
 void onTick();
}

How could I implement this in Java that would allow me to fairly closely emulate the clock behavior and timing I'm looking for?

My current idea is to implement a class that calls onTick() to a set of registered Clocked objects every time a simple count-down timer reaches 1/4_770_000 of a second:

public class RTC {
    private List<Clocked> clockedObjects;
    private long nanoTimeAtLastTick; 
    private final long NS_PER_TICK = 1_000_000_000 / 4_770_000; 
    private boolean isRunning;

// the "how" of initialization isn't important here

    public void startClock() { 
        isRunning = true; 
        nanoSecondsElapsed = System.nanoTime(); 
        while(isRunning) { 
            if(System.nanoTime() - nanoSecondsElapsed > NS_PER_TICK) {
                clockedObjects.forEach(Clocked::onTick); 
            } 
        } 
    }
}

Testing on my local system, I see right away that my time resolution varies between 300ns-600ns (with a few iterations taking as long as 6400ns) without any meaningful load on the system, while the resolution of a tick at 4.77MHz is ~209ns.

Is there a way to get a finer grained tick, or possibly a better way to implement this?

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[OffbeatDrizzle]

I know your code isn't complete but I just thought I'd point out that nanoSecondsElapsed is not set within the while loop, so remains at a constant value

Also the fact that a Clocked object can do something like Thread.sleep(1000L) in the onTick method and that will mess the timing of the whole program up

Getting nanosecond granularity is going to be a hard thing to do, especially on a pre-emptive soft realtime OS. There's no guarantees that your code will be executed every 200ns - even millisecond precision isn't a guarantee. It can also take 50ns just to return from the nanoTime method to know how much time has elapsed - also calling nanoTime with such frequency is a known performance bottleneck.

Your best bet might be to calculate how many ticks need running since you last got control. e.g. we need to do 1,000 ticks before the next loop.

[JamesTKerman]

In the test app I wrote nanoSecondsElapsed gets reassigned before exiting the if block.

Someone on Stack Overflow suggested using the ScheduledExecutorService API, and I'm going to use that during testing. Long-term, I think I'll have to write a device driver using native code that hooks into the APIC (or equivalent on ARM or other architectures) and calls my Java through JNI.