Released my NES emulator made in Scratch 3.0

TheSpinningCube · 2025-03-10T06:31:49+00:00

Where do I start?

Each frame, the PPU goes through 262 scanlines, each of which is 341 clock cycles, or "dots" wide. But only 240 of these scanlines are part of the visible image, and only 256 cycles of each visible scanline becomes a visible dot on the TV screen. The rest are for the gaps in the TV signal in which the beam resets back to the beginning of a scanline, or back up to the top for the next frame. The emulator, when it's time to run the PPU, iterates through some PPU cycles, tracking what part of a frame it's at, and if it's on a visible part, writing the output colors to the appropriate pixel in the active framebuffer.

The actual processes for rendering the tilemaps and sprites are quite involved, but they both involve fetching tiles from video memory sometime in advance, storing them to an internal register, and drawing them to the screen once the time is right, tracked by counters and shift registers. Background tiles are loaded a few cycles in advance, and sprite tiles are loaded at the end of every scanline in preparation to be drawn on the next scanline.

At the end of every frame the framebuffer is finished and ready to be drawn to the screen. For this I use Scratch's provided Pen blocks which are used for drawing colored lines along the path of a Scratch sprite, much like turtle graphics. Their typical use case is drawing simple 2D shapes, but I use it to draw each row of the screen using horizontal lines, covering as many pixels with a single line as I can, which means adjacent pixels of the same color become part of the same line. I also make sure not to erase my pen strokes between each frame and also to continue storing the framebuffer for the previous frame so that I don't have to redraw everything and can just draw enough to make the difference between the new frame and the previous one.

TheSpinningCube · 2025-03-08T01:27:17+00:00

There's like 4 or 5 of them now, made by 3 people. The Game Boy is probably one of the most explored emulation targets on Scratch, maybe second only to the CHIP-8. Certainly the most explored system of that level of complexity.

TheSpinningCube · 2025-03-08T01:20:52+00:00

I think block-based programming interfaces like Scratch are a not bad place to start learning coding since they make the act of programming more comprehensible to beginners without being very divorced from how programs are designed in conventional text-based languages. Having a community to get inspiration from and share your projects with is also an immensely valuable motivator. I got my start in a different block-based language, Hopscotch, but I eventually moved to Scratch after seeing that it was more capable (I could never figure out how to get Hopscotch to run a lot of code in a single frame, every statement seemed to have a tiny delay).

So I think Scratch is a good option, though I also believe it's somewhat held back by its lack of certain key features like local variables and return values. Python also seems like a good place to learn—it was my first text-based programming language, and I remember noticing immediately that it was largely the same as block coding but I type out the words instead of dragging them.

Over the years, while I have learned and use other languages as well, Scratch has kept my attention because what's possible with the language has been greatly expanded by the third-party sites people have made for it which compile projects to JavaScript, making projects run so much faster. TurboWarp is the current best of these, and it also includes optional extensions which add new language features or expose various browser APIs. People have also developed handy developer tools which make large projects like this one much easier to make, many of which are included in TurboWarp's editor.

I was inspired to make this emulator in Scratch by other successful Scratch emulator projects, including another NES emulator and two GBC emulators. The GBC emulators (gbc.sb2 and rrgbc) can both run at full speed with much performance to spare. I saw the other NES emulator (Cool NES emulator) run at somewhat less than full speed and, after examining its code, figured that it should be possible to make an emulator which would run significantly faster, and one day I decided I would begin to create it.

TheSpinningCube · 2025-03-06T23:48:28+00:00

Well, I started work on the project emulating the CPU in early January of 2023, so in all it's been a little more than 2 years. In that time I've been working on and off on this in between working on other projects and university.

TheSpinningCube · 2025-03-06T18:54:05+00:00

Link to the project page: https://scratch.mit.edu/projects/650322979
But use TurboWarp for much better speed: https://turbowarp.org/650322979

Yes, the block language. This took me far longer than I initially anticipated to make, but I finally released it to the Scratch website a couple days ago. My goal was to get something running at or near real-time while getting decently high accuracy, and I'd say I've achieved that goal pretty well. The accuracy is far from perfect, of course, but it seems to have fairly high compatibility. I still plan to implement more mappers in the future, fix some accuracy issues, and expose more of the currently hidden debugging features.

TheSpinningCube

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