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Sega Master System emulation in Silverlight

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I've had to quickly learn Silverlight for work recently, which has been an interesting experience. I've had to write new code, which is fine but doesn't really excite me as far as Silverlight is concerned - it doesn't really matter which language new code is developed in, as long as it gets the job done.

What does interest me more is that Silverlight is ".NET in your browser", and I'm a big fan of .NET technology with a handful of .NET-based projects under my belt. Silverlight therefore gives me the opportunity to run these projects within the browser, which is a fun idea. [smile]


Click to launch emulator in new window.

To this end, I've turned Cogwheel, a Sega 8-bit system emulator, into a Silverlight application. It took about an hour and a half, which was not as bad as I'd expected! (Skip to the bottom for instructions for the demo).

Raster graphics

Silverlight's raster graphics support is somewhat lacking. You can display raster graphics in Image elements, but - as far as I can see - that's about it. If you wish to generate and display images dynamically via primitive pixel-pushing, you're out of luck as far as Silverlight's class library is concerned.

Thankfully, Ian Griffiths has developed a class named PngGenerator that can speedily encode a PNG from an array of Colors that can then be displayed in an Image. Cogwheel's rasteriser returns pixel data as an array of integers so there's a small amount of overhead to convert these but other than that it's easy to push pixels, albeit in a fairly roundabout manner.

Render loop

The render loop is based around an empty Storyboard that invokes an Action every time it completes then restarts itself.

using System;
using System.Windows;
using System.Windows.Media.Animation;

namespace Cogwheel.Silverlight {

public static class RenderLoop {

public static void AttachRenderLoop(this FrameworkElement c, Action update) {
var Board = new Storyboard();
c.Resources.Add("RenderLoop", Board);
Board.Completed += (sender, e) => {
if (update != null) update();
Board.Begin();
};
Board.Begin();
}

public static void DetachRenderLoop(this FrameworkElement c) {
var Board = (Storyboard)c.Resources["RenderLoop"];
Board.Stop();
c.Resources.Remove("RenderLoop");
}

}
}

I'm not sure if this is the best way to do it, but it works well enough and is easy to use - just grab any FrameworkElement (in my case the Page UserControl) and call AttachRenderLoop:

private void UserControl_Loaded(object sender, RoutedEventArgs e) {
this.UserControlRoot.AttachRenderLoop(() => {
/* Update/render loop in here. */
});
}

Missing .NET framework class library features

This is the big one; Silverlight does not cover the entire .NET framework class library, and so bits of it are missing. Fortunately this can be resolved, the difficulty depending on how you want the functionality of the original app to be affected.

Missing types you're not interested in.

These are the easiest to deal with, and this includes attributes and interfaces that the existing code uses that you're not especially interested in. For example, Cogwheel uses some of .NET's serialisation features for save states - a feature I wasn't intending on implementing in the Silverlight version. The [Serializable] and [NonSerialized] attributes are not available in Silverlight, nor is the IDeserializationCallback interface. To get the project to compile some dummy types were created.

namespace System {

class SerializableAttribute : Attribute { }
class NonSerializedAttribute : Attribute { }

interface IDeserializationCallback {
void OnDeserialization(object sender);
}

}

Missing types or methods that you don't mind partially losing.

Cogwheel features some zip file handling code that uses System.IO.Compression.DeflateStream, a class not available in Silverlight. Rather than remove the zip classes entirely (which would require modifications to other files that relied on them) it was easier to use conditional compilation to skip over the DeflateStream where required.

switch (this.Method) {
case CompressionMethod.Store:
CompressingStream = CompressedStream;
break;

#if !SILVERLIGHT
case CompressionMethod.Deflate:
CompressingStream = new DeflateStream(CompressedStream, CompressionMode.Compress, true);
break;
#endif

default:
throw new NotSupportedException();
}

Missing instance methods.

C# 3.0 adds support for extension methods - user-defined methods that can be used to extend the functionality of existing classes that you cannot modify directly. Silverlight is missing a number of instance methods on certain classes, such as string.ToLowerInvariant();. By using extension methods the missing methods can be restored.

namespace System {

public static class Extensions {
public static string ToLowerInvariant(this string s) { return s.ToLower(CultureInfo.InvariantCulture); }
public static string ToUpperInvariant(this string s) { return s.ToUpper(CultureInfo.InvariantCulture); }
}

}

Missing static methods.

These are the most work to fix as extension methods only work on instance methods, not static methods. This requires a change at the place the method is called as well as the code for the method itself.

I've got around this by creating new static classes with Ex appended to the name then using using to alias the types. For example, Silverlight lacks the Array.ConvertAll method.

namespace System {

static class ArrayEx {
public static TOut[] ConvertAll(TIn[] input, Func fn) {
TOut[] result = new TOut[input.Length];
for (int i = 0; i < input.Length; i++) {
result = fn(input);
}
return result;
}
}

}

First, a replacement method is written with Ex appended to the class name. Secondly, any file that contains a reference to the method has this added to the top:

#if SILVERLIGHT
using ArrayEx = System.ArrayEx;
#else
using System.IO.Compression;
using ArrayEx = System.Array;
#endif

Finally, anywhere in the code that calls Array.ConvertAll is modified to call ArrayEx.ConvertAll instead. When compiling for Silverlight it calls the new routine, otherwise it calls the regular Array.Convert.

Demo

The links below launch the emulator with the selected ROM image.
To run your own ROM image, click on the folder image in the bottom-right corner of the browser window to bring up a standard open file dialog.

Zip files are not handled correctly, but if you type *.* into the filename box, right-click a zip file, pick Open, then select the ROM from inside that it should work (it does on Vista at any rate).

The cursor keys act as you'd expect; Ctrl or Z is button 1/Start; Alt, Shift or X is 2 (Alt brings up the menu in IE). Space is Pause if you use an SMS ROM and Start if you use a Game Gear ROM. Keys don't work at all in Opera for some reason, but they should work fine in IE 8 and Firefox 3. You may need to click on the application first!

Issues

There are a number of issues I have yet to address. Performance is an obvious one; it's a little choppy even with 100% usage of one the cores on a Core 2 Duo. Sound is missing, and I'm not sure what Opera's doing with keys. Other than that, I thought it was a fun experiment. [smile]

Once I've tidied it up a bit I'll merge the source code with the existing source repository.
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These are some great tips...specifically, I hadn't thought of using extension methods to fill in functions that aren't in the .net compact framework. This isn't silverlight-specific, either. It's also a good idea to implement for XNA, if you're using core functionality (like string.Split with the StringSplitOptions parameter) that doesn't exist in the CF.

Thanks :)

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Cheers. [smile] When targeting .NET 2.0 from C# 3.0 you'll notice that you cannot use extension methods, but this too can be fixed by adding a dummy ExtensionAttribute class.

namespace System.Runtime.CompilerServices {

class ExtensionAttribute : Attribute { }
}

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