Pecos said:
You don't sound like an idiot. You can use up to 4GB of SDRAM in a 32
bit OS but not all of the 4GB will be useful as system memory. Typically
people report 3.2-3.5 GB of useable system memory with 4 GB installed.
Anything more than 4GB requires a 64 bit OS. I am running a 64 bit
version of Window myself so if there was any real benefit from the +2GB
of extra memory I should have seen it. I don't for what I do.
I don't know why you would want to do that. If you mix DIMMs with
different performance levels you will only get the performance of the
lower rated DIMM(s). At least that's how the pre X38 memory controller
hubs work. I would expect that that the X38 Northbridge works the same
way. It makes more sense to sell the DDR2 DIMMs if you want to upgrade
to DDR3 in the future. In any case, the X38 supports *up to* 8GB, so you
couldn't put 6x2GB of DDR2 and DDR3 SDRAM in the Foxconn and expect to
use all 12 GB. See:
http://www.intel.com/products/chipsets/X38/index.htm
Intel officially *does not* support DDR2 memory with the X38 Express
chipset but that hasn't stopped some motherboard manufacturers from
implementing DDR2 support.
The Foxconn page says up to 8GB of DDR2 1066/800/667MHz memory *or* 4GB
of DDR3 1333/1066/800MHz memory. You might contact their technical
support if you want more info about mixing DDR2 and DDR3 SDRAM.
<snip>
From the very page you quoted, for the X38 - both are supported:
http://www.intel.com/products/chipsets/X38/index.htm
"Dual-Channel DDR3 memory support Delivers up to 21.2 GB/s (DDR3 1333
dual 10.6 Gb/s) of bandwidth and 8 GB
memory addressability for faster system
responsiveness and support of 64-bit computing.
Dual-Channel DDR2 memory support Delivers up to 12.8 GB/s (DDR2 800 dual 6.4 GB/s)
of bandwidth and 8 GB memory addressability
for faster system responsiveness and support
of 64-bit computing."
From the X38 datasheet (31761001.pdf)
"The MCH integrates a system memory DDR2/DDR3 controller with two, 64-bit wide
interfaces. The buffers support both SSTL_1.8 (Stub Series Terminated Logic for 1.8 V)
and SSTL_1.5 (Stub Series Terminated Logic for 1.5 V) signal interfaces. The memory
controller interface is fully configurable through a set of control registers."
That means the interfaces on the Northbridge are dual voltage - only the Vdimm has to be
programmed to the correct voltage, to drive either DDR2 or DDR3, but not both at
the same time.
In this chart, what is mentioned, is that you can use DDR2 with ECC, but DDR3 doesn't
have an ECC option. I still don't understand the details of that, as to why it was
necessary. (I don't know if this is a JEDEC issue, or a chipset specific issue...)
http://compare.intel.com/pcc/showchart.aspx?mmID=29003,143&familyID=10&culture=en-US
X38: DDR3 and DDR2, (ECC on DDR2)
DDR3 runs at 1.5V and DDR2 runs at 1.8V. Mixing them doesn't sound like a good idea.
Especially if you're planning on adding extra voltage to the DDR2 to make it run
better.
http://download.micron.com/pdf/datasheets/modules/ddr3/JTF4C_64x64AY.pdf (DDR3 module spec)
This article has a few benchmarks. Whether the comparison is fair, is debatable.
They use DDR2-1066 CAS5 and DDR3-1333 CAS7. SuperPI 8M performance difference
is about 1.1% faster. How much cash is that worth ?
http://xbitlabs.com/articles/mainboards/display/ga-p35c-ds3r_9.html
I checked a P35 datasheet, as well as X38, and they both state that the
memory buffers support SSTL_1.5 (DDR3) and SSTL_1.8 (DDR2). But you cannot
mix memory types at the same time, on the same channel. Nor, for that matter,
could you put DDR2 on one channel and DDR3 on the other, because chip designers
don't want to put separate power rails for each channel.
This is an example of a board with 6 DIMM slots, supporting some slots with
DDR2 and some with DDR3. Reviews for this mixed DDR2 and DDR3 board aren't very good.
GIGABYTE GA-P35C-DS3R
http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16813128048
As long as DDR2 is cheaper, just use it.
I saw a Powerpoint slide a while back, presented at some conference. Paraphrasing,
it said that basically the memory industry cannot make money, unless a new memory
technology is introduced slightly less than every two years. DDR3 is their
latest money grab. When the two years is up, the price drops to the point,
that they lose money on the memory they make. When you buy DDR3, you're helping
keep those guys afloat.
Paul
