Game Boy

ShizZy · Oct 20, 2011

pradipna: Nice work!
I hope you (and WingsORC) keep your projects up, its nice to see people working on new emulation projects & keeping the programming community active. Looking at this thread brings me back..

pradipna · Oct 22, 2011

Thank you Shizzy.

And Good Luck with your projects that you are currently working on...

WingsORC · Oct 28, 2011

Thanks too. My emulator shows only a glitchy asteroids intro so far

but well the progress is steady!

Drenn · Oct 30, 2011

Hi, I've been slowly plugging away on a gameboy emulator since june. I've made a lot of progress but sound emulation is a very new thing to me. I've emulated the first 2 channels and they sound something like a piano player who always hits the adjacent key... I used SDL_audiospec while following codeslinger's master system sound code and I ended up with this:

Code:

	updateBufferCount += cycles;
	if (updateBufferCount >= updateBufferLimit)
	{
		if (bufferPosition < BUFFERSIZE)
			playbackBuffer[bufferPosition] = tone*3000;

		bufferPosition++;
		updateBufferCount = updateBufferLimit - updateBufferCount ;		
	}

where playbackBuffer is copied into the audio buffer in my SDL_audiospec's callback routine, and updateBufferLimit is the interval measured in clock cycles between the times that the tone is stored into playbackBuffer. The value calculated for updateBufferLimit should make playbackBuffer completely filled by the time SDL requests its audio buffer to be refilled, but it tends to vary. When bufferPosition < BUFFERSIZE equals false, the tone doesn't get put into the playbackBuffer since the array has been filled already, and this seems to happen too often, which I believe is what makes the music sound sloppy.
If there is any better way to program this, I'd love to know, since I've never done anything like this before. Maybe something other than SDL would work better, or I could just be doing it wrong...

pradipna · Oct 31, 2011

I don't have any experience with SDL so I can't really help you on that. I am using xAudio2. The way I am emulating channel 1 and 2 is like this, I have created 4 buffers with different wave duty (12.5%, 25%, 50% and 75%). When game wants to play sound from channel 1 or 2, I just copy the required wave duty buffer to xAudio source buffer and loop it infinitely at given frequency. And stop it according to sound length register, or sweep frequency overflows etc etc...

ShizZy · Oct 31, 2011

Drenn,

Have you checked out:

http://slack.net/~ant/libs/audio.html#Blip_Buffer

Blip_Buffer implements an efficient band-limited sound buffer for high-quality emulation of sound chips. After setting the source clock rate and output sample rate, sound waves are made by specifying the time points where amplitude changes occur.

It's written using SDL, and really helpful for audio emulation of older systems. Blargg also has a really simple example of the Game Boy PAPU sound chip emulator, written in C/SDL (if you're feeling less adventurous).

Drenn · Oct 31, 2011

Thanks for replying

As it turns out,

Code:

updateBufferCount = updateBufferLimit - updateBufferCount ;

should have been

Code:

 updateBufferCount = updateBufferCount - updateBufferLimit;

An honest mistake

It sounds loads better now, just a little... how to say... unclean? Not as crisp as it should be. Blip Buffer sounds perfect for an emulator, I'll probably use that when I'm feeling less lazy. For now I'm happy with the result, I just need to emulate sweep and noise. I may be back to ask about the details of sweep, since my previous attempt at emulating that ended badly.

Edit - gah, now the frequency for channel 3 sounds too low in some cases, like the intro in mario deluxe. Screw it, I'll use blip_buffer!

Drenn · Nov 4, 2011

I am SO close... it sounds nearly perfect, like a real gameboy... except that channel 3 has an annoying tendancy to be a little bit off-tune sometimes. It completely wrecks a few songs

Weirdly, for channel 3 the pandocs say:

Code:

Frequency = 65536/(2048-x) Hz

but the value that works for me is

Code:

frequency = (65536/(2048-x)*32) Hz

Does anyone else have this? Could this be screwing up the frequency just slightly? It's driving me crazy!

Edit - Ohh, the 32 is for the 32 samples. Still, the off-tune channel 3 remains at large... here's my code:

Code:

	// Channel 3
	if (chanOn[2])
	{
		static double polarity[] = { -1, -0.8667, -0.7334, -0.6, -0.4668, -0.3335, -0.2, -0.067, 0.0664, 0.2, 0.333, 0.4668, 0.6, 0.7334, 0.8667, 1  } ;
		if (chanVol[2] != 0)
		{
			// Read the sample from ram
			int wavTone = ioRam[0x30+(chan3WavPos/2)];
			wavTone = chan3WavPos%2? wavTone&0xF : wavTone>>4;

			// Add the sample to the output
			if (chanToOut1[2])
				toneSO1 += (polarity[wavTone])*(0xF >> (chanVol[2]-1));
			if (chanToOut2[2])
				toneSO2 += (polarity[wavTone])*(0xF >> (chanVol[2]-1));
		}
		chanPolarityCounter[2] -= cycles;
		// Update polarity
		if (chanPolarityCounter[2] <= 0)
		{
			chanPolarityCounter[2] = clockSpeed/(65536/(2048-chanFreq[2])*32);

			chan3WavPos++;
			if (chan3WavPos >= 32)
				chan3WavPos = 0;
		}


		// Check that it hasn't timed out
		if (chanUseLen[2])
		{
			chanLenCounter[2] -= cycles;
			if (chanLenCounter[2] <= 0)
			{
				chanOn[2] = 0;
				chanPolarityCounter[2] = 0;
				clearChan3();
			}
		}
	}

Sorry for the mess. It's possible the error's not in there, but unlikely. It's the only channel giving me problems.

pradipna · Dec 4, 2011

Ok, I have finally decided to release the new version of my emulator. Here's a feature list:

Features:
---------------------
-> Can emulate GameBoy and GameBoy Color hardware.
-> Sound Emulation (excluding Vin output)
-> Keyboard and JoyPad input support.
-> MBC1, MBC2, MBC3 and MBC5 support.
-> Real Time Clock emulated.
-> Battery Pack support. Save format is compatible with VBA-M.
-> Save State
-> VisualBoy inspired Turbo Button. Press Space to speed up the game.

I have increased the compatibility too. I am going to keep this project aside for now and work on a new one.
If anyone's interested, download it from here: http://www.pradsprojects.com/dinoboy.html

HyperHacker · Feb 20, 2012

I've been working on a GB emu recently. I've got Tetris and Super Mario Land working great. Now I'm trying out Blargg's tests... It's telling me my "ADD SP,n" and "LD HL,SP+n" are broken somehow, but it doesn't say what the problem is, and I can't find it. Does adding to SP affect flags differently or something?

I tried copying what VBA does, but it doesn't seem to have helped. (I didn't even check if VBA passes that test either... >.>) bgb passes all the tests, but its source doesn't seem to be available.

WingsORC · Mar 8, 2012

Hi guys just wanted to drop by to tell you that im still working /programming emulators. Several months ago i started off with a chip-8 emulator then i moved on to a gb emu. Meanwhile I also worked a lil bit on a NES emulator and now I am back again

. I learned a lot about emulation (escpecially about sound emulation and dsp), considering i didnt konw anything about it when i started off. Recently i looked at my (unfinished) gb emu source code and I noted that it sucks. Well hereby i shall start again with a complete rewrite of my gb emu! I hope there are still some guys here working on emulators, because its rly fun and gives u a great insight into those fascinating machines.

ShizZy · Mar 8, 2012

Hi WingsORC,

I know the feeling

You learn so much writing emulators that you quickly deprecate your own code. The same thing happened to me when I started by GB emu, and it still happens with more advanced projects. Moral of the story: If you've learned enough to realize how you could have done a simple thing better, you've probably grown enough to move on to a more advanced emu project. I'd recommend just diving into a more complex system, you'll learn more that way then by rewriting your other projects.

WingsORC · Mar 21, 2012

ok, well i in the last few days i wanted to give my gb emu a finishing touch before i would follow your advice to move on to a more difficult project (any recommandations?). But i before that i have to resolve one more problem.
Games which scroll a lot have a sometimes a weird scanline drawing but. Some scanlines seems to shift by one pixel right, left, up or down (depending on scrolling direction) while drawing the screen. And this scanlineshift goes like a wave through the screen. Happens with super mario land 1 2 3 , metroid II etc.

ht tp://img6.image shack.us/img6/5994/gbirqtiming.png //remove spaces, how to post pictures properly?
This picture is extremly outdated but the vblank timing is still the same
Vblank seems to be off even though i verified the frequency of my timer (and it is correct) and the number of cpu cycles a screen refresh takes (70224). Any ideas?¨

EDIT:
Ok guys i could fix the scanline glitch. It was simple vertical tearing... so i just synched the screen refresh with the gb vblank interrupt. But the problem with the timer still remains.

ShizZy · Mar 23, 2012

Well, people say the next logical step from gb is nes... because its more involved but very doable. But imo nes isn't more of a challenge than gb, but just more frusterating/time consuming

. Getting a nes emu to boot will be very easy for you, but so many games are very sensitive to timing/accuracy that most people start them to learn then give up because they aren't learning much new. I'd say do a system that's more interesting to you/newer...

WingsORC · Mar 24, 2012

but just more frusterating/time consuming

well i can approve of that because i already wrote a nes emu

. Cpu is quite accurate (luckily NES community has a lot of testroms) but well graphic is glitchy as hell. It runs a few games but to get more games running withought extensive hacks oh my... Like battletoads, your off by one 1 cycle and the screen starts shacking weird etc... So id rather not go back to the nes

Helius · Aug 17, 2012

It's been a long time since I last posted here... 6 years methinks

That time I was struggling with my Gameboy emulator written in C# and finally abandoned it.

And for the sake of completeness I just wanted to say that I'm now finishing it! :blush:

Some weeks ago I decided to give it another try, this time in C++ in order to run it on Windows, Linux, Mac and iOS...

Well, this is the result, it needs more work and lacks sound, but is compatible whith most roms.

You can see all the progress and source code in my github repository: https://github.com/drhelius/Gearboy

Cheers!

roig · Nov 27, 2012

Hello! Fast noob question... Which is the correct implementation?

I have SP = 0xFFFE

If I push a value to the stack it will
A) first decrement SP and then save the value to 0xFFFD
or
B) Save the value to 0xFFFE and decrement SP

??

Thanks

Helius · Dec 3, 2012

A is correct.

Code:

void Processor::StackPush(SixteenBitRegister* reg)
{
    SP.Decrement();
    m_pMemory->Write(SP.GetValue(), reg->GetHigh());
    SP.Decrement();
    m_pMemory->Write(SP.GetValue(), reg->GetLow());
}

djh · Dec 16, 2012

Not sure if this post is still active but I'm working on a GB emulator too. I'm aiming to get a gameboy color emulator going but I've not implemented any GBC features as of yet.

Anyway just wanted to say that after much toil I've managed to get the boot rom working, which is quite an achievement for me as I never thought I would get this far!

Next step is to get some games working (the hard part

)

roig · Dec 16, 2012

Thanks Helius!

djh i'm trying to get the boot rom working, but i don't have any idea how the GB does the video rendering.. Can you share your git repo, I want to take a look at your code

Mine is here if you want:
https://github.com/roig/GBSDL

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