iNes mapper 75 with 256KiB PRG

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mightydidz
Posts: 99
Joined: Wed Jan 24, 2018 12:04 pm

iNes mapper 75 with 256KiB PRG

Post by mightydidz »

I'd like to make an English translation but it oversize PRG to 256KiB.
http://www.romhacking.net/translations/3751/
I need to figure a way to add PRG-A17

I've looked at these references but nothing for VRC1
https://wiki.nesdev.com/w/index.php/Use ... r_thoughts

any one knows what to decode?

Thanks
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Quietust
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Re: iNes mapper 75 with 256KiB PRG

Post by Quietust »

The Konami VRC1 is itself incapable of supporting more than 128KB of PRG ROM - it only has 4 data input pins, so all of its registers are 4 bits wide.

Supporting a game larger than that would require an additional "outer bank" register, either implemented outside the chip (similar to MMC3-using multicarts) or by stealing one of the CHR ROM bank lines (similar to how certain MMC1 boards were made to work with 512KB of PRG ROM).

However, this would have required the ROM translation to have been written with one of those modifications in mind - since they just treated the bank registers as being wider, the only way to make it work on real hardware would be to design a better VRC1 clone chip from scratch, either in an FPGA or by using a whole bunch of discrete logic chips (e.g. some 74139s for address decoding, several 74161s for CHR banking, a few 74174s for larger PRG bank registers, and a few more chips to glue it all together).
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krzysiobal
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Re: iNes mapper 75 with 256KiB PRG

Post by krzysiobal »

Try this:
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Quietust
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Re: iNes mapper 75 with 256KiB PRG

Post by Quietust »

krzysiobal wrote: Fri Jun 04, 2021 9:50 amTry this:
Huh, that's pretty clever - slightly wasteful (using a 4x4 register file but only using slightly less than a quarter of it, though I suppose you could connect the other 3 bits in order to boost it all the way to 2MB), but it takes advantage of the VRC1's register layout (with the PRG banks being at nicely-aligned locations) and also handles the fixed bank (by putting a pull-up on the output bit and tristating it when accessing $E000-$FFFF).

For reference, the diodes+resistors between R/W+A12+ROMSEL and GW look to be a 3-input OR gate, and the ones between A13+A14 and GR appear to be a 2-input AND gate.
Last edited by Quietust on Fri Jun 04, 2021 10:43 am, edited 1 time in total.
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lidnariq
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Re: iNes mapper 75 with 256KiB PRG

Post by lidnariq »

Could use something like my oversize m82 bodge hardware again, although the specific signals need to be changed. viewtopic.php?p=246438#p246438
mightydidz
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Re: iNes mapper 75 with 256KiB PRG

Post by mightydidz »

whatever is cheaper would fit my need, the 4x4 register seems to be pretty expensive although it would be a compact addon.

The 74'138+74'259+74'153 seems also a nice option, can someone let me know what specific signals are needed?

Thanks
lidnariq
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Re: iNes mapper 75 with 256KiB PRG

Post by lidnariq »

Something like this:
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mightydidz
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Re: iNes mapper 75 with 256KiB PRG

Post by mightydidz »

Something like this?
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vrc expand.JPG
lidnariq
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Re: iNes mapper 75 with 256KiB PRG

Post by lidnariq »

For example, yes.
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