8k to 32k SNES VRAM schematic

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nintendo2600
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8k to 32k SNES VRAM schematic

Post by nintendo2600 » Fri May 29, 2020 5:20 am

Hey All,

Where would the extra address lines need to be routed to expand the VRAM from 8k to 32k? Would
it be different on lorom VS hirom? The 8k footprint being 28pin is nice and a little easier for placement
but I was also considering if a 2k could be replaced with a 32k as well? I have a 1a1m lorom with 2k
that I would like to beef up and a 1j3m with 8k that I would like to beef up as well. I already know
how to rewire 2k to 8k but I've never seen, nor do I understand how to go higher than that for sram
as large as 32k in the VRAM slot. Hope everyone is healthy and well during this trying time in the world.

Thanks,

Shawn

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Re: 8k to 32k SNES VRAM schematic

Post by dougeff » Fri May 29, 2020 6:12 am

Are you sure you mean SNES?

SNES has 64kB VRAM, and is not expandable.

Edit... Oh, you mean 0x8000. That still doesn't make sense. That's 32768 decimal. Although there are 0x8000 word addresses, which is 64k bytes.
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Re: 8k to 32k SNES VRAM schematic

Post by nocash » Fri May 29, 2020 8:34 am

dougeff wrote:
Fri May 29, 2020 6:12 am
Are you sure you mean SNES?
I think the post is about SRAM in SNES cartridges... but...
VRAM is short for Video RAM.
But there is no video memory in SNES cartridges (unlike NES cartridges).
SNES cartridges that do contain SRAM are using it for saving game positions.
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Re: 8k to 32k SNES VRAM schematic

Post by tepples » Fri May 29, 2020 5:25 pm

The closest thing to "video memory" in a Super NES Game Pak is the work RAM used by the Super FX GSU to hold the frame buffer that it renders, which the S-CPU copies to VRAM through DMA once the frame is done. And even this is already much bigger than 8 KiB (0x2000 = 8192 bytes).

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Re: 8k to 32k SNES VRAM schematic

Post by lidnariq » Fri May 29, 2020 6:43 pm

Assuming, for the moment, that you mean "cart save RAM" when you wrote "VRAM" ...

LoROM carts connect extra address lines in order, skipping A15. (So a 2KiB RAM is connected to A0 through A10; 8 KiB to A0-A12; 32 KiB to A0-A14; 128 KiB to A0-A14 and then A16 and A17)
HiROM carts connect address lines in order, skipping A13 through A15. (So a 2KiB RAM is connected to A0-A10; 8 KiB to A0-A12; 32 KiB to A0-A12 and then A16 and A17)

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Re: 8k to 32k SNES VRAM schematic

Post by Fisher » Sat May 30, 2020 4:04 pm

AFAIK, according to the SNES' schematics pin 46 of PPU-1 is one extra address pin that could double the available VRAM.
Pin 20 of the S-DSP is also labeled A15, wich could do the same on the sound system.
I'm not sure how would existing software work with extra RAM added.
Just imagine how great (and expensive) the console would be and how awesome the games with this extra RAM could be back in the day.

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Re: 8k to 32k SNES VRAM schematic

Post by nintendo2600 » Sun May 31, 2020 9:57 pm

lidnariq wrote:
Fri May 29, 2020 6:43 pm
Assuming, for the moment, that you mean "cart save RAM" when you wrote "VRAM" ...

LoROM carts connect extra address lines in order, skipping A15. (So a 2KiB RAM is connected to A0 through A10; 8 KiB to A0-A12; 32 KiB to A0-A14; 128 KiB to A0-A14 and then A16 and A17)
HiROM carts connect address lines in order, skipping A13 through A15. (So a 2KiB RAM is connected to A0-A10; 8 KiB to A0-A12; 32 KiB to A0-A12 and then A16 and A17)
Yes, I meant SRAM. The homebrew\hack game Super Mario 2D Land seems to be asking for 1024k (AKA 32K). This is the readout from NSRT v3.4 - Nach's SNES ROM Tools. I'd be very happy to hear it's an inacurate readout and doesn't need the extra ram :)


s.png
s.png (11.35 KiB) Viewed 1537 times

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Re: 8k to 32k SNES VRAM schematic

Post by lidnariq » Sun May 31, 2020 10:35 pm

nintendo2600 wrote:
Sun May 31, 2020 9:57 pm
Yes, I meant SRAM. The homebrew\hack game Super Mario 2D Land seems to be asking for 1024k (AKA 32K).
well, for one, 1024kibit = 128KiB, not 32KiB...

the best way to find out is to load the hack in an emulator with a debugger (bsnes-plus, mesen-s, no$sns) and set a breakpoint for access to the RAM region. As you tickle breakpoints, you can keep moving the window until you find the highest point that it doesn't use again.

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Re: 8k to 32k SNES VRAM schematic

Post by Fisher » Mon Jun 01, 2020 3:34 pm

Oh, I can see clearly now what you mean.
Wouldn't be enough to get the extra address pins straight from the ROM, since the SRAM's address when decoded just sets its /CE (or /OE) pin?

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Re: 8k to 32k SNES VRAM schematic

Post by nintendo2600 » Thu Jun 04, 2020 9:22 am

lidnariq wrote:
Sun May 31, 2020 10:35 pm
well, for one, 1024kibit = 128KiB, not 32KiB...


[/quote]

Oops! Well bad math on my part. That is way too much work then. I'm not pissing around trying to route that many address lines. The game isn't that interesting. :)

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