Cartridge Rom Size (32x vs. Genesis)

[3vix6]

Just curious, could the 32x handle larger (in space) carts than the Genesis could? or was this another thing that limited the 32x?

[oompa loompa]

Biggest native cartridge size even for 32x is 4mbyte without any funny stuff or bankswitching

[jlf65]

While there is space in the 32X memory map for 32MB (IIRC), you can't actually address that much from the 32X side as you don't have all the address lines going to the cart port. It might be possible to address 16MB - it all depends on if the 32X asserts the rom enable for any access to rom space, regardless of the address. That would be a nifty hardware test to perform. On the MD side, you would need to bank switch, of course. The 32X actually has two levels of bank switching... testing that would also be an interesting task.

[GiGaBiTe]

If the SH2s have a 32 bit address bus, they should be able to address 4 GB if all of the address lines were connected. I wonder if you could mod the 32x and multiplex the extra 16 address lines onto the lower 16 address lines and use some mechanism to switch between them.

[jlf65]

Jan 27, 2011 20:35:59 GMT -5 GiGaBiTe said:

If the SH2s have a 32 bit address bus, they should be able to address 4 GB if all of the address lines were connected. I wonder if you could mod the 32x and multiplex the extra 16 address lines onto the lower 16 address lines and use some mechanism to switch between them.

Uh, no. The SH2 breaks the address space into four regions of 32MBytes each. Each region has its own chip select out, and its own characteristics (like handling burst, having a certain number of wait states, or being 8/16/32 bits wide).

You only get A0-A26 and four chip select lines. While 27 address lines implies 128MB, the top two bits (A25 and A26) denote the region, so each region is 32MB. On a custom system, I suppose you could combine them all together to make one 128MB region, but that wouldn't be possible on the 32X. Region 0 is the hardware, region 1 is the ROM, region 2 is the frame buffer, and region 3 is the SDRAM.

[GiGaBiTe]

That's a weird addressing system. I'd figure it would be a sane flat memory map like a 680x0 or x86.

[jlf65]

Feb 2, 2011 21:53:58 GMT -5 GiGaBiTe said:

That's a weird addressing system. I'd figure it would be a sane flat memory map like a 680x0 or x86.

It's even weirder - the top three bits in the 32bit pointer control the bus response... 000 means cached, 001 means cache-through (uncached), 010 means flush the cache line, 011 addresses the cache address array, 100 and 101 are reserved, 110 addresses the cache data array, and 111 addresses the internal hardware.

In fairness to Hitachi, the SH series is meant for embedded usage where you don't address ridiculous amounts of memory for anything. 32MB each for four decoded spaces was probably considered overkill for such usage.

[defzonoc]

i love this i just love this
32 millon addresses only means 32MB if your data is 1 byte wide.
if your data is 16 bit each address can access 2 bytes so 64MBs
and so on.
;D
ha
and while the top three lines are for the purpose you say they are still data lines. so the ram the SH2 can read is 32 million addresses by 4 bytes or drum rolllllllll.........128 MBytes ding ding ding.
and there are 2 SH2s so 265MBs and here is the kicker. the SH2 is a general purpose RISC microprocessor those so called regions are segas ideals more then anything. i have a hd4470 lcd on my apple2E and i use some of its charecter ram to store data.
if you had the patiance and drive to do so the 32X has massive potential but its just to much work for me and to little benifit as i couldent share anything i made for the custom monster it would become.
the cpu in the 32X is easy its the propriatary way its rigged that gives me the headache.
like the dang boot up commands and other shit sega put in its rom.
but you can also if you had the patiance and were willing to risk breaking it. solder in ram chips and address them. but those SMD surface mount componets are a pain and are easy to damage.
stick with a cart.
also i havent realy studied it but i think a bypass may be plausable to let the genny have its own cart ram and the 32X run over that with its own cartridge in place.
but if some people think my planed projects arnt plausable they should see the ones i throw out. ;D

[Tiido]

you forget that lower address lines get unused or become individual byte strobes so you will not get 64 or 128MB range, but you are stuck with 32MB, with lower lines not doing anything useful.

[defzonoc]

i said it will be to hard and would be difficult and could probly ruin it so it should not be done but at the same time i was trying to answer the if i want to link straight to the address lines bit.
if you had lots of patiance it can be very posible to get a lot of ram and even at 25 address lines you can call 8 million addresses and if you connect to 32bit or 4byte ram you can get 32MBs but there are also two SH2s if you gave each its own well thats 64MBs right there. yup i did fudhe my math a little and sorry for that but the regions dont realy matter so much so long as your code in the games you make for your modded 32X doesnt write to the addresses with hardware attached. there are tones of old computers with ram linked with the whole address bus but some cant be used as ram because hardware is mapped to that area as well. thats all im saying.
and thanks for not being mean about my number fudge. ;D

[defzonoc]

also what about the ram cart bypass?
how much memory map does the pertanet inteface to the 32X take?
ive never thought about looking at it before.
but what would happen if you made something between the sega and the 32X to give the genny some of its own ram cart and first load the gennys ram cart then using the needed addresses to talk to the 32X load its ram cart. its only a hypothetical. like i said i dont realy like thing thats hard to share.
but it would be awsome to see like the video players on commodores and atari computers. a sega genny super combuter bwaa haaaa haaaaaaaaa

[jlf65]

Sticking something in between the 32X and the MD is probably a bad idea. It would be better to make a new cart that had a little flash (for a menu or something), a bunch of ram, and an SD or CF interface. The NeoMyth got close, but it's not quite perfect and rather expensive. The Everdrive-MD was better in some ways, but didn't include any ram, making it almost useless for advanced homebrew. So the ultimate MD cart still has yet to be made.