Could the Sega CD have run Doom ?

[Blzut3]

Yeah I would imagine for a pure CD only game the amount of RAM could be a problem since IIRC the Jaguar version needs about 2MB (that's RAM memory mapped ROM) and the Sega CD doesn't even have 1MB to work with.  However I would imagine given the 32X+CD was a thing, it would be possible to pair a CD with a base system cartridge as well.  Then the question is if the currently loaded level data can fit into the RAM for the right CPU.  There's still a lot of architecture details that could easily get in the way that I'm not qualified at all to comment on.

 

Ultimately the homebrew that Individualised has been linking suggests that it would be possible to run a version of Doom on the base hardware.  The author of that mentioned in the comments in one of the videos that Doom level geometry would need to be cut down but I would imagine you could get some decent results.  I think whether the Sega CD could be used to store additional levels or improve performance really would come down to some technical details about the connection between the units.

[Individualised]

In case anyone was wondering, here's actual Doom ported to the Mega Drive: https://github.com/krikzz/doom-68k

Runs in greyscale and small screen size. One frame per every 2-3 seconds, which is actually better than I would have expected. Unfortunately cannot be emulated or ran from a normal cartridge as it expects an Everdrive to be able to read a .WAD file, however it doesn't take advantage of any extra power of the Everdrive unless you specifically use that version, the game runs on the stock Mega Drive hardware

Edited July 15, 2023 by Individualised

[Vic]

9 hours ago, Individualised said:

however it doesn't take advantage of any extra power of the Everdrive unless you specifically use that version, the game runs on the stock Mega Drive hardware

It does not on stock MD hardware, It runs on the MEDPro itself.

 

And no, Doom would not be possible on the SegaCD alone. Even a CD32X combo would be severely hadicapped by the lack of RAM on the 32X and would still require a cart to store most of the game's assets.

[Marsguy]

Does this mean the Mega Drive or 32X uses the cartridge storage space as slow RAM?

[Vic]

10 minutes ago, Marsguy said:

Does this mean the Mega Drive or 32X uses the cartridge storage space as slow RAM?

It does

[Individualised]

4 hours ago, Vic said:

It does not on stock MD hardware, It runs on the MEDPro itself.

I think you're thinking of a different port (which I'm also aware of). That one runs in full colour and full screen with sound. This is different and allegedly everything runs on the 68k unless the readme.txt is a fabrication. It must be using the Everdrive for RAM though? It's obviously not fitting everything into the stock 64kb

4 hours ago, Vic said:

And no, Doom would not be possible on the SegaCD alone. Even a CD32X combo would be severely hadicapped by the lack of RAM on the 32X and would still require a cart to store most of the game's assets.

It really depends on how open your definition of Doom is. I don't think anyone was expecting actual Doom (as in id Tech 1) to run, but if you include things such as custom engines (think RL/SNES Doom) and simplified levels the question becomes more interesting. I demonstrated that such is possible in theory, or at least the rendering part of it is.

Edited July 16, 2023 by Individualised

[TasAcri]

4 hours ago, Marsguy said:

Does this mean the Mega Drive or 32X uses the cartridge storage space as slow RAM?

 

I thought all cart based consoles use the carts this way in all their games. That's why they always have a tiny amount of RAM, they don't need much if they stream assets directly from the cart.

 

The Neo-Geo CD had a massive (for the time) 7 MB RAM because of the transition from carts to CD. The Rom based AES/Arcade didn't need that.

Edited July 16, 2023 by TasAcri

[Individualised]

8 minutes ago, TasAcri said:

 

I thought all cart based consoles use the carts this way in all their games. That's why they always have a tiny RAM, they don't need much if they stream assets directly from the cart.

 

The Neo-Geo CD had a massive (for the time) 7 MB RAM because of the transition from carts to CD. The AES didn't need that.

This is correct. In fact the NES was built around this kind of design so much that it doesn't even have any proper VRAM and reads video data directly from the cartridge, from a separate ROM chip (CHR-ROM) than the game program (PRG-ROM) meaning all graphics assets are static unless you include VRAM (called CHR-RAM in Nintendo's official terminology) on the cartridge. It also only has 2kb of RAM in general which was hilariously small even for the time, so much so that a good portion of games had to include extra RAM (PRG-RAM) on the cartridge because certain types of games (think big scrolling games with worlds that you can modify, like Super Mario Bros. 3) are just impossible to do on 2kb. That's why you can't scroll back in Super Mario Bros. 1.

Edited July 16, 2023 by Individualised

[Vic]

11 minutes ago, Individualised said:

I think you're thinking of a different port (which I'm also aware of). That one runs in full colour and full screen with sound. This is different and allegedly everything runs on the 68k unless the readme.txt is a fabrication. It must be using the Everdrive for RAM though? It's obviously not fitting everything into the stock 64kb

Regardless, it's still unplayable.
 

13 minutes ago, Individualised said:

It really depends on how open your definition of Doom is. I don't think anyone was expecting actual Doom (as in id Tech 1) to run, but if you include things such as custom engines (think RL/SNES Doom) and simplified levels the question becomes more interesting. I demonstrated that such is possible in theory, or at least the rendering part of it is.

My definition of Doom is inclusion of the core gameplay, all weapons, the majority of Doom1 monsters, textures and sounds, no geometry distortions and levels consisting more than a few rooms and corridors and a window size larger than a post stamp. Anything else would be a Doom facsimile.

[Individualised]

9 minutes ago, Vic said:

My definition of Doom is inclusion of the core gameplay, all weapons, the majority of Doom1 monsters, textures and sounds, no geometry distortions and levels consisting more than a few rooms and corridors and a window size larger than a post stamp. Anything else would be a Doom facsimile.

That's already kinda excluding Jaguar Doom which has a very small selection of textures. If by geometry distortions you mean low precision geometry rendering then SNES Doom has that issue. I do think everything you say would be possible based on my limited knowledge of the hardware though. You'd have to mask some limitations though, while you may be able to have a large texture roster you wouldn't be able to have more than a few textures used at a time, but PlayStation Doom and I'm assuming other Jaguar-based ports is like that. Same with monsters.

 

The engine I showed off is open source: https://github.com/ehaliewicz/Manifold

Might have a go at doing something with this as a proof of concept. Because of the limitations of the engine (convex sectors only) levels could never be automatically converted/remade 1:1, but I don't think that's a huge detraction from it being Doom as Saturn Quake also has remade, not 1:1 levels (it uses a Build-style engine) yet that game is still very clearly Quake. This would be similar to that.

Edited July 16, 2023 by Individualised

[TasAcri]

33 minutes ago, Individualised said:

That's already kinda excluding Jaguar Doom which has a very small selection of textures. If by geometry distortions you mean low precision geometry rendering then SNES Doom has that issue. I do think everything you say would be possible based on my limited knowledge of the hardware though. You'd have to mask some limitations though, while you may be able to have a large texture roster you wouldn't be able to have more than a few textures used at a time, but PlayStation Doom and I'm assuming other Jaguar-based ports is like that. Same with monsters.

IMO if the game looks and plays close enough to DOOM, then it's DOOM. Jaguar DOOM and the ports based on it have some simplifications, sure, but it's still much closer to the original DOOM than, say, Wolfenstein 3D or any basic orthogonal level design shooter. Even the SNES version that lacks textures on floor/ceiling is still a lot closer to the original DOOM than that so it earns the title.

 

On the other hand, Duke Nukem on the Genesis is just Wolfenstein 3D with a Duke sprite mod on top of it. Because technically, it does the same things the original Wolfenstein 3D does and almost none of what Duke Nukem 3D does. Same with other DOOM projects where the only thing similar to Doom is some art assets and the fact that it's in first person.

 

Think of it as a slider where one end is Wolf3D and the other is the original DOOM. If it's closer to the original DOOM, i can accept it as a DOOM port/remake. Though in almost all cases, the results are clear as day to me. Ports and remakes so far are either too close to Wolf 3D or too close to DOOM, there is no example i can think of where it sits close to the middle and i can't decide if it's really a DOOM game for me or not.

[Herr Dethnout]

1 hour ago, TasAcri said:

The Neo-Geo CD had a massive (for the time) 7 MB RAM because of the transition from carts to CD. The Rom based AES/Arcade didn't need that.

Just imagine a Doom Port on the Neo Geo...

[Individualised]

32 minutes ago, Herr Dethnout said:

Just imagine a Doom Port on the Neo Geo...

I could be totally wrong with this as I'm not familiar with the Neo Geo at all but as far as I know this may be impossible due to the way Neo Geo graphics work - it's not possible to have a bitmap display at all - and I don't just mean there's no bitmap modes, the SNES or Mega Drive doesn't have any either, it's literally just not possible at all, you can't even make a bitmap out of tiles or sprites like you can on the aforementioned consoles. I'm not sure why or what the specifics are as I'm not familiar with the hardware so for all I know I might be completely wrong, but I do remember someone bringing this up (maybe even on this forum) as one of the reasons why the Neo Geo isn't as powerful of a platform as many are lead to believe, and how certain game genres are just impossible to develop for it.

 

Found what I was thinking of in the comments section of this Reddit thread:

TL;DR while Neo Geo games look impressive it's mostly due to the games using massive ROM sizes that allows them to fake and pre-render cool effects, and the platform itself is in many aspects worse than the SNES or Mega Drive. The things that it does excel at, other than raw power, are only good for very specific use cases and game genres. No Doom on it any time soon

Edited July 16, 2023 by Individualised

[Marsguy]

John Carmack has described that the Jaguar's M68K is insufficient to run the Doom 1 engine on its own, the Sega CD has roughly the same CPU with the same clock speed. Even the CPUs from SAT/PS1 and 3DO are weak here, the problem is not the graphics but the game logic of the engine, which doesn't really run well on the Risc CPUs of the time.

 

This is probably also the reason why the Doom Engine never had a great future on the Playstation. While the Quake II survives and is used to this day. Also on consoles ...

 

Certainly a maze FPS would be possible on the Sega CD, but not DOOM! Something better than Bloodshot would certainly be possible.

Edited July 16, 2023 by Marsguy

[Herr Dethnout]

1 hour ago, Individualised said:

TL;DR while Neo Geo games look impressive it's mostly due to the games using massive ROM sizes that allows them to fake and pre-render cool effects, and the platform itself is in many aspects worse than the SNES or Mega Drive. The things that it does excel at, other than raw power, are only good for very specific use cases and game genres. No Doom on it any time soon

Well, techtechnically it can run Doom it you count the whole NG family (cuz Neo Geo 64 exist although got the same success as the CPS-3).

 

One workaround would be doing the same as The Super Spy... But it would take a ridiculous ammount of size just for one level... So is kinda a yes and no (More No than yes heh)

[Roebloz]

More than likely not, I mean it struggles to run on the 32x. (I would know ;D)

However, I think one thing that could be funny is trying to turn Doom into a like, rail shooter for the Sega CD, that would definitely work.

[Eurisko]

I hope this thread prompts someone to have a go at getting it to run on Sega CD. Would love to see it. 

Edited July 27, 2023 by Eurisko

[Individualised]

38 minutes ago, Eurisko said:

I hope this thread prompts someone to have a go at getting it to run on Sega CD. Would love to see it. 

From here what can be done is very open ended. We've already ruled out the possibility of running actual, original Doom on the Sega CD so it's unlikely this thread will really prompt anyone to do anything in that regard. Maybe someone will try and port Erik Haliewicz's BSP engine to the Sega CD as a result of seeing this thread. Otherwise any attempts of doing a Doom game on Sega CD will probably be done by someone who was already passionate about trying to make that happen.

[Individualised]

After playing the ROTT remaster, it occurred to me that a more interesting question would be can ROTT be done on the Sega Mega Drive/Mega CD? Wolfenstein 3D has been fully ported already, using the original source code hand-converted to 68k assembly and optimised. Given that ROTT's engine is based on Wolf 3D's, maybe it's not out of the question?

Edited August 1, 2023 by Individualised

[aboyes1989]

9 hours ago, Individualised said:

After playing the ROTT remaster, it occurred to me that a more interesting question would be can ROTT be done on the Sega Mega Drive/Mega CD? Wolfenstein 3D has been fully ported already, using the original source code hand-converted to 68k assembly and optimised. Given that ROTT's engine is based on Wolf 3D's, maybe it's not out of the question?

 

It's based on Wolf 3D's, but there is so much more going on in ROTT that I think a lot of simplifying to the levels and gameplay would have to be made. I've not played it for a while so I can't remember how big the enemy count is or the texture variety, etc. The gameplay itself, however, is bonkers and fast paced, so I wonder how that would translate to a Mega Drive/Sega CD port.

[Herr Dethnout]

18 hours ago, Individualised said:

After playing the ROTT remaster, it occurred to me that a more interesting question would be can ROTT be done on the Sega Mega Drive/Mega CD? Wolfenstein 3D has been fully ported already, using the original source code hand-converted to 68k assembly and optimised. Given that ROTT's engine is based on Wolf 3D's, maybe it's not out of the question?

Based but highly modified, it probably will need a lot of cuts to be playable (let's remember the gibs system, and that ammount of sprites on screen). Not to mention the need to manage the fake sprite RoR and Height.

[Blzut3]

I have no idea where the particular limits/bottlenecks of that port are so it's hard to say, but given that a lot of ROTT is masked textures and a ton of sprites I would assume the massive amount of overdraw would prove to be a problem.  In many ways the engine changes are less crazy than they appear, but there's a lot of reliance on brute force.

[Individualised]

I've been thinking about it more and Rise of the Triad seems like it would be interesting to try and port to older systems. As far as I know it never got any console ports (other than the upcoming ones of Ludicrous Edition for modern systems) but the game, engine complexity wise, seems like it would be perfect for hardware such as the Super FX which could just barely run Doom.

 

Apparently there's a homebrew DS port, will try and look into that (though it's hard to find anything ROTT-related other than Ludicrous Edition content at the moment due to that version's well-deserved critical acclaim)

Edited August 6, 2023 by Individualised

[Blzut3]

Once again I think it comes back to half of ROTT's level design being based around brute forcing masked textures and sprites.  I will also note that for performance reasons the SNES version of Wolf3D switched to using Doom style rendering using BSP tree and wall segments.  Likewise I don't know what the specific bottlenecks were on the SNES Wolf3D and Doom ports so it's hard for me to speculate on possibility, but I do know the console ports of Doom avoided masked textures which leads me to believe ROTT's level design wouldn't mesh well.

[Boaby Kenobi]

There's an interesting section in this video on Doom on the 32x.

 

 

Edited August 17, 2023 by Boaby Kenobi

[Dark Pulse]

Bit late to this topic... but in short, no. Or at least, not without some serious compromises somewhere.

 

The reason ports of Doom were possible to systems like the 32X and SNES is simple: They are ROM. With ROM, you can page stuff in and out very quickly, on the nanosecond scale - SNES, for example, was about 120-200ns depending on the game, so you generally do not have to worry about RAM limitations nearly as much - just need enough of it to keep the game state going, render the current view, and so on.

 

On a CD-based system, this is very, VERY different. Seek time, at best, is about 80 milliseconds. This works out to... oh, EIGHTY MILLION nanoseconds. We're talking literally seven orders of magnitude slower. And again, that's "at best." Gotta go across the disc? Make that more like 300ms. This is an eternity, so basically at this point, you are forced to stuff everything in RAM, and that brings us to RAM of the system, which is... six megabit, or if you prefer bytes, 768 KB.

 

Now, even that isn't entirely impossible. Most individual maps, even on PC, are a couple hundred of kilobytes. But hold on a second, buckaroo - that 768 KB must fit EVERYTHING. Game engine. Sounds. Graphics data. Sprites. Then... the level itself.

 

And that is basically the dagger - there is just not enough RAM. The engine would easily take up probably about half of that, and then good luck cramming everything else into the remaining half.

 

Cartridge-based system? No problem, sprites can be paged in and out fast depending on what's being rendered (or about to be rendered). Sounds can just get blasted to the RAM of the soundchip. You only need to keep in RAM what is in view, and in a case like this, that 768 KB of RAM would be plenty - even generous.

 

But on a CD-based system, it's an absolute killer.

 

Since the Neo-Geo was also mentioned in here earlier, I would like to point out the difference between a Neo-Geo game in its cartridge form (MVS/AES) and the same title in its CD form. But since it wound up being a hell of a tangent, I will spoiler it for the brave.

Spoiler

 

SNK, understanding the costs of the games were huge because of the price of ROM chips, sought to use CDs as a medium to let people play "the same" games at a much cheaper price. And indeed, if you get some of the earlier titles for the system, the game might only load once at startup, or only very infrequently, and be nearly the same experience.

 

Great, right? Sure... and then  the late 90s rolled around.

 

Enter King of Fighters '98. The arcade hardware wasn't anything special:

• 64 KB of SRAM for the main 68000
• 84 KB of VRAM (broken up into 64, 16, and 4 KB for the main VRAM, palette RAM, and fast sprite RAM)
• A little extra RAM for the Z80 (sound processor) and battery backup, neither of which is important here.

Obviously, the games don't really have load times per se, everything plays nice and fast just like you expect it would.

 

Now, along comes the Neo-Geo CD. It has a whopping SEVEN MEGABYTES of RAM, broken down like this:

• 4 MB: Graphics memory
• 2 MB: Main 68000 program code
• 1 MB: Sound sample memory
• Some other miscellaneous amounts for things like the "Fix" layer (think things like lifebars or other mostly static elements), Z80 program, NVRAM, etc.

Great, everything should be just flawless, right? Wrong!

 

Enter later games, like The King of Fighters '98. First, we have to load to the title screen - something understandable, all games do that. Then we load into the character select screen. Then we load into the fight itself. And the fun is just beginning.

 

See, every time the stage changes (multiple stages have things like time-of-day shifts as the rounds progress), or it's a new fighter's turn... that's a load. Picture, if you will, you barely won the last round, by a sliver of your health. The game now has to load again, for your opponent... and it's a 15-second load. The round begins, he hits you right out of the gate, KO... and now the game has to load your next fighter. Time to wait another 15 seconds!

 

Don't believe me? There's video footage on YouTube that, thankfully, did not cut the load times out. Here you go!

 

 

34 seconds from power-on to start loading the main menu. Another 15 seconds or so before that loads. Want to watch the attract demo? Add another 13 seconds. Loading into the character select screen isn't too bad (about 5 seconds), but once you've actually got a fight ready, the load time is damn near a full thirty seconds. There's often a short pause of a couple of seconds before the actual fight begins in the "Ready... go!" phase (changing out the sounds, maybe?). Changing out a character is about another fifteen seconds. Oh, and when a match is won - or lost - that's a load too, a ten-second one.

 

SNK realized around 1996 or 1997 that the load times were starting to become a problem, and they did release an updated Neo-Geo CD model, the CDZ. This one had a 2x drive, while the original had a 1x drive. This did shorten the load times in some games, but in KOF '98, it only saves a couple of seconds. (It should also be noted that there are also now aftermarket mods that will let the images load off an SD card, and in turn, that brings times down dramatically - a first round load of about 13 seconds, and extra rounds are like seven seconds to load the next fighter. The pause in "Ready... go!" seems to be gone too.)

 

Basically, the main drawback with CD-based systems in the 90s is that in the end, they were limited by their RAM. The tradeoff for that massive storage space increase was that it was literally thousands of times slower than a cartridge, and that meant systems went from being held back by ROM capacity to being held back by RAM capacity. You could do a lot on the more advanced 32-bit systems - plenty of great PS1 games ran in its comparatively meager 2 MB of RAM - but it took until the following generation of consoles - Dreamcast and PS2 - that both had 16+ MB of RAM (and separate VRAM; consoles didn't really reach unified memory architectures until the generation after that) which, along with their faster data transfer speeds, generally made things like this even possible.

 

 

Anyway... this post was way longer than I intended it to be. Bravo to you if you made it this far.

 

TL;DR Peeps: Nope, not enough RAM. Every CD-based version of Doom had a system with at least 2 MB of RAM, and the Sega CD had less than half of that (768 KB), and unlike the later systems, everything had to fit within that space, including the game engine, sound data, and graphics.

Edited August 21, 2023 by Dark Pulse

[Individualised]

Doom wouldn't be possible on the cartridge based Neo Geo either as it's PPU just isn't up to the task - it is impossible to do bitmap graphics on it. Not just that it doesn't have a bitmap mode - the only thing the Neo Geo can do is draw static sprites and that's pretty much it, sprite graphics are read directly from ROM (similar to early NES games) and therefore cannot be modified to allow for a bitmap display like you could on the SNES or Mega Drive (or even NES games that have cartridge VRAM instead of CHR-ROM).

 

The Neo Geo is a budget system designed to give the illusion of being a powerful system by pretty much just throwing large amounts of ROM and RAM at the task. You don't need to worry about graphics capabilities when you can pre-render everything. But of course you can't pre-render Doom. The SNES and Mega Drive both outclass the Neo Geo in pretty much most ways that actually matter for non-arcade style games, with the exception of raw CPU power (but when you can't utilise that power it doesn't really matter). The Neo Geo is like maybe good at Street Fighter clones and that's it. It's a symptom of the "fake it and don't make it" attitude the gaming industry had in the 90s, it was impressive then but in modern times we can see it was just a gimmick. Also see the 3D craze in the 90s and how the gaming press would criticise games for being 2D (and Sony would ban 2D games from their platform altogether in certain markets for a short period of time), even though the 2D games would often play better (and even look better too; because let's face it, a lot of early 3D games didn't look very good):

^{Right, this is a topic about Doom. Sorry about that.}

Edited August 21, 2023 by Individualised

[Dark Pulse]

1 hour ago, Individualised said:

Doom wouldn't be possible on the cartridge based Neo Geo either as it's PPU just isn't up to the task - it is impossible to do bitmap graphics on it. Not just that it doesn't have a bitmap mode - the only thing the Neo Geo can do is draw static sprites and that's pretty much it, sprite graphics are read directly from ROM (similar to early NES games) and therefore cannot be modified to allow for a bitmap display like you could on the SNES or Mega Drive (or even NES games that have cartridge VRAM instead of CHR-ROM).

 

The Neo Geo is a budget system designed to give the illusion of being a powerful system by pretty much just throwing large amounts of ROM and RAM at the task. You don't need to worry about graphics capabilities when you can pre-render everything. But of course you can't pre-render Doom. The SNES and Mega Drive both outclass the Neo Geo in pretty much most ways that actually matter for non-arcade style games, with the exception of raw CPU power (but when you can't utilise that power it doesn't really matter). The Neo Geo is like maybe good at Street Fighter clones and that's it. It's a symptom of the "fake it and don't make it" attitude the gaming industry had in the 90s, it was impressive then but in modern times we can see it was just a gimmick. Also see the 3D craze in the 90s and how the gaming press would criticise games for being 2D (and Sony would ban 2D games from their platform altogether in certain markets for a short period of time), even though the 2D games would often play better (and even look better too; because let's face it, a lot of early 3D games didn't look very good):

^{Right, this is a topic about Doom. Sorry about that.}

Yeah, I'm not trying to suggest the Neo-Geo could render Doom. It couldn't, for both the reasons you said as well as other ones.

 

The main thrust was that to even begin to try, Sega CD would likely need a lot more RAM, or a hell of a lot of compromises. Neo-Geo CD had a hell of a lot more RAM, but it was done in by that its games were literally dozens of megabytes by the late 90s. (I think the largest official game for it clocked in at nearly 70 MB or so?)

 

Basically, if it had the system core of the Sega CD but the RAM of the Neo-Geo, then it might be workable in a way similar to PSX Doom, where you load sprite, sound, and texture data into the RAM, and page it to VRAM as needed (not sound, obviously). The rendering would then become the next bottleneck, and seeing as the processor was only 50% faster than that of the Genesis, I think that is where you'd start to hit problems. It might only do about 10-15 FPS unless you shrink the view. Bloodshot has a hell of a lot less to render (no flats, for example), but it can't really achieve much more than that.

Edited August 21, 2023 by Dark Pulse

[Herr Dethnout]

On 8/4/2023 at 8:52 PM, Blzut3 said:

I have no idea where the particular limits/bottlenecks of that port are so it's hard to say, but given that a lot of ROTT is masked textures and a ton of sprites I would assume the massive amount of overdraw would prove to be a problem.  In many ways the engine changes are less crazy than they appear, but there's a lot of reliance on brute force.

It's similar a how Power Drift manages his "3D". And you can look the Turbografx-16 port to see how much they need to change to make it into an underpowered system (Not a really good port).

[Blzut3]

10 hours ago, Dark Pulse said:

TL;DR Peeps: Nope, not enough RAM. Every CD-based version of Doom had a system with at least 2 MB of RAM, and the Sega CD had less than half of that (768 KB), and unlike the later systems, everything had to fit within that space, including the game engine, sound data, and graphics.

This is of course the answer for strictly the question the OP asked, but it would be possible to use a cartridge with the Sega CD (at least I assume that's the case since I haven't been told otherwise).  This of course goes back to it would look more or less the same as trying to get Doom running on the base system (which home brew suggests that something that could qualify as Doom is possible even if severe edits would be required).  There is the open question on if the Sega CD coprocessors could be used to accelerate the game in any meaningful way (i.e. the transfer from the coprocessor to the base system doesn't render any improvement moot), and if the CD could be used to meaningfully expand the game content (i.e. load levels from the CD).