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  • Joined: 12 Nov 2019
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SMS on Megadrive carts
Post Posted: Tue Nov 12, 2019 6:05 pm
Hi all - had a search around but couldn't find any distilled information on this question, sorry if it's been asked before - is it possible (and what needs to be considered in addition to the obvious burner, pcbs, chips etc) to develop a SMS game with a view to burning it/using it on a new Megadrive cart?

Does the adapter play an active role in the conversion process, or is it possible to use, or adapt without too much bother one of the common MD PCBs I see for sale in Europe to run a SMS game on a MD directly?

Thanks in advance :)

EDIT: I should specify that I mean a new game, so any needed software hacks are on the table.
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  • Joined: 23 Aug 2009
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Post Posted: Tue Nov 12, 2019 6:24 pm
IIRC, the PowerBase Converter injects several instructions onto the bus that forces the Genesis/MD to run in SMS mode. The re-release of Phantasy Star I for the MD did this, but I'm not sure if they did it in software or via hardware. This page indicates it "used a pin" to accomplish this, but if memory serves, the PBC has a little PROM that actually executes a handful of instructions:

https://segaretro.org/Phantasy_Star#Sequels_and_re-releases
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Post Posted: Tue Nov 12, 2019 6:31 pm
SMS mode is controlled by a pin, that's how converters can work. Some games rely on the BIOS to set up the stack, that's what the PBC does but it's not necessary for most games.

You can make a Mega Drive game that uses the SMS graphics mode, don't use the YM2612 and try to avoid using the 68000 CPU, but that seems a bit restrictive.
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  • Joined: 28 Sep 1999
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Post Posted: Tue Nov 12, 2019 7:21 pm
Pin B30 on a MegaDrive cartridge can be tied low to put the system in SMS mode. In that mode some of the cartridge pin functions change so it isn't a 1:1 mapping to the existing documented pin functions that regular Genesis games use, though it's close. I don't have a good resource for those assignments but I can verify those for you on a Phantasy Star MD cart in a few days if you'd like.

This means the ROM that's in the cartridge can't be used as-is, such as directly replaced with a 27C322 EPROM that has been programmed with your SMS software. You'll have to make some modifications to the cartridge.

On some MD carts the cartridge contacts along the edge are physically present but not used by a particular game so they aren't routed to the ROM socket. You may need to run jumper wire from those pads to your EPROM to make up for those missing connections.

As SavagePencil pointed out the Power Base Converter runs a snippet of code to set the stack pointer up before passing control to the game. If your new SMS software sets up the stack pointer appropriately you'll be OK. Other than that the PBC is a passive device that doesn't do anything special to make SMS games work on the Genesis.

On the flip side, Mega Drive software running natively on the 68000 can enter and exit the SMS graphics mode without any issues by writing to a VDP register. It could be fun to make a game with SMS visuals but the processing power of the 68000. :)
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Post Posted: Tue Nov 12, 2019 9:52 pm
Thanks for all the replies.

Charles MacDonald wrote
I can verify those for you on a Phantasy Star MD cart in a few days if you'd like.


That's really nice of you - If you think it's a useful thing for the general furthering of The Knowledge, go for it, but I think on my behalf it sounds a bit past what I'd consider trying to do right now, but thanks so much.

I just wondered if it was a known thing that people are already doing, seems it would open up a lot of MD owners to the SMS and possibly make it easier to source pcb and cases for new games. Am I committing heresy in my first post!? :)
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Post Posted: Tue Nov 12, 2019 10:32 pm
Maxim wrote
You can make a Mega Drive game that uses the SMS graphics mode, don't use the YM2612 and try to avoid using the 68000 CPU, but that seems a bit restrictive.


Slightly different tack:

I know that the 68000 is put in idle when the MD is running in proper SMS mode, but do you happen to know to what degree you can just program the MD 'as if' it were running SMS software without being in SMS mode? I mean, the Z80 is programmed separately and is done in MD development, you can set the graphics mode and just avoid the FM chip as you say...is there any real need to consider 'conversion' at the cart level, or are there things you simply can't make it do in software alone, even if you keep the 68000 involved for housekeeping?

I can see it would (mostly) be a case of 'why would you bother?' back in the day....
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  • Joined: 04 Jul 2010
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Post Posted: Wed Nov 13, 2019 5:00 am
I've make a MD / SMS pcb year ago.
(forced Z80 mode == SMS)
IMG_20191113_055214.jpg (481.91 KB)
MD PCB (sms/z80 mode)
IMG_20191113_055214.jpg

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Post Posted: Wed Nov 13, 2019 7:40 am
I'm not super familiar with the Mega Drive but I think that in MD mode the Z80 can't do everything, being the "sound CPU". However you could write some minimal "drivers" to have the 68000 do things commanded by the Z80.

I think Mega Drive owners will look on an SMS mode game similarly to how SMS owners would look at a game running in TMS9918a graphics mode - not very favourably.
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  • Joined: 29 Mar 2012
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Post Posted: Wed Nov 13, 2019 9:24 am
I would say that, at least on the spanish forums, most SMS homebrew is already played on Megadrive's using the everdrive.
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