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What is inside ?!

Kaneda - Feb 27, 2004

		Kaneda	Feb 27, 2004
		Hi! for some research i'm looking for pictures of inside a genny cartdrige (good quality and front back if possible) if you have some, can you share them with me ? they will be displayed on my web page, until you don't want i do some research on how several games handle the even/odd byte and save ram Thanks Kaneda

		Respectible Face of Engineering	Mar 3, 2004
		Will look into taking the odd one apart some have cross heads on so easy to take apart

		ExCyber	Mar 4, 2004
		Unfortunately my Genny carts are in storage so I can't help you directly, but generally how it works is that A21 (and A20 if there are two ROMs) is connected to the select inputs of a 74xx139 or similar decoder chip, and SRAM reads are decoded just like ROM reads (i.e. !C_CE enables the 139 which enables the appropriate chip for the address range). Since the SRAM is generally only 8 bits wide, it is only connected to half of the data bus. For reads, the reads are simply done in byte mode so that the 68K masks off the unused half of the bus. For writes, there are two write signals called !UDSW and !LDSW (Upper Data Set Write and Lower Data Set Write) that each correspond to one half of the data bus (and thus even and odd addresses). I don't recall what the correspondence is among !UDSW/!LDSW, the halves of the data bus, and even/odd addresses. I haven't seen how this signal is used, but it's probably just connected directly to the SRAM's !WE input. Typically, what you will see for ROM is that official Sega carts have one or two 16-bit ROMs, while unlicensed and bootleg carts (which are usually the ones with standard screws; especially common are early Electronic Arts games with the yellow tab) tend to have two or four 8-bit ROMs arranged so that they act like one or two 16-bit ROMs (i.e. they put one on each half of the data bus and connect their !CE and address pins together so that they activate and read just like a 16-bit ROM would). edit: minor detail I guess, but it's worth noting that when you connect an 8-bit memory to a 16-bit system, the memory address shifts left one bit relative to the system address, i.e. if you read from $200006, you're actually reading SRAM address $000003. Likewise, the signal names for the address bus will be off by one - SRAM A0 = 68K A1, and so on. This is because 16-bit systems don't require an A0 output - the selection is made by only reading half of the data bus as mentioned above. edit: further edited to remove some faulty reasoning regarding !UDSW/!LDSW

		Kaneda	Mar 4, 2004
		in fact, what is strange is that it read UDSW an LDSW aren't used to choose odd/even... it seems it must be the cartdrige itself who must give the proper information.... it's why i want to look inside a genny cart i have a pic of a beta of NBA Jam... where it seems pins b28/b29 (UD/LD SW) aren't used.... and DevSter eprom cart... doesn't use it too so are these 2 pins while connected to the 68k not used ? or is there several way to hande odd/even ? it's what i want to confirm anyway, thanks for the SRAM stuff, i didn't try to figure it yet Kanead

		ExCyber	Mar 4, 2004
		Your NBA Jam link seems to be broken (HTTP 403 Forbidden it looks like), and Devster's flash (not EPROM; EPROMs have a Vpp pin rather than a !WE pin since they can't generate the write voltage internally) cart doesn't seem to permit writing to the Flash ROMs while they are installed on the cart (WE is connected to VCC, which disables all writes). edit: What kind of save chip is in the NBA Jam cart, and does it have anything connected to pin B31?

		Mask of Destiny	Mar 4, 2004
		NBA Jam has a serial EEPROM if I'm not mistaken. EDIT:     Originally posted by Kaneda in fact, what is strange is that it read UDSW an LDSW aren't used to choose odd/even... it seems it must be the cartdrige itself who must give the proper information.... On the 68K an 8-bit read is treated as an 16-bit read, but it only looks at either D0-D7 or D8-D16 depending on whether it's an odd or even read. It only differentiates between 8-bit and 16-bit when doing writes (because for obvious reasons, writing 16-bits to a 16-bit wide device when you only want to write 8-bits would be bad.)

		ExCyber	Mar 6, 2004
		That's what I thought; the reason I ask is that serial EEPROMs look nothing at all like SRAM/ROM interface-wise. I'd be interested in hearing how it's hooked up (most likely it's not using B31), as it's probably something interesting, perhaps generating the clock signal with an equation like NOT !C_CE AND A22. edit: silly me, just needed to copy/paste the image URL... I don't see a save chip on there at all, just the 4 EPROMs and a 74139 decoder. Are you sure it actually saves anything?

		Mask of Destiny	Mar 6, 2004
		Originally posted by ExCyber I don't see a save chip on there at all, just the 4 EPROMs and a 74139 decoder. Are you sure it actually saves anything? Pretty sure, but maybe that's just the Tournament Edition. I suppose I could pop my cart open and confirm.

		ExCyber	Mar 7, 2004
		Well, I was just referring to Kaneda's proto. I know the production version saves.

		Respectible Face of Engineering	Mar 11, 2004