MD-NICCC - a 3D Mega Drive experience by TiTAN

[notaz]

800000...9FFFFF. This area is used by 32X and SVP for example.

Not by SVP AFAIK.

[TmEE co.(TM)]

Then I remember wrong. I recall that it used one such area that 32X didn't pass through and possibly used it for itself thus SVP cannot run through it but seems only A15xxx range is used by SVP outside normal stuff. But it could just be the DMA latency issue I just read about from your docs. 32X does alter timings of the bus accesses, reducing headroom a bit.

[HardWareMan]

$A14xxx has the TMSS-related ports (on systems with TMSS, that is).

Systems without TMSS still don't assert !DTACK at those range.

...but seems only A15xxx range is used by SVP outside normal stuff.

Ok, then it will be A16000...BFFFFF without !DTACK to use more or less freely.

[notaz]

seems only A15xxx range is used by SVP outside normal stuff. But it could just be the DMA latency issue I just read about from your docs. 32X does alter timings of the bus accesses, reducing headroom a bit.

I think it's just the A15xxx range. Running a !DTACK wire from 32X's cartridge slot to it's cartridge connector makes the SVP Virtua Racing work fine.

[TmEE co.(TM)]

That's good to know, another mod to add to a 32X to make it more useful. Just need to figure out how exactly should overriding stuff work lol

[Sik]

Just looked up, SVP registers cover $A15000-$A15009 while 32X registers are all from $A15100 onwards. In other words, the exact addresses don't overlap.

That makes the whole part about leaving out SVP support particularly bad if /DTACK was indeed the only issue…

[Chilly Willy]

Yeah, that's just silly. I can understand not handling SMS pass-thru, but not passing through /DTACK?

There's a number of things I wish they had done on the 32X, like supporting 16MB flat addressed carts. The cart port has all the address lines, and the 32X maps the cart into a 32MB space. They could assert the cart select along with a full address rather than just 4MB. For the MD side, it's already using bank selecting, so you just have 16 banks instead of 4. Having a flat address space on the SH2 side would have made it a LOT easier to program big games. No weird mapper handling and worrying about switching banks to reach data. This is the one thing the GBA has on the 32X - a larger flat addressed cart space. Passing the write strobe to the cart port would have also been great. The 32X can only READ the cart space, not write it. That's why the MD side has to handle save ram and mapper handling.

[Sik]

The write strobe is definitely passed to the cartridge slot, isn't it? I mean, otherwise games wouldn't be able to save. That's a completely different issue.

I wonder if not wiring /DTACK was an attempt to thwart Game Genie and the like (what addresses are used in those cases?).

[TmEE co.(TM)]

Write strobe is passed but in only works from accesses from 68K side, SH2s can only produce read cycles in cart area. I don't know if this is a limitation due to bus management chip or SH2 configuration registers caused problem. You're not supposed to change those registers after initial setup but I'm pretty sure you can change them afterwards. I don't remember anymore what they get set to or if I even got that far...

[Sik]

I believe it's just how they decided to handle the SH-2 side for the cartridge slot.

My point was more that they did leave the strobe connected and hence they didn't remove the ability to write back to cartridge (even if only the Mega Drive side can do it). It's not like with /DTACK where not wiring it ruins it even for non-32X things.

[TmEE co.(TM)]

It actually is connected, just on VA0 32X. on VA1 it is replaced with GND connection on the cartridge end. I wonder why...

[maxwell_ln]

don't know if this is the right thread but -> NTSC version? https://demozoo.org/productions/205249/