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Understanding dual channel memory

I'm trying to understand how motherboards read dual channel memory. I have a Biostar board (Acer M1100 Desktop) with 4 memory slots. I understand that one set is identified as one channel and the other pair is the 2nd channel. I currently have 2 1GB 667MHz chips in the first set of slots. I added a 3rd 1GB 667 chip in the next open slot and I noticed that the Windows Experience Index in W7 dropped my score down from a 5.5 to a 5.0 after the upgrade. My question is; did putting just one memory chip in the open slot instead of two matching chips cause the MB to revert to single channel access for each chip? I was assuming that the MB would read 2GB on one channel and 1GB on the 2nd channel.

I know that if you have 4 slots on a board that supports DDR3 populated with 4 equal size DDR3 chips, the board will revert to dual channel instead of tripple channel. Wonder if this is the same behavior for Dual channel.

Basically I was wondering if I had to use 4 1GB chips to maintain dual channel performace. Sorry for the long post.

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Comments
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My understanding is that you do it in pair..always.
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What did you MB manual say ?

In most instances.....it's best to install matched pairs of DIMMs in the respective channels. Same capacity, same speed etc., and same brand if possible.

"My question is; did putting just one memory chip in the open slot instead of two matching chips cause the MB to revert to single channel access for each chip?

Answer: Yes

"I was assuming that the MB would read 2GB on one channel and 1GB on the 2nd channel."

Answer: You sssumed wrong. You could have put 1GB in but it would have had to have been 2 @ 512MBs for a total of 1 GB.

"I know that if you have 4 slots on a board that supports DDR3 populated with 4 equal size DDR3 chips, the board will revert to dual channel instead of triple channel. Wonder if this is the same behavior for Dual channel."

Answer: Adding a DIMM matching to go with the single 1GB DIMM will put you back in dual channel mode

VAPCMD

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Thanx for the confirmation

I installed another 1GB module with the same specs (667Mhz) but a different manufacturer. Could not find the same Kingston Value RAM stick locally! Anyway, I think I'm experiencing what someone called "the law of diminishing returns" with this setup. I'm not noticing any significant boost in performance at all with 4GB of RAM vs 2 or 3GB of RAM. I guess I'd have to upgrade the speed of the RAM and the processor for that.

Also, when I re-ran the Windows Experience Index, my memory scored lower with the 4GB of RAM vs the 2GB setup. (5.5 vs 5.1) Makes me wonder if it is in fact in dual channel mode. How can I check to see if the MB is in fact in dual channel mode?

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Start by checking your MB Manual..it should be the

authoritative source for info re the operation your MB product.

Once upon a time, running 4 DIMMs (4x1Gb=4GB) versus 2 DIMMs (2x2GBs=4GB) caused the system to run the RAM at a lower speed. Again...your MB manual should indicate that condition/limitation if applicable.

Also, when the PC starts up (showing system detail vs the mfg logo screen), the memory mode is often seen in one of opening screens. It says something like '128-bit Memory or Dual Channel Mode'. There's probably another screen in the BIOS setup that shows that too.

Last....try running PC Wizard 2010. It has a RAM speed test that should help show you the speed difference between 2 DIMM, 3 DIMM or 4 DIMM operation.

Re diminishing return...the gain from 2GB to 4GB for most is not nearly as significant as 512MBs to 1GB or 1GB to 2GBs. And that depends on the single or dual channel operation too...that is whether you're running 1GB...1-1GB DIMM or 2-512MBs DIMMs, 2-1GB DIMMs or 1-2GB DIMM. My approach, buy the max amount of RAM you anticipate you'll need when you buy the system. That helps avoid the 4 DIMM problem for some systems and finding/installing matching RAM later.

Let us know what you find.

VAPCMD

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Found the issue

I just downloaded a program called CPU-Z that gives you info on your CPU and memory setup. It says that I'm in single mode. The problem is with the last chip that I bought and put in. It's clocking at 400MHz while the others are clocking at 333MHz although the specs are exactly the same as the other 3 chips on the package. I'll be taking this back tomorrow.

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If you want to run your system

memory in 'dual-channel' mode, instead of 'single-channel', you have to install 2 pairs of identical DDR3 memory into the 4 DIMM slots.
Otherwise your computer will only recognize all your memory combined as 'single-channel'

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