Gameboy Emulator

If your VBA, Game Boy Advance emulator is not functioning as expected, our troubleshooting guide may be able to help you diagnose and resolve any common errors. Should you require additional assistance, please don't hesitate to reach out to us directly.

I'm writing a gameboy emulator, and am struggling with making sure opcodes are emulated correctly. Certain operations set flag registers, and it can be hard to track whether the flag is set correctly, and where.

Gameboy Emulator

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Write some sort of logging framework that logs a stacktrace at each operation and compares it to other established emulators. This would be pretty quick to do, and allows a fairly rapid overview of what exactly went wrong. The log file would look a bit like this:

The highly-anticipated scripting feature, which has been in development for the past several months, has now been merged and is available in development builds.With this merged, users can now write and run scripts in Lua, as is possible in some other emulators.Currently, there is only preliminary support and many features are not yet exposed.These builds include an example script that shows how to interact with the emulator, and can pull information about the party from the US releases of the first three Pokmon generations.There is also documentation on the current API available on its own page.

A new release of mGBA, version 0.8.4, is available. This version is a bugfix release, which contains many stability and accuracy fixes. Notably, an issue in the 3DS version that would lead to the 3DS crashing when exiting the emulator, and an issue that would lead mGBA to appearing in a language other than English by default on macOS despite the system language being set to English, have been fixed. The previously mentioned bug with Advance Wars is still present in 0.8.4. It has also been discovered to affect Final Fantasy I. This can be worked around by using a dump of the official BIOS, or using a development build, also available on the download page, which has the bug fixed. An extensive list of changes follows after the cut.

I am currently at address 0x0064 - 0x0068 and I am confused how to pass the JR NZ at address 0x0068. I am using these websites as a guide (gbdev & realboyemulator.wordpress) and this is my understanding so far.

What I'm stuck on is how to initialize the state of the emulator. When I started out, I assumed that the gameboy's own boot rom (some call it the BIOS) was responsible for setting the initial state of the various registers and places in memory. But each emulator I've looked at seems to be doing something different.

We have a lot of gameboy emulation projects, and it would be wise to collect the wisdom of all our brainstorming. It seems that most projects are using peanut-gb as a base, and branching off that to add support for Playdate.

Regarding JIT ("just in time" compilation), the idea is to translate the gameboy SM83 instructions into ARMv7-M assembly, so that the Playdate CPU runs the code natively -- blazingly fast by gameboy standards. What's more, we can statically optimize out most flag updates.

That's already what is currently done by some implementations. There's a disagreement about semantics though (is that 30 or 60 fps?). The actual implementation also differs -- it can be handled as two gameboy frames in one playdate update, or one gameboy frame per update with only the odd numbered frames flushing to the display. In the former case, playdate reports 30 fps maximum; in the latter, 60 (and the updates, we presume, are different lengths of time each, depending on parity.)

Just to clarify: efficiency around memory usage is everything on Playdate. The amount of work most emulators do per pixel just doesn't seem to fly on device. This is why I've been thinking about rendering and caching all backgrounds and sprites ahead of time, and only update them when we see new sprite or bg data loaded. This may break a few games but would work on most.

Also! Not really sure what other emulators are doing, but I switched from my own emulation layer to PeanutGB early on as I just didn't feel it necessary to add yet another GB emulator code base into the word when Peanut was pretty good and being actively worked on with goals that aligned with mine.

There are some test ROMs online, but at the moment I don't have a way to launch an arbitrary ROM to debug it. I'm using qemu to debug the transpiler on PC, which lets me step through the ARM code instruction-by-instruction to find bugs. But I don't yet have the ability to run this via the Playdate Simulator, so I can't use the existing ROM loading interface, and indeed I don't have an actual gameboy emulator connected to this (only an 'emulator' (transpiler) for gameboy CPU instructions.)

Probably the simplest way to proceed would be to modify the test ROMs so that they don't require an entire gameboy emulation context to run, and then embed them right into the C source for the test suite of the transpiler.

Oh also, while the Playdate frontend of my emulator won't be available until start of 2024 (I can't share it yet, so nobody can look at its magic yet, sorry!), I can tell you that you should load the ROM from the .pdx instead of a binary include into the binary. I have fought with this code a lot, and this one works:

You inspired me to take a stab at getting cpu-instrs working in the JIT-based emulator. I now have it going all the way into a basic test case and returning. I'm not running Blargg's tests yet, but I am running the preamble to them. Already I found and fixed a couple of bugs.

I've started writing a GameBoy emulator in JS for the fun of it (I know there are hundreds of such emulators, it's for the practice and understanding how a GB works, not for bringing anything new to the scene).

When readin most tutorials or blog posts about people writing GB emulators, it seems that no one has any trouble reaching that screen (even before interrupts and some other slightly more advanced features). What am I missing here ?

Back in 2014 I was messing around with the idea of converting Gameboy .GBS soundtracks into Amiga .MOD files for a game I was doing. If you don't know what a .GBS file is, it's basically just a Gameboy ROM with Z80 code but with the graphics routines and gameplay stripped out, so it's just the sound driver and music data, so in order to play it back you essentially have to emulate the full Gameboy processor and sound chip. At the time I was sort of infatuated with FreePascal and Lazarus, and wanted to find a Gameboy emulator written in Pascal that I could maybe yank the CPU and sound code from.

Somehow after scavenging over a bunch of old forum posts, I found (apparently) the only Gameboy emulator written in Pascal, ever: UGE (edit: it turns out the famous BGB emulator is also written in Pascal... Too bad it's not open source!). It was written by a guy named Christian Hackbart with Delphi and released in 2000, and the only available download was from a mirror on Zophar's Domain. I started it up, loaded a ROM, was met with some ear-piercing noise, and then the emulator crashed. I canned the project pretty shortly after and that was pretty much that.

By a stroke of luck, I figured out that the reason the emulator was crashing: the sound emulation. If you untick the sound box in the menu, it stops running the sound code and it can be played for more than 5 seconds before blowing chunks. It still runs insanely fast and the graphics are upside down though. I tackled the speed issue first... should be as simple as just throwing in a spinloop to waste time and lock it to 60fps or whatever.

The emulator actually does render the screen right-side-up when using DirectDraw, but I can't record that with OBS for some reason. It renders flipped graphics when drawing with GDI-- let's fix that. In dib_out.pas I changed biheight := 144; to biheight := -144; and....

Holy shit, it's perfect! Well, the audio is clipping a little bit. But all I did was change like 20 characters! It really strikes me that all Christian Hackbart had to do back in 2000 was do 5 lines of fixes and he would have had a near perfect Gameboy emulator on his hands. Also let me just say that it's super impressive that not only does near 20-year-old code work unmodified on Windows 10, but also that it's compiled by a totally different compiler and totally rewritten GUI toolkit, almost with zero changes. My hat is off to the FreePascal folks.

For me, the most favorite type of a computer program is an emulator. Being able to run code from a completely different hardware architecture always seemed like a magic. The old computers are great on their own, so this kind of connection between historic machines and the modern computing environment feels almost like a time travel.

I think that forcing the emulator to pass all of these tests took my longer than writing the untested version of the code in the first place. At some point I integrated the tests with the build process and Travis, so the ROM-based tests are now being run in the cloud every time I push a change to GitHub.

Picking a system to emulate isn't an easy choice; the standard first emulator project seems to be a CHIP-8 emulator. Reading about CHIP-8 definitely helped me to understand a lot of emulation concepts, but it seemed a bit too basic. I felt that I got enough out of just reading through other people's emulators, and that writing my own would be a pointless exercise.

While I wouldn't recommend trying to learn about emulation from someone as inexperienced as me, I wanted to outline the process of starting an emulator for the first time, talk about which documents were most helpful to me, and also talk about some of the design choices I went through.

Most of the Game Boy specific code that I wrote in the beginning, such as loading a ROM, was based heavily on other emulators. I looked at how two or three different emulators did it, and then wrote it into Cinoop in my own style. It wasn't worth trying to write code on my own just yet, I needed to have a base to work with first, before I could experiment with doing things my own way. ff782bc1db