[Sega Genesis to Sega CD] ROM converter

[Sean697]

I would rather think that a complete emulation of a CD Drive would be possible. But would require a chip to run that emulation. Taking ISO input for CD data and outputting the same data as a CD drive to the sytem. No small task. But would be cool to have an Everdrive like device running of of an FPGa or something that plugged into the CD drive ribbon cable and stored ISO files on an SD card or hardrive. What if you could just hijack the interface between the laser and the CD data itself. Directly feed the data from a file as bits to to the laser. Would not have to emulate the entire CD drive. Would just have to have a good understanding for the way Data is handled and read for the CD spec and emulate the physical spinning of the disc and know what sector the CD was trying to locate. I'm not a real technical person but but I've never seen anyone take that approach. Probrably why the easiest way is to just make a Bios modification than try to emulate all of that.

[Drakon]

I would rather think that a complete emulation of a CD Drive would be possible. But would require a chip to run that emulation. Taking ISO input for CD data and outputting the same data as a CD drive to the sytem. No small task. But would be cool to have an Everdrive like device running of of an FPGa or something that plugged into the CD drive ribbon cable and stored ISO files on an SD card or hardrive. What if you could just hijack the interface between the laser and the CD data itself. Directly feed the data from a file as bits to to the laser. Would not have to emulate the entire CD drive. Would just have to have a good understanding for the way Data is handled and read for the CD spec and emulate the physical spinning of the disc and know what sector the CD was trying to locate. I'm not a real technical person but but I've never seen anyone take that approach. Probrably why the easiest way is to just make a Bios modification than try to emulate all of that.

I've asked tiido about this. He says it would be easier using a fpga to recreate the entire cd device and I guess interface at the console expansion port. A re-written bios is an excellent idea.

[FuturePrimitive]

I've asked tiido about this. He says it would be easier using a fpga to recreate the entire cd device and I guess interface at the console expansion port. A re-written bios is an excellent idea.

Oh wow, that would make dreams come true! An FPGA device interfaced to the expansion port would allow Sega CD games to survive much longer!

[Mask of Destiny]

So the only Game will fit in the Sega CD RAM is Sonic for the Mega Drive?

Not exactly. The Sega CD has 768KB of RAM. There are probably a fair number of Genesis/MD games that would fit in that much RAM (though a lot of the more notable ones are too big), but it's not that simple. The first problem is that the RAM is split into two parts. The bigger one is 512KB (Program RAM) and is intended to be used by the CPU in the Sega CD only. The rest (256KB) can be easily used by the Genesis CPU, but is too small for all but a handful of games (Columns is one of the few that would fit). There's a couple of ways to use the whole 768KB, but they all bring their own challenges. There's the way that's been mentioned in this thread (disabling the sub CPU and using the banked access mechanism) which is problematic because Genesis games are not generally designed for banked access to RAM. Another approach is to run most of the code on the Sega CD CPU, but this is even more problematic as all the hardware the game would normally access is not directly accessible from that CPU.

On top of that problem, there are other challenges. One is that making a perfect disassembly of a game is incredibly difficult to do automatically and a perfect disassembly is pretty much required for any of the approaches a converter would take. Additionally, there are some quirks with doing DMA from Word RAM (the 256KB chunk) to the VDP which need to be accounted for. Sound code needs to be modified to deal with the fact that there's no RAM the Z80 can samples from IIRC.

[Drakon]

Oh wow, that would make dreams come true! An FPGA device interfaced to the expansion port would allow Sega CD games to survive much longer!

I'd prefer to still use the original segacd unit and just replace the cd drive. But if it's easier to plug something into the expansion port....fine.

[GhaleonX]

Interesting.

Be cool to see another Sega CD Volume, like Volume 2 Classics series with SOR2, Vectorman, Comix Zone, Midnight Resistance and Strider or something.

Does the original 5 in 1 volume use cdda wavs from the composers mastered studio recordings put on that cd, or just the genesis rom/cart music dubbed onto cd ?

What would be even cooler is some of the home-brew hacks we could see from it. I'd love to make a Sega CD version of Ys III that combines the best elements between the Genesis and Turbo CD versions of the game!

[Scorpion Illuminati]

256KB of Word RAM in one place, and 128KB of Program RAM in another that you can use if you disable the CD 68000 after loading the game. So you can see, the games can't be very big. CD targeted games split everything by levels... you aren't loading the whole game, just one level. ROM games don't work like that, and tend to be hard to split into separate levels.

Does that include the genesis's original ram:

Memory
System RAM: 136 KB
Main RAM: 64 KB (repeated over the upper 2 MB of address space)
VRAM: 64 KB
Audio RAM: 8 KB
VDP internal RAM: 152 bytes [24]

Or is that inaccessible while the sega cd is attached?

Cite: SegaRetro

[Mask of Destiny]

Those RAMs are accessible, but a Genesis game will be already using them for things.

[xelement5x]

What would be even cooler is some of the home-brew hacks we could see from it. I'd love to make a Sega CD version of Ys III that combines the best elements between the Genesis and Turbo CD versions of the game!

Mask of the Sun was slated for the MegaCD as well before the Sega/Falcom partnership ended. At least we got Popful Mail out of it.

[thegrreat]

Converting sega genesis to SegaCD is possible "Not all games tho" But it requires alot of work and coding knowledge.

[GreyFox]

If there is 256kb easily accessible by the Megadrive/Genesis might it be more feasible to port smaller Master System games?

Wouldn't 1 Megabit and 2 Megabit games fit in this?

The Megadrive/Genesis is capable of running them as the Powerbase is purely a passive pin adaptor I believe?

Obviously not saying it would be easy but some CD compilation disks of smaller Master System games would be ace.

I am likely barking up completely the wrong tree though