Large ROM images and Sega CD + 32X

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Morden
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Large ROM images and Sega CD + 32X

Post by Morden » Sat May 20, 2017 7:53 pm

Okay, so I got my Everdrive X7 and all was well until I tried running some large homebrew images on a Wondermega. As soon as tried an image exceeding 32 megabit, I got an error message prompting me to unplug the CD and 32X add-ons. The problem is, Wondermega, just like CDX and X'Eye, has the CD module integrated.

I do understand that regular cart space is mapped up to $3FFFFF, but I thought that space above that, up to $7FFFFF, could also be used by a flat mapped cartridge without a CD present. After all, the drive stays inactive when a cartridge is inserted. What would happen if this check was disabled and Everdrive would try to run large ROM images on a Wondermega [or CDX, or X'Eye]? Would this guarantee a bus conflict?

I have posted this question on the Mega Everdrive subforum, but unfortunately it remains unanswered. I guess I never thought of the bus conflicts, because no "normal" game is this big, and the modern big stuff, like Overdrive II, uses bank switching. But there are things like What Is Love, which are huge, but flat mapped. Anyway, I'd appreciate any kind of definite clarification when it comes to systems with an integrated CD component and bus conflict issues. Thanks!

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TmEE co.(TM)
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Re: Large ROM images and Sega CD + 32X

Post by TmEE co.(TM) » Sun May 21, 2017 8:04 am

When MCD is connected the 400000...7FFFFF range will have MCD BIOS and other bits of it visible there. When you disable the check stuff will probably run but sooner or later there will be hardware damage due to bus fighting that happens betwees MCD and the everdrive cart. Cart side tends to win over CD side, but it will still be harmful.
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Re: Large ROM images and Sega CD + 32X

Post by Sik » Sun May 21, 2017 12:25 pm

Note that it's not "when no CD inserted" but rather "when no Sega CD is inserted" (or for that matter, anything going in the expansion port). So huh yeah, Wondermega is a no-go for this.

And yes, this problem is precisely why we should rather discourage people from doing that kind of huge ROMs and instead use the SSF2 mapper. I guess the big issue is that a lot of emulators still hardcode the mapper emulation explicitly to the SSF2 ROM and nothing else (amusingly, Fusion doesn't, it just looks for the appropriate register writes). You'd think we could just, huh, not hardcode emulators, seeing as they would need to explicitly be modified to handle large ROMs anyway.

EDIT: and maybe to be more explicit
Morden wrote:I do understand that regular cart space is mapped up to $3FFFFF, but I thought that space above that, up to $7FFFFF, could also be used by a flat mapped cartridge without a CD present. After all, the drive stays inactive when a cartridge is inserted. What would happen if this check was disabled and Everdrive would try to run large ROM images on a Wondermega [or CDX, or X'Eye]? Would this guarantee a bus conflict?
(emphasis mine) No, it doesn't, the ROM and the rest of the hardware facing the Mega Drive side are indeed active. The only difference is that the cartridge and expansion port swap places, but both of them are always active regardless of the situation.
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Morden
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Re: Large ROM images and Sega CD + 32X

Post by Morden » Sun May 21, 2017 5:54 pm

Sik wrote:No, it doesn't, the ROM and the rest of the hardware facing the Mega Drive side are indeed active. The only difference is that the cartridge and expansion port swap places, but both of them are always active regardless of the situation.
This shows how much I know about the inner workings of Wondermega, I guess. Or MD + CD combo for that matter. When you start a Wondermega without a cartridge in the slot, the console will boot to the Wondermega BIOS and initialize the laser. Also, two status LEDs will blink. One for access, during the initialization, and one for ready, since the CD drive is ready to be operated. Since the drive seems to be "dead", I figured it's inactive, but now that I think about it, didn't Pier Solar allow CD music to be played during gameplay from the cartridge? Heh ... Not that inactive after all.

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