Technical aspects of the Sonic games

For a while now, I've had a few questions poking at my brain, with no answer available to calm them down. Questions about intricate aspects of the Megadrive Sonic titles, to whom which I had insufficient knowledge or no answer at all to solve. I haven't posted them before mainly just because I'm lazy, and because I keep forgetting them. Today, I remembered some, I'm near my computer, I have a cold, it's over 2 AM -- it's the perfect time. Without further ado:


1. Sonic 2 Special Stages

There's two particular aspects about the special stages in Sonic 2 that I could never quite understand:

1) Why does it use the 256x244 display mode rather than the full 320x224 resolution? The only other things that use the mode in Sonic games are the Sonic CD title/special stage, with the intent of using the "Mode7" display mode for the clouds and level track, respectively. Sonic 2's special stage, however, doesn't use it, so I can't see any justification to it except that it requires less 8x8 tiles to be displayed in order to fill the screen. This is really the only explanation to make sense, but then again I think about Sonic 3's special stage, which is pretty busy itself, but manages to use the full 320x224 mode. Is the Sonic 3 special stage less intensive, or is the reason for using 256x224 in Sonic 2's a different one?

2) Why are the graphics for the stage doubled up in scale? Are the graphics themselves stored in the ROM like that, or does the game blow them up via software scaling to be displayed on screen? If so, what's the reason for that? Saving ROM space, or is it a video memory limitation of sorts?

In both cases, would it be technically possible to implement the Sonic 2 special stage in full 320x224 resolution with un-blown up graphics?


2. 2P Versus mode oddities

Again, these are actually two questions in one. Sonic 2's 2P mode works perfectly with the selectable stages (Emerald Hill, Casino Night and Mystic Cave), however, the same cannot be said about the remainder of the stages:

1) Most stages do not display correctly. Most of them have weird artifacts floating across the entire stage, and in some the 2P scene is not drawn correctly and the camera doesn't scroll the level fluidly, but rather in "chunks" (which I assume is the visible area of the level as it's loaded to memory). What I'm wondering is why does this happen? Why does, say, Chemical Plant not draw correctly, but Casino Night does? And about the 2P scene not behaving correctly - does it have to do with special level settings such as vertical wrapping, etc? In any case, would the levels be fixable in order to work properly in 2P mode? And if so, would it be at the expense of removing features from a level?

2) Levels with water do not load at all. This must be a condition set by the game itself, as water must seriously fuck up 2P mode. Why does water not work, could it be made to work without major alterations to the level itself, and if so, how?


3. The Megadrive NTSC display border

This isn't really a Sonic question, but it's Sonic-related. I've noticed that in Sonic 1 and 2, the little border on the top and bottom of the screen (covering the empty space left from displaying a 320x224 image in a 320x240 screen) seems to take the color of the first index in pallete line 1 (or something to the effect), whilst as in Sonic 3 (and Knuckles) this border is always black (save for the Sega screen and the level select, which is hangover from Sonic 2). It's obvious that the color the border displays can be conditioned by software, but how exactly? Can you directly tell the system which color to use, or does Sonic 3 go around this and arranges it so that the border is always black?

I'll post more as they come back to me.

 

Thanks for the answers so far, guys. You've helped a bunch.

This doesn't answer one of my questions, though: is it possible to make all the other stages "work properly" (as far as resolving all drawing issues goes, anyway), save for the ones with water? And if so, would it be necessary to cut down something in the stages themselves?

You say that "the unsupported levels don't have the code". Would the camera behave correctly if you just added the code to the remaining stages/made it global to all stages?