Genesis/MD special chip question

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Markfrizb
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Genesis/MD special chip question

Post by Markfrizb »

The chip on the Super Street Fighter II is a mapper chip for games larger than 4 mB. SSF2 is 5mB in size. The cart has 3 chips, 2mB + 2mB + 1mB = 5mB
On the pcb, under each chip there is silk 32m/16m mask rom which says to me that each chip could be 32mb (4mB) each for a max of 12mB. All the pins of each chip are bussed alike except for pin12 which is the /CE pin.
Sooooo here’s the questions: what do the SL’s 1-4 do?
Currently on this pcb, SL3 SL1 are bridged which puts 5v on pins 23&24. SL4&2 (unabridged) would ground those two pins of the special chip. Anyone know why the SL’s and their functions?
Could it be that those SL’s select a larger rom map?
My goal was to see if I could get the Ultimate Mortal Kombat Trilogy to run on the cart which is 10mB.
Any insights would be appreciated. Thank you
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TmEE
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Re: Genesis/MD special chip question

Post by TmEE »

This is all untreaded territory. It is likely the jumpers choose !CE size for particular ROMs but you will not get that 10MB hack working because it requires 10MB *linear* unbanked space in the 68K address space while this chip banks up to 32MB (theoretically) inside 4MB of normal ROM area in 68K space, in multiple 512KB windows.
Markfrizb
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Re: Genesis/MD special chip question

Post by Markfrizb »

TmEE wrote: Mon Aug 29, 2022 4:34 pm This is all untreaded territory. It is likely the jumpers choose !CE size for particular ROMs but you will not get that 10MB hack working because it requires 10MB *linear* unbanked space in the 68K address space while this chip banks up to 32MB (theoretically) inside 4MB of normal ROM area in 68K space, in multiple 512KB windows.
So linear as in, just A0-23? The address numbering is a little weird because Gen/MD use 16 bit addressing. Edge connector B8- B11 being the higher addresses and no extra chip needed for higher than 4mB mapping?
lidnariq
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Re: Genesis/MD special chip question

Post by lidnariq »

Remember that we tried to get the 10MB "flat map" working? I didn't (still don't) have any ideas why it didn't work, but it's the right idea.

The problem on the SMD is that by default it'll stall forever on a read from $800000-$9FFFFE and external hardware has to drive the /DTACK pin at that moment. (For normal cart ROM, it automatically handles that)
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TmEE
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Re: Genesis/MD special chip question

Post by TmEE »

Markfrizb wrote: Mon Aug 29, 2022 5:05 pmSo linear as in, just A0-23? The address numbering is a little weird because Gen/MD use 16 bit addressing. Edge connector B8- B11 being the higher addresses and no extra chip needed for higher than 4mB mapping?
Yes, all of the address bus is available on the cartslot. You do need to make a !DTACK generation circuit for accesses into $800000...$9FFFFF as mentioned by lidanriq. You can only drive the signal low, you are not allowed to drive it high (open collector/drain only). 32X uses this range, and it will not allow the flat map to work.

Flat map also won't work when MegaCD is connected, it lives in $400000...$7FFFFF range, right above the first 4MB where cartridge ROM space is and with it connected there will be a bus fight with MegaCD boot ROM and few other things in it.
Markfrizb
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Re: Genesis/MD special chip question

Post by Markfrizb »

lidnariq wrote: Mon Aug 29, 2022 6:27 pm Remember that we tried to get the 10MB "flat map" working? I didn't (still don't) have any ideas why it didn't work, but it's the right idea.

The problem on the SMD is that by default it'll stall forever on a read from $800000-$9FFFFE and external hardware has to drive the /DTACK pin at that moment. (For normal cart ROM, it automatically handles that)
Right. My interest in solving this issue has been renewed but I still lack any meaningful answers. I *thought* that the SSF2 special chip might be the answer if the /CE could be modified to switch on 4mB mapping instead of 2mB….. buuut from these posts, sounds like it’s not an option.

I don’t have a CD drive.
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