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SRAM location
patroclus02 - May 8, 2006
  patroclus02 May 8, 2006
Hi!

Back to my devcart building project, I wonder if all save-game-SRAM carts map the SRAM in the same area. If you want to make a universal cart, with SRAM support for back up save games, how should be done?

I check all carts I have, and all map SRAM at $200000

Thanks!

 ExCyber May 8, 2006
$200000 is the standard. I've never seen or heard of a game that places SRAM at a different location. Even the 24MBit games with SRAM (Beyond Oasis, Phantasy Star 4) use a bankswitching scheme just to place it there. <_<

That being said, to make it reasonably compatible you need to be able to connect SRAM to either the upper half or lower half of the data bus (i.e. even/odd byte addresses) within that region. I think even addresses are more common, but they are definitely both out there. A game that maps to odd addresses will have $200001 in the SRAM location field of the header instead of $200000. If games are generally coded properly (i.e. they don't do word-wide reads and barf if the "empty" half returns something other than $FF)., you might be able to get away with just wiring the data outputs to both halves of the bus and switching the USDW/LDSW signals to the write enable input of the SRAM.

 patroclus02 May 9, 2006

  
	
	

		
		

			you might be able to get away with just wiring the data outputs to both halves of the bus and switching the USDW/LDSW signals to the write enable input of the SRAM.


Wire both halves, but using some kind of multiplexer I suppose. You cannot just wire them together...

What about EEPROM cartridges like Sonic3? is it mapped at $200000 too??

 cgfm2 May 9, 2006

  
	
	

		
		

			Originally posted by patroclus02@Tue, 2006-05-09 @ 09:40 AM



Wire both halves, but using some kind of multiplexer I suppose. You cannot just wire them together...



What about EEPROM cartridges like Sonic3? is it mapped at $200000 too??




[post=146158]Quoted post[/post]


Sonic 3 has a FM1280S FRAM which I think is 4Kx8.

I don't know which address it's mapped to, but if you just look at the ROM image header there is a field that describes the type of memory backup device (EEPROM, SRAM, etc.) and the address it's at. The format of the header is described in Sega's manuals:

http://www.antime.org/sega/md.html...

You can check the header for any game that you need SRAM information for.

EDIT:

For the SRAM data bus issue, you could always use a 16-bit RAM chip to handle both even and odd addresses.

 patroclus02 May 9, 2006
I've downloaded the manuals, but I see no info about how to detect type of RAM (or EEPROM, or whatever).

Indeed, I just looked at the header of Sonic 3 and Shining Force (which uses SRAM) and I see no difference about backup RAM info.

Anyway, acording to Charles Macdonald, Some cartridges use a Xicor X24C01 EEPROM chip, and have

0001B0: 52 41 E8 40 00 20 00 01 00 20 00 01

So sonic 3 is not an EEPROM cart?

Also, is there any info about how to write to these kind of memories? I suppose going to datasheet and see how it works..?

 ExCyber May 9, 2006
Right, the Xicor chip is actually a serial EEPROM and so only needs the one byte of address space. From an access standpoint FRAM is basically identical to SRAM. I'm pretty sure the Xicor datasheet is on Charles's site......

 patroclus02 May 10, 2006
Yes, I know about serial EEPROMS.

What I never heard about is FRAM.. neither find much info in google. Is it just the same as any SRAM?

 ExCyber May 10, 2006
From an external perspective it's basically identical; the main difference is that it is nonvolatile without needing a battery. The innovation is in how the actual memory cells work. You might get more relevant stuff out of Google with "ferroelectric RAM" rather than "FRAM" (which also happens to be the name of a major manufacturer of automotive components), or check out the Wikipedia article....