Yeah, there's all kinds of things you can make use of. Of course, there's CD audio playback, but there's also 8 channel PCM, graphics operations (both scaling/rotation and also bitmap -> tilemap conversion), and reading any kind of disc data (i.e. FMVs, extra content (dare I say, DLC?), etc.). There's also more generic uses like offloading tasks to the Sub CPU, or just using the additional 256 KiB of Word RAM for whatever. You can have all that without needing to make a disc game, provided that you can initialize the Sega CD hardware and get the Sub CPU up and running, since the BIOS isn't booted into to do that for you (*cough*). I'm hoping that the usage of mode 1 outside of CD audio playback gets more attention, because there really is so much more you can do. Kega Fusion and Genesis Plus GX support it (albeit VDP DMA from m68k memory is wacked out in Kega Fusion in mode 1, because it still treats memory as if it was in mode 2 for them, and without the cartridge actually being accessible either). With Sega CD support being added to BlastEm as of late, I'd imaging that mode 1 will be supported if it isn't already. Of course, this also works on hardware, all you need to do is have a Sega CD plugged in and just run the cartridge ROM. ...but for now, this is what I got.
Blastem's nightlies already support Mode 1 (your Mode 1 Demo runs flawlessly in it), and already it seems to be more accurate than Genesis Plus GX at this point. GX actually appears to be terrible with regards to the Sega CD's finer hardware details; testing a Mode 1 tool I wrote revealed a couple of severe inaccuracies in its Sega CD emulation, one of which, as far as I'm concerned, makes it utterly useless for any Sega CD development (both of which I'll be discussing when I release the tool in the next few days).
Question about the whole mode1 thing. I recall tech members here talking about the bus between MD and MCD being slow as fuck. I see on your video that the game lags like there's no tomorrow when the cliff breaks, is that because of said bus (or other bottlenecks that can happen while using the Sega CD), or is it just cos it's a demo and there are unpolished stuff? Other than that, Mode1 really looks like literally an arcade machine hardware. You get scaling, rotation, another powerful CPU (if you can use it without too many drawbacks that is), you keep 6 synth, gain 8 damn PCM channels, you still have the PSG... save for the still limited colour palette, that's some killer hardware for real.
It's very unpolished, and Sonic 1's engine wasn't designed for this. Currently, I'm manually copying the rendered tiled to be arranged for sprites, but I'm probably gonna port the DMA queue and do a series of DMAs straight from Word RAM, and also optimize Sonic 1's engine some. I also decreased the size of the sprite canvas after I made that post and got better performance, because less data to copy.
While you can get scaling to work, it should be noted that Chaotix uses the 32X to render sprites, and all 4 Genesis palette lines for the stages. The Sega CD's graphics capabilities only involve transforming/converting an image, which the programmer then transfers into VRAM, so you are still limited by what the Genesis VDP can handle. A conversion from the 32X to the Sega CD would still require a lot of work to get *all* of the graphics to fit within the Genesis limitations and look okay.
For what it's worth, if you set Word RAM to 1M/1M mode (Word RAM is split in half with the Main CPU having access to one half and the Sub CPU having access to the other), the graphics operation changes into a sort of bitmap to tilemap converter. The idea is that a Word RAM bank will contain a plain 2D array of pixels, with each byte still containing 2 pixels. The Sub CPU side provides a mirror of its Word RAM bank in which each pixel/nibble is assigned into separate bytes, making the drawing process easier to deal with. The Main CPU side provides its own mirror of its Word RAM bank in which the data is arranged into tiles that can be directly copied over into VRAM. There are multiple canvases to work with. At the start of a Word RAM bank is a 256px height canvas, followed by a 128px height one, then a 64px height one, and then 2 32px height ones. The output tile data is arranged vertically (i.e. the tile #2 is to be placed below tile #1), allowing for a variadic width (up to 512px), since you can just copy up to only what you need. (btw, it's meant to say "cell", not "dot" in those 2 sections) Basically, the idea is that on the Sub CPU side, you write your pixel data into the Word RAM bank mirror, which will then write your pixels into the appropriate nibbles. After drawing, you swap Word RAM banks, and on the Main CPU side, you copy the output tile data from its Word RAM bank mirror into VRAM. Basically, this was Sega's solution for software rendering on the Genesis before the 32X.
Way too many star posts in the game now. It is time to spice things up a bit (and BTW bring back coherency with other classic Sonic games).
Had a burst of inspiration over the weekend and cooked this up. This mod overhauls the Boost mechanic to be more similar to the dash from Spark the Electric Jester. Take a look:
