Your dual channel knowledge here

Please tell me more about dual channel DDR.
What memory characteristics are best, most important?
How/where does dual mode tell you it's active?
How closely do memory sticks have to "match"? same size (MB), same speed (MHz), same layout (#x#), same model number, I can't imagine any other realistic requirements but...


Does anybody know any of this or is it all hyped? I have a dual DDR mobo with two sticks bought a year apart from 2 manufacturers and the mobo flies and is stable as a rock but gives no indication of actually being in dual mode. Frankly it reads like hype.

In a motherboard, the CPU is connected to the memory modules (one or two or more) via a memory controller (inside the north bridge).

Single channel memory modules and memory bus

CPU <---- fsb ----> Memory Controller <---- memory bus ----> Memory Module (dimm1/dimm2/dimm3)

maxFsbBandwidth = fsb x 2 x 8 MB/s
maxMemoryBandwidth = memoryBus x 2 x 8 MB/s
x 2 because data is pumped at twice the bus frequency (using both rising and falling clock edges) in DDR (double data rate) form,
x 8 because the memory data bus is 64 bit or 8 Byte wide.
E.g. fsb = 200 MHz,
memoryBus = 200 MHz,
maxMemoryBandwith = 200 x 2 x 8 = 3200 MB/s
(This is why fsb 200 MHz, DDR 400, PC3200 refer to same thing)

In a motherboard with dual channel, two memory dimms can be connected to the memory controller(s) in parallell to provide in theory twice the bandwidth to the memory controller(s)

Dual channel with two memory modules, memory buses in parallel

........................... Memory Controller <---- memory bus ----> Memory Module (dimm1/dimm2)
CPU <---- fsb ---->
........................... Memory Controller <---- memory bus ----> Memory Module (dimm3)

singleChannelMemoryBandwidth = memoryBus x 2 x 8 MB/s
maxMemoryBandwidth = 2 x memoryBus x 2 x 8 MB/s

So indeed, in dual channel, the memory controller(s) sees twice the memory bandwidth if two memory modules are put into the correct dimms (e.g. dimm1+dimm3, or dimm2+dimm3 for A7N8X). So far so good.

For the current AMD nforce2 MB, the fsb for dual channel is still running as the SAME frequency as a single channel memory bus. Even the memory modules are in parallel to give twice the bandwidth to the memory controller(s), but the fsb CANNOT take advantage of getting twice the data rate from memory controller(s)!!!!

Things are actually more complicated than that for nforce2 memory controller, there is some speculative prefetch data cache in the north bridge to make use of the dual channel bandwidth, so that some data are cached in the NB and can get to the CPU faster than getting from the memory. But the net is that until AMD can provide quad rate (or double fsb speed) than now, there is little or no advantage to make use of the dual memory bandwidth for the CPU. There are some MBs implementing integrated video (a diff version of the NB) which has 2x64 bit datapath and can take advantage of the dual channel memory bus bandwidth.

For current nforce2 MB, maxMemoryBandwidth = fsb x 2 x 8 MB/s
At fsb = 166,
maxMemoryBandwidth = 166 x 2 x 8 = 2656 MB/s.
If one can overclock hard (for nforce2) to 220 fsb, one would get around 3300 MB/s which will close in to Intel quad rate w/ DDR (but not there yet 4000-5000 MB/s).

For Intel, the fsb is quad rate, i.e. the fsb is running TWICE as fast as the memory bus speed at DDR.
E.g.
fsb = 166 * 2
memory bus = 166 (DDR 333)
maxMemoryBandwidth = 2 x 166 x 2 x 8 = 5312 MB/s
In general, the controller efficiency is not 100%, and also due other bus traffic. So one cannot get twice the memory bandwidth, the effective is around 70-80% = 4000 MB/s (guessing some what here, have to look up for details).

It's getting a bit long here. The net is

.......

(to continue)

(continue from last post)

The net is

1. For current nforce2 dual channel, little or no advantage until AMD fsb is twice as fast as the memory speed (in the future ?).

For Intel P4 chip set, since fsb is twice as fast as memory speed, minus some overhead in the memory controller, the effective bandwidth is about 70-80% of dual channel bandwidth, (guessing some what here, have to look up for details). It is clear 2 modules in dual channel should be used whenever possible.

2. Overclocking memory module(s) vs dual channel vs bank interleaving

IMHO,
one single bank memory module, can clock few MHz higher than
one double bank memory module, can clock few MHz higher than
two modules in single channel mode, can clock few MHz higher than
two moudles in dual channel mode

For AMD nfoce2, I found that for two memory modules, putting in as dual channel (dimm1+dimm3 for A7N8X) cannot be clocked as high as putting them in as non-dual channel (dimm1+dimm2), by a few MHz. (This does not mean the dual channel is slower in nature, actually should deliver almost twice the bandwidth. I suspect it is due to the current memory controller implementation, driver, ..., in a way that the fsb is lacking in dual channel SYNC mode. Exact reasons TBD, it is unknown to me???) ANY DIFFERENT OPINION ON THIS ??

So for AMD MB, the few MHz gain by overclocking a single memory module along that line translates into higher memory bandwidth and 3D performance and outweighs the intrinsic advantages for dual channel and multi-bank memory modules.

Dual channel, pro: the double memory bandwidth minus some overhead loss (if the fsb can use it as in P4), and also the speculative prefetch cache in the NF2 memory controller.

Bank interleave using 1 or 2 modules, pro: to reduce RAS overhead when read/write large blocks/pages of data that cross over memory banks, as in video applications

This is just a tradeoff between the performance gain in overclocking (say 5-10 MHz), and the few % performance gain from dual channel (AMD NF2) and bank interleaving. It seems to me that the MHz gain by overclocking has a slight edge over the dual channel/bank interleaving for AMD MB (exclude the NF2 version that has integrated video).

But if for non-overclockers or like manufacturers who run at a fixed frequency, then using two modules in dual channel and bank interleaving will clearly get the advantages mentioned above.

Again, for Intel P4, it is clear that using two modules win.

3. Whether running as duall channel or not, should use memory of the same size (MB), same number of banks/module, same RAS/CAS timing (those magic numbers that look like 6-3-3-2 1T) into the different dimms (basically, the same part number).

4. 256MB vs 512 MB vs 1GB

Currently, most modules are using 32Mx8 = 256 Mb DRAM chips, regardless of speed (6ns, 5 ns or 4.5 ns).

1 module with 1 bank overclocks the highest, currently it is 256 MB. But 256 MB is usually not enough to run smoothly in XP (too much paging).

256 MB = 1 bank in 1 module (which contains 8 chips on 1 side (nonECC), (9 chips for ECC module))
512 MB = 2 banks in 1 module (which contains 16 chips on 2 sides (nonECC), (18 chips for ECC module))

If you want 512 MB total, two choices:
2 modules 256 MB (1 bank/module) ---- prefer for Intel P4 for dual channel advantage
1 module 512 MB (2 bank) ---- prefer for AMD since dual channel no advantage yet !!!

If you want 1 GB total, you have to get 2 modules of 512 MB (no choice)

5. To test whether dual channel is active:

For Intel P4 MB, you will see a noticeable drop in memory bandwidth when one channel is taken out.

For nforce2 MB, you won't see a big difference w/ or w/o dual channel when running in SYNC mode.
In order to see the dual channel effect in nforce2, you can test by running ASYNC at 50% or 75% with memory speed being slower than fsb. Then you will see a noticeable drop in memory bandwidth when one channel is taken out.

Gotcha, hype (almost) all the way. That's what I thought. Both my modules LOOK the same so they're probably running in dual mode.

I originally thought dual mode was with the two close together and had it that way. I inexplicably lost 50 points from my posted 3DMark score, the stick arrangement could resplick it. I'll check later. Thanks for the explanation.

Great post.


On a sidenote, Corsair XMS sells specially matched modules for dual channel based systems (http://www.corsairmicro.com/xms/xms_modules.html). They actually test them on A7N8X to do up to 400mghz in 2-2-2-6-T1.

A fantastic buy for A7N8X. My KingMax PC 3200(256x2) cant even hit 140mghz in dual channel mode



Ugmore Baggage I highly disagree with you. My A7N8X scores over 1000 more 3DMarks (2001SE) in dual channel mode. No the memory benches arent 2x that of single channel. But they still are higher.

"A fantastic buy for A7N8X. My KingMax PC 3200(256x2) cant even hit 140mghz in dual channel mode"

That doesn't sound right. I mean that's what you should expect from crummy PC2100.

I tried moving my sticks. Still inexplicable, my score's the same.

To put A7N8X into dual mode, you have to select agressive mode in Bios, and actually @ startup it tells you that it is using agressive dual timings.

Thanks T2O, Nothing like that happened when I put my memory into "High Performance".

So T20 You have seen inprovement in your dual channel. I am just getting my Epox 8RDA in a day or two and I was thinking about getting some matched pair corsairs.

Here is a good example:

My Rig:
A7N8X Deluxe
XP1600+ @ 333FSB
2x PC2700 Samsung DDR ram
Ti4200 300/500


333FSB mem in sync (333Mhz): 12536 3DMarks
http://service.futuremark.com/compare?2k1=6077092

333FSB mem @ 400Mhz: 12181 3DMarks
http://service.futuremark.com/compare?2k1=6079750


As you can see even though in dual channel mode, the mem is running MUCH slower, I get 400 more 3DMarks.

i ahve a question. i plan on buying an 8rda+ in the next couple of months...
dimm1 is closest to the processor, correct? suppose i get two matched corsair 512MB modules... if theyre in dual channel mode, will it read that i only have 512MB of ram (like raid, but for ram), or will it read 1GB no matter what combo i have them in?

oh, and dual channel has higher 3dmark scores than single channel?

Ok, I had to chop the screenshot down quite a bit so theforumisdown's 50KB limit would accept it, but...

This is all you need to know about dual channel: Pentium4 system, high-fsb overclock, and this screenshot from my setup:



Oh yes, dual channel is where it's at!

AshlarZiven said:

if theyre in dual channel mode, will it read that i only have 512MB of ram (like raid, but for ram), or will it read 1GB no matter what combo i have them in?

Click to expand...

It will be 1GB, no matter what combo.