After having a lot of fun writing a GBA emulator in C# a while ago, I decided it was too slow for my liking. I took the time to rewrite it in C++ with an added challenge: I wanted to do all rendering on the GPU. I do some minimal parsing of video memory (mostly in OAM, but also some IO registers) to optimize GPU usage a bit, but other than that, the PPU is entirely emulated in shaders!
It runs pretty fast (on my system, i7 2600 and GTX 670 I have reached framerates of over 5 to 10k fps in some menus, generally in game it reaches framerates of 800fps). It can frameskip, although that causes some graphical glitches sometimes because I might not be syncing correctly. The frameskip feature is mainly there to save GPU usage (less data transfer). I noticed this more on lower end hardware (on my laptop with intel HD graphics for example).
As for accuracy: it’s fairly accurate, it passes all “sane” AGS tests (so not the prefetch buffer ones, or the ones that require cycle accuracy). It also passes a lot of endrift’s test in her mGBA suite.
I spent a lot of time profiling it, checking out the assembly it generated and optimizing based on that. Some major optimizations I made are:
- Templated function handlers
- Faking the pipeline (saving a lot of time spent reading memory on branches and such). I “reflush” the pipeline whenever a write close to PC happens.
- Faster DMAs
- A better scheduler (based on a heap, and it resets its own timer so that I could use a 32 bit int clock instead of a 64 bit one. This might not sound like a big improvement, but it really did save a lot of time)
- Pagetables for memory reads (software fastmem)
- Compiler intrinsics
And on the graphics side, to save resources (and time spent memcpy’ing to buffer video memory), I did some stuff as well:
- Batching up scanlines to whether or not VRAM is “dirty” (writes happened), and only buffering the “dirty” regions of VRAM.
- Object batching/palette batching (basically the same thing but for OAM or palette RAM)
To run the emulator, you need an opengl 3.3 compatible graphics card. It should be cross platform, though I haven’t tried that myself, but it works on windows for me.
Anyway, here is the link to the repository, feel free to check it out and let me know what you think!