Sega Genesis rom/sram address mapping

Discussion of development of software for any "obsolete" computer or video game system.
Sik
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Re: Sega Genesis rom/sram address mapping

Post by Sik » Sat Sep 13, 2014 12:04 am

Byte-wide access only (try doing a word access on an odd address and the 68000 will yell at you at the very least). Though looking again at Tiido's diagram, it *is* just using the bottom bit... (honestly I don't know any game that actually cares about the SRAM protection anyway)

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getafixx
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Re: Sega Genesis rom/sram address mapping

Post by getafixx » Sat Sep 13, 2014 11:47 am

lidnariq wrote:Maybe I'm confused, what exactly are you trying to do?
Is it "use a single 4MiB/2MiW ROM on a board while retaining this battery-backed save RAM" ?
In that case, you'd use either socket—as far as I know, all of D0…D15 and A1…A19 are connected to both—and connect the new PROM's /CE from the output of the 74'08. You may also need to connect A20 and A21 from the cartridge edge to the PROM.

i.e. ROM1 has /CE
and ROM2 has /CE
and you want something that is low if either are low.


Yeah I was just trying to get a 32Mbit cart to run just as one chip instead of needing two of them. I ordered a few 74HC08 chips, so I'll try it out hopefully next week.

Thanks again for your help guys!

Sik
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Re: Sega Genesis rom/sram address mapping

Post by Sik » Sat Sep 13, 2014 2:22 pm

Not an expert in electronics but a random thought: with a flip-flop (for the $A130xx register) and a 4-way multiplexer, couldn't you just get away with using the multiplexer as a decoder?

00 = low half of ROM (enable ROM)
01 = low half of ROM (enable ROM)
10 = high half of ROM (enable ROM)
11 = SRAM (enable SRAM)

(where bit 0 is the latch and bit 1 is the MSB of the address)

Although I'd imagine that if there wasn't something wrong with this that they'd have probably used that method for starters (unless 4MB ROMs were more expensive than two 2MB ones back then... I know many games are 2MB+1MB, including some games using SRAM, and in that case you indeed need two ROMs).

lidnariq
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Re: Sega Genesis rom/sram address mapping

Post by lidnariq » Sat Sep 13, 2014 4:55 pm

Looking at the depopulated boards at romlab ("ps4cart1", "ps4cart2"), the schematic does seem to be what TµEE drew.

Only problem I can see with using a '153 in this way is that we have three inputs (/CE0, Q0, A21), and when the '153's /E input is deasserted it drives all its outputs low, not high: the opposite of what we need.

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TmEE
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Re: Sega Genesis rom/sram address mapping

Post by TmEE » Sun Sep 14, 2014 3:48 am

Image

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getafixx
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Re: Sega Genesis rom/sram address mapping

Post by getafixx » Tue Sep 16, 2014 12:30 pm

Ok so parts came in, and TmEE's design works great! The rewiring makes it not worth it, but it proves the concept works. So, if its really just changing how the 74'74 and 74'139 is wired, why didn't Sega do this? It would have cut down on the cost of the extra maskrom and PCB size (slightly), unless of course 32Mbit roms were just too expensive back then.

Thanks for the all the help guys!

Sik
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Re: Sega Genesis rom/sram address mapping

Post by Sik » Tue Sep 16, 2014 4:27 pm

getafixx wrote:unless of course 32Mbit roms were just too expensive back then.

Very likely the reason. I already mentioned that many games were actually 3MB (basically 2MB+1MB).

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MottZilla
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Re: Sega Genesis rom/sram address mapping

Post by MottZilla » Sat Sep 20, 2014 12:14 pm

I don't recall too many Genesis games being 32 megabits anyway. There are some but there are far more 24 megs and less. I had a list once as I was curious because you could find some older Copier units that had 24 megabits of DRAM but not as many with 32.

Sonic 3D Blast, Ultimate Mortal Kombat 3. Those are the only two games that spring to mind that were that large on Genesis. Oh and then you have Super Street Fighter 2 which was 40 megs I think.

There are far more that are 24 megs. Perhaps because the SNES seemed to be supported longer, there are alot more 32 meg games that could be listed for it.

Bretmi
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Re: Sega Genesis rom/sram address mapping

Post by Bretmi » Wed May 29, 2019 10:04 am

Did someone still play Sega Genesis games? I download the games I extract the games and the emulator can't find them or if he sees the file it cant be played. I use only RetroArch emulator.


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