Hi,
Yep, there always is.
I dont have any information on SD-Ram slots that can hold more than 512MB
per slot. (Thats SD-Ram, not DDR SD-Ram, which is already up to 1GB a DIMM)
This doesnt mean your slot can take 512MB though, designers can only design
the slots to take whats available (Or proposed, but even then the standard
changes and boom, your design is shot) so if 128MB DIMMS were the best on
the market at the time thats the max size it can take
As for a maximum memory for the CPU it depends on one thing, how much
address space your willing to set a side just for the memory, 32Bit CPU's
have 4GB address space, but you cant have 4GB of RAM because the other bits
in your computer need address space. (Like the USB chip, PCI slots, IDE
interface, all of these need address space)
This is why 32bit motherboards only normally have up to 2GB of memory, it
leaves enough room for those graphics cards with 256MB of memory and such
like.
The other guy who replied said about the controller chip being the limit,
well yeah but you would design/buy one to suit, for example I havent seen a
64Bit AMD motherboard with more than 4GB of memory allocated for RAM, well
it can take a lot more than that (a lot lot more) but you would have to add
a second controller chip to add extra memory, not a big problem.... there
are even PCI SD-Ram controller chips although I dont know a sole who uses
one in a design.
He also said about the cost of memory, I dont know how much of a difference
it would make to the design, I mean I would do a design that would take the
biggest DIMM available but then go and stick the right one in for the price,
and as such letting the user upgrade at a later date.
Finally he said a memory controller that has more address bits is more
complicated.. again I will say I have never designed an SD-Ram controller
(Just used the built in ones found on embedded CPU's) but the 72pin SIMM
controllers just needed another 74LS258, ok the design would use FPGA's or
CPLD's but I doubt the cost would be effected much by adding the extra
address bits as it still works on a CAS/RAS system... could be wrong though,
as I said I have never designed such a controller.... I would love to
though, just gotta get some free time.
Wow this post is getting long, better end here.
Oliver Hannaford-Day