A question about sdram memory

[Gaetan Mailloux]

Hi

My BCM PIII motherboard can have up to 512 meg sdram dimm.

Why, for some sdram memory in sales, they say that if a motherboard can
have up to 512 meg you can only put 256 meg ?

They say that you need a motherboard who can have up 1 gig sdram if you
want to put on it a 512 meg stick..

I whant to upgrade to 512 meg, is it need a special kind of sdram dimm
memory stick ?

Thank

Bye

Gaetan
--

 

[Oliver Hannaford-Day]

Hi,

Why, for some sdram memory in sales, they say that if a motherboard can
have up to 512 meg you can only put 256 meg ?

Long Version:
Not 100% but it could be that the slot doesnt have enought address bits to
use a 512 MB DIMM so you need to use two 256MB ones.
I have done a few DRAM designs and with them the SIMM size usable is limited
to the size of the address bus (Yeah, shocking isnt it, lol), so you could
find that a 128MB SIMM wouldnt work in a computer that can take 128MB of
total memory but only by using 4 32MB SIMMS. (This is because the extra
address bits needed were used for selecting the bank/slot)
The same must be true for SDRam so maybe thats it, its not that a
motherboard that can take 512MB cant take the full memory, just you cant do
it with one DIMM.

Short Version:
You probably cant use 512MB DIMMS in a computer with a max memory of 512MB,
but you could probably use two 256MB DIMMS.

Oliver Hannaford-Day
www.cdtv.org.uk

 

[[email protected]]

there is a max ??

 

[PeteS]

The maximum amount of memory supported is a function of the SDRAM
controller, and is fixed in internal hardware.

That's the reason a slot may have a maximum of 256MByte, for instance -
for a x32 configuration, that would in a hypothetical example of a
stick that has 8 64M x 4 parts yield a typical configuraion (address
wise) of 13 x 11 x 2 (Where that's Row by Column by Bank).

If the maximum number of rows/columns/banks (any one of them) are at
the above limits (and that's internal to the controller), then that's
the maximum amount on that particular chip select. To get more memory,
extra chip selects are used, thus increasing total *system* memory to
max (controller) x number of chip selects.

The maximum amount per chip select varies widely (depending on
application). Most PCs nowadays can deal with up to 1GByte per slot.
High end servers can handle a great deal more, although the necessary
size on sticks has not yet appeared.

Note the reason those high end parts are used in servers only right now
is cost - it's more difficult to make the controller work with more
signals, basically (to gloss over an exceeedingly complex subject).
Initially, those parts cost more, and eventually will drop in price,
but the PC business is cut-throat, so the cheapest availble parts that
do the job are used. Currently, that's about 1GByte per slot.

Cheers

PeteS

 

[Oliver Hannaford-Day]

Hi,

there is a max ??

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

 

[Jamie]

Hi

My BCM PIII motherboard can have up to 512 meg sdram dimm.

Why, for some sdram memory in sales, they say that if a motherboard can
have up to 512 meg you can only put 256 meg ?

They say that you need a motherboard who can have up 1 gig sdram if you
want to put on it a 512 meg stick..

I whant to upgrade to 512 meg, is it need a special kind of sdram dimm
memory stick ?

Thank

Bye

Gaetan

they most likely mean the number of sockets you have.
some mother boards don't have enough sockets using
low scale SDrams boards.
and on top of that, you normally can not put in
a larger type than what it was designed for.
for example
if your board has 2 sockets and its designed for
512 megs total, then 2, 256 types is most likely
all you can put in.
but putting in a 512 in one slot and 256 in another
most likely isn't going to work.
in other words, you must go with the combination of
sizes designed for your mother board to achieve its
max size of memory.