melonDS aims at providing fast and accurate Nintendo DS emulation. While it is still a work in progress, it has a pretty solid set of features:

• Nearly complete core (CPU, video, audio, ...)
• OpenGL renderer, 3D upscaling
• RTC, microphone, lid close/open
• Joystick support
• Savestates
• Various display position/sizing/rotation modes
• (WIP) Wifi: local multiplayer, online connectivity
• and more are planned!









Download melonDS

If you're running into trouble: Howto/FAQ

If you're feeling generous: melonDS Patreon
Why there is no 32-bit build of melonDS
The main reason is that I don't have an incentive to provide 32-bit builds. Most people already have 64-bit OSes.

That being said, melonDS can currently run on 32-bit platforms. It may be less performant, as the 3D renderer does a lot of 64-bit math, but it is still possible.

But if I ever decide to implement a JIT, for example, there will be no 32-bit version of it.


If you're stuck on a 32-bit OS for hardware reasons, your computer will not be fast enough to run melonDS at playable speeds.

melonDS will be optimized, it will run faster, but it will also tend towards more accuracy. So I can't tell how fast it will be in the end. But I highly doubt it will run well on a PC from 2004. Maybe it will, if a JIT is made, but that's not a high priority task.

If you are stuck on such hardware, NO$GBA is a better choice for you. Or NeonDS if you don't mind lacking sound. Or hell, the commonly mentioned method of running DraStic in an Android emulator -- those who bash DeSmuME at every turn claim it's fast.


Truth is, emulating the DS is not a walk in the park. People tend to assume it should be easy to emulate fast because the main CPU is clocked at a measly 66MHz. Let's see:

There are two CPUs. ARM9 and ARM7, 66MHz and 33MHz respectively. Which means you need to keep them more or less in sync. Each time you stop emulating one CPU to go emulate the other (or to emulate any other hardware) impacts performance negatively, but synchronizing too loosely (not enough) can cause games to break. So you need to find the right compromise.

The ARM7 generally handles tasks like audio playback, low-level wifi access, and accessing some other peripherals (power management, firmware FLASH...). All commercial games use the same ARM7 program, because Nintendo never provided another one or allowed game devs to write their own. This means that in theory the ARM7 could be emulated in HLE. In practice, this has never been attempted, unless DraStic happens to do it. It's also worth noting that it would be incompatible with homebrew, since they don't use Nintendo's ARM7 program.

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melonDS 0.4 -- It's here, finally!


melonDS 0.4 was long awaited, and finally, it's here!

So here's a quick rundown of the changes since 0.3. I'm keeping the best for the end.


The infamous boxtest bug that plagued several games has finally been fixed. The bug generally resulted in missing graphics.

The boxtest feature of the DS lets you determine whether a given box is within the view volume. Several games use it to avoid submitting geometry that would end up completely offscreen. A prime example would be Nanostray, which uses it for everything, including 2D menu elements that are always visible.

Technically, you send XYZ coordinates and sizes to the GPU, which calculates the box vertices from that. The box faces are then transformed and clipped like regular geometry, and the test returns true if any geometry makes it through the process (which would mean that it appears onscreen). This also means that the result will be false if the view volume is entirely contained within the box.

I had no idea where the bug was, as melonDS did all that correctly, and some tests with the libnds BoxTest sample revealed nothing. It turned out that the issue lied within the initial calculation of the box coordinates. When melonDS calculated, say, "X + width", it did so with 32-bit precision. However, the hardware calculates it with 16-bit precision, so if the result overflows, it just gets truncated. And, surprise, games tend to rely on that behavior. Getting it wrong means you end up testing a different box, and getting different results. Hence the bug.


There have been various other improvements to the 3D renderer. Things have been revised to be closer to the hardware.

As a result, the Pokémon games look nicer, they don't have those random black dots/lines all over the place anymore. The horrendous Z-fighting observed in Black/White is also gone.

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Nightmare in viewport street
And another bug bites the dust... a quite old one, by the way.

Namely, it was reported two months ago in issue #18: missing character models in Homie Rollerz's character select screen. Actually, if you look closely, you can see they were there, but they got compressed to one scanline at the top, which was caused by a bug in the viewport transform.

In 3D graphics terms, the viewport defines how normalized device coordinates of polygons are transformed to screen coordinates, which can be used to render the polygons.

Most games specify a standard fullscreen viewport, but there are games that pull tricks. Homie Rollerz is one of them, the character select screen uses a 'bad' viewport. But, unlike high-level graphics APIs, the DS has no concept of bad viewport. You can input whatever viewport coordinates, it doesn't reject them or correct them.

So how does it behave? Well, if you've been following this, you surely know that the DS looks normal and friendly on the surface, but if you look deeper, you find out that everything is weird and quirky. Viewports are no exception.

That's why the bug stayed unfixed for so long. GBAtek doesn't document these cases, so it's a matter of running hardware tests, observing results, and doing it again and again until you figure out the logic.


For example, here's a test: the viewport is set to range from 64,0 to 128,192. Nothing special there.

shitty triangle

Now, we change the viewport to range from 192,0 to 128,192, which results in a width of -64 (which, by the way, OpenGL would reject). One would say that such a viewport results in graphics getting mirrored, like this:

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To make it clear
There is no Patreon version of melonDS. There will never be such a version.

Donating doesn't entitle you to anything. Not donating doesn't make you miss out on anything. You get the same package in all cases.


(this post used to be directed towards a particular news post, but it has since been corrected, so I will leave this here as a general note)


While it's nice that there are emu sites spreading the news, it is really better when they do some basic research and fact checking instead of posting mere assumptions that only cause confusion.


Thank you.
Opening to the outer world
If you have followed melonDS from the beginning, you'd know that wifi was one of the goals. And well, it's getting there.


The first melonDS release, 0.1, already included some wifi code, but it was a very minimalistic stub. The point was merely to allow games to get past wifi initialization successfully. And it even failed at that due to a bug.

Chocolate waffles to you if you can locate the bug, by the way ;)

But well, at that stage, the focus wasn't much on wifi.


It was eventually fixed in 0.2, and some functionality was added, but it still didn't do much at all. Games finally got past wifi initialization, but that was about it.

It wasn't until 0.3 that some serious work was done. With the emulator core getting more robust, I could try going for the wifi quest again. Not that 0.3 went far at all -- it merely allowed players to see eachother, but it wasn't possible to actually connect. But it was something, and infrastructure for sending and receiving packets was in place and working, as well as a good chunk of the wifi hardware functionality.


You may already know how it went back in the DeSmuME days. As far as local multiplayer was concerned, I kept hitting a wall. Couldn't get it working, no matter how hard I tried. WFC is a separate issue.

It didn't help drive motivation knowing that my work was doomed to stay locked behind a permanent EXPERIMENTAL_WIFI wall, requiring a custom wifi-enabled build, and that the DeSmuME team's public attitude is to sweep wifi under the carpet and pretend it doesn't exist, but the main issue was the lack of documentation as far as local multiplayer is concerned.


The DS wifi hardware isn't a simple, standard transceiver. It is a custom, proprietary wifi system made by Nintendo for their purposes. It has special features to assist local multiplay communication at a fast rate.

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So there it is, melonDS 0.3


So what's new in this version?

A bunch of bugfixes. This version has better compatibility than the previous one.

This includes graphical glitches ranging from UI elements folding on themselves to motion blur filters becoming acid trips, or your TV decoder breaking.

But also, more evil bugs. As stated in previous posts, booting your game from the firmware is no longer a gamble, it should be stable all the time now. An amusing side note on that bug, it has existed since melonDS 0.1, but in that version, the RTC returned a hardcoded time, so the bug would always behave the same (some games worked, others not). melonDS 0.2 started using the system time, which is what introduced the randomness.

The 3D renderer got one-upped too. Now it runs on a separate thread, which gives a pretty nice speed boost on multicore CPUs. This is optional, so if it causes issues or slows things down, you can disable it and the renderer will work mostly like before.

When I had to implement the less interesting aspects of this (controlling the 3D renderer thread), I procrastinated and implemented 3D features instead. Like fog or edge marking. You can see them demonstrated in the screenshots above.

Then I went back and finished the threading. I'm not a big fan of threaded code, but it seems to be completely stable.

However, resetting or loading a new game is still not completely stable, it has a chance of freezing. Oh and the UI still sucks. I plan to finally get at the UI shit for 0.4, and I want to ditch wxWidgets, so I don't really feel like pouring a lot of time into the current UI.

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Threads!
In the previous post, I said I wanted to run the 3D renderer on a separate thread. Well, we're going to see how all that works in detail.


Ever since 3D rendering was added into melonDS, it's been one of the bottlenecks whenever games use it. On the other hand, 2D rendering, while not being very well optimized, doesn't make for a big performance hit.

2D rendering isn't very expensive or difficult though -- the 2D renderers are oldschool tile engines, essentially drawing raster graphics onscreen at the specified coordinates, optionally with a bunch of fancy effects, but nothing too complex. In comparison, the 3D renderer is a full-fledged 3D GPU. It basically turns a bunch of polygons defined in 3D space, into a 2D representation that is then passed to the main 2D renderer and composited mostly like a regular 2D layer.

Transformations by various matrices, culling, clipping, viewport transform, rasterization with perspective-correct interpolation... it's a bunch of work.


The approach originally taken was to render a whole 3D frame upon scanline 215. I'm not sure whether rendering should start upon scanline 214 or 215 (GBAtek says it starts "48 scanlines in advance"), but melonDS starts at 215.

Which basically meant that the emulator had to wait until the whole 3D frame was rendered before doing anything else.


However, in emulation, you can't just throw everything on separate threads. Considering a component, whether you can put it on a separate thread depends on how tightly it is synchronized to other components.

This excellent article from byuu explains well how all this works.

In the case of our DS 3D renderer, threading is feasible because tight synchronization isn't required. Once the renderer starts rendering, you can't alter its state, the polygon and vertex lists are double-buffered and all the other important registers are latched. The only thing you can do is change VRAM mapping, but I have yet to come across a game that pulls stunts like swapping texture banks mid-frame.

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Slicing the melons!
So here are the two main goals for melonDS 0.3: threading the 3D renderer, and starting work on wifi connectivity.


The first goal basically aims at running the 3D renderer in parallel with the rest of the emulator. On the hardware, the renderer's state can't be altered while it is rendering, so the timing doesn't have to be precise, and we can use it to our advantage. As the current 3D renderer is a bottleneck, threading it should give a nice speed boost for multi-core CPUs (which are quite the norm nowadays).


The second goal doesn't mean wifi will work, but hey, we need to start somewhere.

Well actually wifi emulation has already been upped a notch compared to 0.2. The wifi RAM and associated registers are functional, as well as most of the timers. What remains to be done is functionality for sending/receiving packets. Power management and specific multiplayer features also need proper investigation. Then we have things like the RF and BB chips, which will likely never be fully emulated since they control very low-level aspects of wifi, like "how much energy should it take to consider we're receiving data".

So where does this get us? I have tested Pictochat and NSMB multiplayer, and in both cases, the host sets up a beacon and attempts to send it regularly, which is a good sign.

(The beacon is a packet regularly sent by wifi access points to advertise their presence. Since the DS doesn't support ad-hoc communication, multiplayer games use a similar scheme to communicate, typically with the first player acting as a host and other players being clients.)

Anyway, don't get too hyped over this, there's nothing too new here. DeSmuME and NO$GBA both get atleast this far if not further.


In the meantime, I've been implementing another obscure feature of the DS: writable VCount.

Old consoles typically have a register that reflects which scanline is being drawn onscreen. There are various names it can be called (LY on the GameBoy, VCOUNT on the GBA/DS), but it's essentially the same thing.

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More fun fixes
So as I said in the last post, I've been fixing the firmware boot issues. Basically, when booting from the firmware, there was a high chance that the game would crash upon boot. But the bug and its effects were random. Sometimes it would simply hang on a white screen, sometimes it would jump to an invalid address... and sometimes it would work fine.


So we're going to see how we try to fix a random bug.


First thing to do is finding a way to reproduce the bug consistenly. So, hoping my bug would be affected by the time taken before clicking the health/safety warning screen, I set up a quick hack to automatically click the screen after a fixed number of frames. That didn't cut it, so I made the RTC return a fixed time. Which finally allowed me to reproduce the bug reliably.

I landed on a variation where the ARM9 jumped to an invalid address, which made it easy to find where it was doing that. I then started backtracking, which eventually led me to find out that some data were being copied from the wrong place, but it appeared the bug had more consequences than anticipated, making the backtracking long and tedious.

So instead, I dumped the RAM at the time the game booted, and compared it with a RAM dump from before a direct boot. It appeared that a chunk of code got accidentally erased. The range that got erased was within the cart's secure area, so I checked the code that handled secure area reads, but the bug wasn't there.

So I tracked where that code region was getting erased. The bug appeared, and it was another stupid bug. It turned out to be a DMA from the ARM7 accidentally doing that.

The bug was that during cart transfers, melonDS tried to start DMA for both CPUs. The cart interface can only be enabled for one CPU at a time, and thus DMA should only be checked for that CPU.

The bug resulted from a combination of factors:

1. The ARM7 BIOS loads the cart secure area, using DMA. When it's done, it leaves the DMA enabled(!). So when the ARM9 went to load something else from the cart, it accidentally triggered the ARM7-side DMA.

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Fixing Pokémon White
This post will get into the bugfixing process for a particular bug. Bugfixing in emulation may look like black magic, but it's not so different from general bugfixing-- it boils down to understanding the bug and why it occurs. Of course, emulation makes it harder when it involves blobs of machine code for which you have no source code, but nothing insurmontable.


Anyway, the issue with Pokémon White (and probably others) was that the game wouldn't boot unless it was launched from the firmware. Not really convenient, especially as at the time of writing this, firmware boot in melonDS is unstable.

I first suspected the RAM setup done prior to a direct boot (NDS::SetupDirectBoot). There are several variables stored in memory by the firmware, which games can use for various purposes. For example, the firmware stores the cartridge chip ID there, games then check the cartridge chip ID against the stored value, and throw an exception if it has changed (which typically means that the cart was ejected).

However, some testing revealed that there was nothing the direct boot setup was missing that could have broken Pokémon, atleast regarding the RAM.


So I had to dig deeper into the issue. It turned out that during initialization, the ARM9 got interrupted by an IRQ, but for some reason, it never returned to its work after processing the IRQ.

DS games often use multiple threads, so it isn't uncommon to switch to a different task after an IRQ. But that wasn't the case here, it wasn't even picking a thread to return to. It got stuck inside the IRQ handler.

The IRQ occured upon receiving data from the IPC FIFO. In particular, the ARM7 sent the word 0x0008000C. The ARM9-side handler was coded to panic and enter an infinite loop upon receiving this word.

More investigation of the FIFO traffic showed that the ARM9 first sent the word 0x0000004D, which is part of the initialization procedure. To which the ARM7 replied with 0x0008000C. But it appeared that the ARM7-side FIFO handler was coded to do that. For a while, that stumped me. I couldn't understand how it was supposed to work.


I then logged FIFO traffic when booting the game from the firmware, whenever it successfully booted, to see where the exchange differed. The ARM9 sent 0x0000004D, to which the ARM7 replied 0x0000004D. But it appeared that in that case, the ARM7 was using a different handler.

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