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- DDR high and low density?
July 30, 2006, 9:59 pm
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Re: DDR high and low density?
This is about more than density.
Each generation of RAM starts with a base array size. The chip package
pinouts are designed, so that several output data bus widths are
supported, giving flexibility in how modules can be constructed.
The same silicon die can be used to satisfy several different
pinouts, simply by wire bonding choices when the memory IC is
At the 512Megabit level, some chip types might be 16Mx16,
32Mx8, 64Mx4. The "x16" and "x8" types are defined as
acceptable by JEDEC, for making unbuffered modules. With the
"x16" chips, four of those chips on one side of the module,
makes a bank.
With the "x8" type, it takes eight of those chips on the side
of a module, to make a bank. Basically, it takes as many
chips as is necessary to make a 64 bit grouping (as the
bus interface on a DIMM is 64 bits wide).
Now, x4 chips are legal to build, but they are typically
used on registered DIMMs. JEDEC standards would allow
registered DIMMs to be built with x4 chips. It takes sixteen
chips, enough to fill both sides of the DIMM, to make a
single bank that way. That places sixteen loads on the shared
chip select signal for the bank, and for any other "global"
control signals for the bank on the DIMM. The registered
DIMM, via the register chip on the board, buffers the
control signals, so the motherboard does not see sixteen
loads on a bank that is on a registered DIMM. A registered
DIMM makes it possible to overload the signals, since
they are driven by the register chip on the DIMM itself.
The motherboard never sees the overload, since the
register chip sits between the motherboard and the
It seems that the x4 chips are cheaper to make. I don't
have any figures to back that up, except to observe that
the market is flooded with generic (unbranded) unbuffered
DIMMs that use x4 chips. JEDEC doesn't want the modules to
be built that way, so the companies who make them, don't
put their name on them.
The unbuffered modules with x4 chips do work (kinda) on a
limited set of chipsets. Some adverts on the web, actually
list the chipsets where the module might work. The question
they don't answer, is whether filling all memory slots on
the motherboard, with those products, would be successful
or not. Mushkin used to have a section of their web site,
where they tested various motherboards, and I think the
result was, they found exactly _one_ motherboard that could
be filled with those kinds of sticks.
At the 512MB DIMM level, there seems to be no incentive
to screw around. You are fairly safe buying 512MB DDR
DIMMs, as they'll usually be made with (16) 32Mx8 chips.
That is two banks or double sided DIMMs. I don't even know
if anyone makes 64Mx4 chips or not, they just don't seem
to be used to make "bad" 512MB modules.
There are also single sided 512MB DIMMs, which are safe to
use on a motherboard, as long as the motherboard claims
to be able to use 1GB modules. Those DIMMs would use 64Mx8
chips, and only have 8 chips total. I don't even know if
I could find you an example of a module like this now, so
they would be reasonably obscure.
At the 1GB DIMM level, both 64Mx8 chips and 128Mx4 chips
are in play. A company can shave $20 off the price, by using
the 128Mx4 chips. But the trick is, the module will be
unbranded, since the (small) company making the modules
doesn't really want to get in trouble with JEDEC. And the
memory companies cannot police this, since the (small)
company could tell them that they are being used to make
registered 1GB DIMMs.
The modules with the 128Mx4 chips are a favorite on Ebay,
and as a result, I would never buy a 1GB module off Ebay.
As a consequence, when you are shopping for 1GB DIMMs, you
have to check (somehow) how they are constructed. If the
advert says "we use 64Mx8 chips", then you are safe. If
the company admits they are 128Mx4 chips or if the company
gives that weird list of "this module works with SIS648,
Via xxx" type of list, then that is an admission of the
use of 128Mx4 chips as well. You will notice that
there are no Intel chipsets in the list of acceptable
So they can call them "high density" if they want, but
in fact they are improperly constructed modules that
should not have been offered for sale in the first
place. If you cannot fill a motherboard with those
modules and be guaranteed that they work, what good
are they ?
On the previous generation of SDRAM, it truly was
a density issue. The modules were all legally
constructed and broke no rules. The difference between
low and high density chips, was the width of the
multiplexed row and column addresses. A high density
chip had one more address bit, than there were drivers
on the Northbridge (of chipsets like 440BX). As a consequence,
only half of a high density chip could be addressed by
the Northbridge. A module with 8 chips would be half
recognized, while a module with the "lower density" 16
chips in two banks, would be fully recognized. That is
why you had to be careful when buying 256MB SDRAM for
Re: DDR high and low density?
Cool.Great explanation.But,This 64x64 thing and 128x64.Same as 128x4? The
link I proved seerms not to go where I thought.
Anyway,the dont recommend these for Intel.I wouldn't buy them for anything
anyway.I was just curious. Alot of people just look at price when buying,as
do I,but I'd like to know a bit more about these things.Thanks Paul.
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