Java Emulator Framework

[swpalmer]

CPU opcode formats usually have some sort of semantic logic to them.

Yes, true.  But unfortunately to decode the instruction in software based on that structure is just many more operations.  

The native CPU hardware would rip the instruction apart in parallel (e.g. the register #s would be tied directly to some sort of 'register address lines')  so the elegance of the structure shows through...
When you don't have all those different hardware units running in parallel I'm afraid that model can't be exploited efficiently.

[MaVi]

Xbrain,
Although the VM could possibly identify which opcodes are used more frequently, I'm afraid there's not much it can do to optimize those particular code paths.

It can't inline anything a that point based on frequency of access, it has to handle all possible situations. So there's not much optimization possible other than a jump table a this point.

Even the method I proposed in the previous post, the hierarchical decode, is probably slower, I only like it because it seems like an elegant solution, but that would have to be seen in an actual implementation.

Seb

Maybe using some kind of short circuits on a range check....

[javatypo]

why dont u guys make an emulator api.

Games published by our own members! Check 'em out!

[erikd]

That's basically what JEF is.
https://sourceforge.net/projects/jef/

[javatypo]

o..

are there any that have been written with that?

[erikd]

Yes, CottAGE:
http://cottage.emuunlim.com

and there's a demo space invaders emu included in JEF.

[CornedBee]

C or C++ emulators tend to have highly optimized inline assembly.
A pure C/C++ emulator is not likely to be that much faster than a Java one.

[erikd]

A pure C/C++ emulator is not likely to be that much faster than a Java one.

In my experience, emulators are not really java's cup of tea. While it's true that most emulaters written in C/C++ have some inline assembly, it's also true that because you have much more low level control in just C/C++, C/C++ based emulators are generally quite a lot faster than the java ones. This doesn't necessarily make C/C++ quite a lot faster than java *per definition*, but a better choice for creating emulators if performance is a major concern.

Look at the emulator 'Modeler' for example (emulates Sega System32 arcade machines). It's completely written in C++, no fancy dynamic recompiling CPU cores (just interpreters), and it emulates possibly the most complex 2D video hardware ever. It runs quite acceptably on a 450Mhz P2.
You probably won't come close to that kind of performance if you did it in java.

Erik

[yandy.ah]

I have waited for this emulator for a Long time, I wanted to code a emulator with java long before,but have no time ,the work is too much and busy at all,and team-force,dear all,would you start this project with me?

[erikd]

To be honest, I'm not much into emulators anymore. I've written a couple of emulators around my own CPU cores and started an open source MAME-like effort, but to start a new project now.... No, I don't have the time I'm afraid.
You *really* need a lot of patience. Really rewarding if the emulator starts working, but a major pain to debug.