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Slicing the melons!
May 10th 2017, by StapleButter
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.
The DS has the particularity that the VCount register can be written to. This feature is typically used to synchronize consoles during multiplay, but there are other possible uses -- ZXDS uses it to bring the refresh rate to 50Hz.
GBAtek doesn't provide a whole lot of details about this and how it works, besides a recommended VCount value range of 202-212. The I/O map listing also hints that VCount is only writable from the ARM7, but that seems to be a copy-paste error, I observed that VCount is writable from both CPUs.
So I set out and investigated it, which gave fun results, as always with the DS.
I immediately tried writing to VCount during active display (scanlines 0-191). And, surprise, it works. The GPU then continues drawing from the updated VCount, but the screens keep going from their old position as the LCD controllers can't skip forward or backward.
It's also worth noting that VCount writes are delayed until the next scanline, which has the advantage that it makes timings reliable.
The effects of VCount writes only last for one frame, things go back to normal after the end of the frame (unless you write to VCount every frame).
Implementing this feature in melonDS was also the perfect occasion to make the framerate limiter suck less. It had to be upgraded anyway in order to support adjusting the framerate cap based on how long the frame lasted. So for example, if you ran ZXDS, melonDS would run at 50 FPS instead of 60. The new framerate limiter is also more accurate, which makes for nearly perfect audio output.
(ZXDS wouldn't run currently due to the lack of DLDI support, but you get the idea)
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