What is the difference between Dual Channal DDR and DDR.
Am I able to use the same memory.
The DIMMs do not change, between Dual Channel and Single
Channel.
There are several ways to construct a Dual Channel interface.
On an Nforce2 chipset, for example, the memory modules are
virtually independent of one another. There is no matching
requirement on that chipset. The memory controller works in
dual channel mode, for as long as there are matching
quantities of RAM on the bus. If there is 600MB of memory
on one channel, and 456MB of memory on the other channel,
the first 2x456MB would operate in dual channel (128 bits)
mode, and the last 600-456=144MB would operate in single
channel. (I've tested this with a three line mod to
memtest86, and it really works.)
On Intel chipsets, I think the very latest chipset has
requirements almost as relaxed as the Nforce2.
For the 875 and 865PE, DIMMs sitting in equivalent spots
(like A1 & B1) have to match in their addressable
dimensions. That is because the address bus on each
channel uses identical address information, so for the
modules to operate in "lock-step", the modules have
to be addressed the same way.
Each memory chip has a multiplexed address bus. Row and
column address bits ride on that bus. The memory is a
rectangle inside the memory chip, and the row and column
address pick a location in the chip. The memory also is
chopped into four separate banks (logically speaking),
so we are talking about three dimensions from a logical
addressing perspective, even though the memory is planar.
At the next level, is the fact that a module can be
single or double sided. While that concept is easy to
remember, it is not exactly precise. DIMM modules have
64 bit wide data busses, and if you put 8 chips side by
side, each one having 8 data bits, that is enough to
make the required 64 bit width. Another way to make a
"rank" of memory, would be to use 4 chips that are
16 bits wide. Or, an illegal configuration would be
to put 16 chips of 4 bits wide each together (due to
bus loading issues).
A DIMM has enough control signals for up to 2 ranks of
memory. A standard 512MB DIMM has two groups of 8 chips
each. That is (8) x8 on one side of the module and
(8) x8 on the other side. The memory chips used are
32million locations by 8 bits, or 32Mx8 in a shorter
notation.
A second way to make a 512MB module (not too common yet),
is to use 64Mx8 chips. In this case, we have a single
rank of (8) x8 chips. The other side of the DIMM is blank.
But each chip is 64Mx8, so the row and column addressing
cannot be the same as for a 32Mx8 chip. If I mix an 8 chip
512MB module with a 16 chip 512MB module, they do not
match in their addressing dimensions. They cannot run
dual channel on the Intel chipset.
Since the dominant way of making 512MB modules results
in many (16) x 32Mx8 designs, odds are very good that
you can mix a Samsung module with a Micron module etc.
The 8 chip DDR modules are more expensive, and you have
to look hard to find them.
Timing is another issue. But a good BIOS will read the
speed info from each DIMM, and simply pick the slowest
timing from all the DIMMs, when setting them up. Timing
should not really be an issue.
When you buy dual channel memory, the modules themselves
are no different than any single channel memory. But, the
manufacturing process used to make them is very similar.
At the very least, the rows/columns/banks/ranks are
matched. The timing ("2-3-2-6") matches as well, although
that is not a strong technical requirement. The contents
of the SPD EEPROM on the DIMM, that holds the speed info,
will match (except for the serial number).
With 128MB and 256MB DIMMs, the situation is more fluid.
There are different ways of making modules, in many cases
visible from the outside (if there are no heat spreaders
in the way), that will make the modules unmatched. The
Intel chipset will use them in "virtual single channel"
mode, meaning you won't get the same level of performance
as you would if they were dimensionally matched.
The matching requirement on Athlon64 seems to be different
than on Intel chipsets. It seems the BIOS, at least, is
demanding more parameters match, than would be used on
a board like an 875/865 based chipset. As you can see from
recent postings to this newsgroup, people are still trying
to figure out what will work, and perhaps later BIOS will
improve things. Technically, revision E silicon for 64 bit
AMD processors, have similar capabilities to the Intel
chipsets, but it may be a while before a good BIOS makes
this apparent. (Document 26094 on the AMD site is the
best reference available.)
So, to summarize, if you are using two 512MB DIMMs, chances
are very good they will match well enough to be used in
dual channel mode on a new motherboard. You could use, for
example, a pair of 512MB DIMMs, and a pair of 1GB DIMMs.
In the previous example, there are slight performance
advantages to using four matching DIMMs, versus two
pairs of matching DIMMs. In the first case, the memory
controller can be put in an interleaved mode, which means
logically adjacent memory locations are in physically
different DIMMs. Using pairs of DIMMs that aren't the
same size, while the memory is still dual channel,
means interleaving cannot be enabled. The performance loss
is not worth fretting about, and is quite miniscule in
real world terms.
The memory requirements are also stated in these two Intel
documents. Have a look at this, and compare to the inventory
of modules you have in your hand.
865P memory guide (for P4P800 type boards)
ftp://download.intel.com/design/chipsets/applnots/25303601.pdf
875 memory guide (for P4C800 type boards)
ftp://download.intel.com/design/chipsets/applnots/25273001.pdf
Also, when shopping for a board, do not get an 848P board, as
I believe that is a single channel version of the 875/865 and
will be no more useful than your P4V8X-X board to you.
In terms of buying "dual channel memory", where two DIMMs are
packaged together, you should not have to pay a premium for
the fact that two DIMMs were pulled from a big barrel and
slapped into a retail package. The price of two individual
DIMMs should match the price of two DIMMs packaged together.
There is no "secret matching process" being used. There is
also nothing contained inside the DIMM that says "dual channel
only" or the like.
Since the DIMMs came from the same big barrel of parts, we
know their physical construction is identical, and that is what
you are getting when the DIMMs are packaged as duals.
If getting 1GB modules, get ones with (16) 64Mx8 chips. Ask
your vendor if in doubt.
If getting 512MB modules, (16) 32Mx8 are the most common design.
For smaller modules, there are too many construction techniques
to list. Memory is too cheap now, to worry about such small
modules.
HTH,
Paul
