melonDS changes to teakra

[Arisotura]

teakra has been retrofitted into melonDS back in 2021, to emulate the DSi's DSP.

I've had to modify teakra to make it work in this context. I was thinking maybe some of the modifications could interest you? If so, I could work out something and make a PR.

Namely:

1. I added the retd instruction, which is used in aac.a (DSi Sound App ucode). The way I did it is a hack (it executes two instructions before returning -- should be two cycles).
2. Similarly, commented out an exception in OffsetAddress(), because it was getting hit. The way the function works seems to be correct for this use case.
3. aac.a kicks DMA by writing to 0x8184, so I had to add support for that.
4. I added support for driving the BTDMP with an external clock -- on the DSi, it is driven by the I2S clock, not the internal timer.
5. I also added support for BTDMP input -- on the DSi, it's used for mic input. I don't know if it's used on the 3DS. It seems to function mostly the same as BTDMP output, except the FIFO empty bit is backwards.
6. I modified teakra to take callbacks to read/write memory, so that it could directly access melonDS's NWRAM blocks. Handling it this way is probably not the most efficient, but it was less likely to cause problems than trying to keep teakra's internal buffer in sync with melonDS's memory.

I've also observed a bug with the graphics ucode's scaling function, which suggests that the AHBM functions differently wrt alignment on the DSi, but I'll need to investigate that one.

That's about it.

[wwylele]

I'll give a more technical thought and might reply more on that later. This comment is more like venting...

After losing the original parent, Teakra didn't really have a driving force for development. I planned to soft-archiving it, forget about the lost and sad stories, and move on. Now I realized Teakra still has active user, and what's worse, there are multiple of them, each with their good ideas. As I have lost a lot of the context, I don't think I have the capacity to host the discussion and choose the right design for everyone.

Perhaps I should wait and let other users chime in.

[Arisotura]

since I saw recent commits, I didn't guess its history was rocky like that, sorry to hear

as far as melonDS is concerned, teakra is mostly a reference (that we may adapt to fit our needs). The performance doesn't make it a very practical solution from an end-user perspective... I've been working on DSP HLE, and also have some plans for a DSP JIT later down the road...