If you can't play ROMs, could you...?

[arromdee]

If you can't play ROMs, could you...?

Would it be feasible to have a [SM]ega CD which worked similarly to the Super Hu-Card for the TG-16/PCE (http://www.zeograd.com/super_hucard_download.php)? It doesn't play plain ROMs, but it can play ROMs that are specially hacked to work with it and that are small enough to fit in the Super CD memory. Is memory laid out in a [SM]ega CD in such a way that it could be possible there too?

(I'm aware that whether it's feasible or not it doesn't actually exist...)

 

[Mask of Destiny]

If you can't play ROMs, could you...?

The problems they had to overcome with the TG-16 aren't all that different from the ones with the Sega CD; however, there are a few things that make it a little more difficult with the Sega CD.

First is the fact that the 6502 derivative in the TG-16 has a segmented memory space whereas the 68000 in the Genesis/MD has a flat memory space. The flat memory space is convenient for programmers, but makes it more difficult to fix all the pointers since you have to track down every single absolute memory reference whereas in the TG-16 you just have to find the places where it changes banks.

The second problem, which I don't believe to be a problem on the TG-16 (though I could be mistaken), is that the Sega CD uses part of normal Genesis RAM for an interrupt jump table. If the hacked Genesis game tries to write to this area, the system will crash.

The third problem, is that unlike with the TG-16, very few Genesis games will fit in 256KB of RAM. 512KB ROMs could be handled by using Sega CD Program RAM for the last 256KB, but the Genesis 68K can only access that 128KB at a time so you'd have to hack in some bank switching code. Games that have more than about 700KB or data or so, will have the added complexity of having to read off the cd drive from time to time.

None of these problems are completely insurmountable, but producing an automated solution is not an easy task. The closest we have so far is a modified version of Gens that will work with IDAPro to produce a very accurate disassembly, but even this results in an disassembly that needs some work before it can be used and it doesn't make any attempt to hack in solutions to the problems listed above.