A question on Dual Channel RAM

[HOOfan 1]

I have seen people state that most motherboards will allow 3 sticks of equal size, while one channel will be double the size of the other channel.

I have seen people say that it will only be dual channel if you have two sticks in one channel that add up to the size of the stick in the other channel (some motherboards may not support)

I have seen people say that if you stack 2 sticks in one channel of equal size to the stick in another channel, that you will have two sticks running in dual channel and a third running single channel...although this doesn't make as much sense, because that third stick would have to be in the same channel as one of the sticks running in dual channel mode.

The first explanation makes sense, that you could have dual channel working, but one channel would be twice as large as the other channel, but what type of performance consequences would that bring?

Edit: as a follow-up. Some things I am reading suggest that Intel's Flex Memory Technology allows for the assymetric dual channel RAM (e.g. 3X1GB sticks), yet AMD has not answer to this, meaning that AMD motherboards would require matched channels.

Or does it just mean that Intel has found a solution to speed up assymetric dual channel arrays, while AMD boards can still work with assymetric dual channel, yet be nearly as slow as single channel?

 

[taltamir]

AFAIK the only way to get dual chanel is to have two identical sized sticks in the MATCHED pair of slots (weather they alternate or are near each other varies by mobo).

 

[HOOfan 1]

Here is a page about how Intel motherboards achieve dual channel

http://www.intel.com/support/m.../sb/cs-011965.htm#dual

Here is an excerpt from an Asus M4A79T Deluxe motherboard

You may install varying memory sized in Channel A and Channel B. The system maps the total size of the lower-sized channel for dual-channel configuration. Any excess memory from the higher-sized channel is then maped for single-channel operation

I am not sure if that makes sense though.

That would imply that if you have two 2GB sticks and a 1GB stick, and you place the 1GB stick and a 2GB stick in one channel and the other 2GB stick in the other channel...the lower-sized channel is 2GB the larger-sized channel is 3GB, so the system will run dual channel with 2GB and single channel with 3GB. Or if you have a 2GB stick a 1GB stick and a 4GB stick, if you place the 1GB stick and the 2GB stick in one channel and the 2GB stick in another channel, then the system will run 3GB as dual channel and 4GB as single channel.

So if you have three 1GB sticks and you place two in a channel and the remaining DIMM in the other channel, you will have 1GB running in dual channel and 2GB running in single channel. But I would think you would have 2GB running dual channel and 1GB running single channel...but the quote above, doesn't really suggest that UNLESS they meant the lower sized pair (meaning lowest sized span of two sticks between channels)

 

[taltamir]

wow... i stand corrected...

 

[HOOfan 1]

see my edit above...I added a lot.

Long story short, there is some dude on another forum, making absolutely ludicrous claims about his system..and I am trying to catch him in as many lies as possible.

He claims to have a Phenom II X4 955 and an AM3 790FX Asus motherboard..not to mention a bunch of other equipment that would cost several thousand dollars, but then he claimed to be using three 1GB sticks of DDR3. I pointed out that Phenom II does not support triple channel, and asked why he would handicap himself by not getting matched pairs and going with 4GB. He stated he didn't have the money....after bragging about all of the extremely expensive (and some of it obviously imaginary) hardware that he has.

Every time I catch him in a lie, he tries to spin something out of it.

He claimed to have a 750W Enermax...I told him Enermax doesn't make 750W PSUs...so he claimed they custom built it for him.

Edit: Actually Intel's description of Flex Mode sounds the same as the above quote from Asus's manual

Asus M4A79T Manual:
You may install varying memory sized in Channel A and Channel B. The system maps the total size of the lower-sized channel for dual-channel configuration. Any excess memory from the higher-sized channel is then maped for single-channel operation

Intel about Flex Mode:
The bottommost DRAM memory (the memory that is lowest within the system memory map) is mapped to dual channel operation; the topmost DRAM memory (the memory that is nearest to the 8 GB address space limit), if any, is mapped to single channel operation. Flex mode results in multiple zones of dual and single channel operation across the whole of DRAM memory. To use flex mode, it is necessary to populate both channels.

I thought I read somewhere that only Intel had anything like Flex Mode..either I misunderstood, or AM3 has rectified that situation.

 

[taltamir]

He claimed to have a 750W Enermax...I told him Enermax doesn't make 750W PSUs...so he claimed they custom built it for him.

Wow... that must have been expensive to get them to custom build a psu for him...

 

[HOOfan 1]

Originally posted by: taltamir

He claimed to have a 750W Enermax...I told him Enermax doesn't make 750W PSUs...so he claimed they custom built it for him.

Wow... that must have been expensive to get them to custom build a psu for him...

I am sure if they were even willing to do that...it would cost thousands of dollars...of course he claims it has no fan, and no heatsinks...which is basically impossible.

 

[fffblackmage]

Asus M4A79T Manual:
You may install varying memory sized in Channel A and Channel B. The system maps the total size of the lower-sized channel for dual-channel configuration. Any excess memory from the higher-sized channel is then maped for single-channel operation

Intel about Flex Mode:
The bottommost DRAM memory (the memory that is lowest within the system memory map) is mapped to dual channel operation; the topmost DRAM memory (the memory that is nearest to the 8 GB address space limit), if any, is mapped to single channel operation. Flex mode results in multiple zones of dual and single channel operation across the whole of DRAM memory. To use flex mode, it is necessary to populate both channels.

I think these two things are saying the same thing.

For example:
Channel A has 2x1GB
Channel B has 1GB

The smallest amount of ram in any single channel is 1GB, so it takes 1GB from each channel and uses them in dual channel mode. The leftover 1GB in channel A is in single channel mode.

Note the the Asus manual states using the excess memory from the higher sized channel (the excess 1GB in channel A) in single channel mode.

Intel's Flex mode seems to say the same thing to me.

 

[taltamir]

no heatsinks? it just... magics the head away?

 

[TemjinGold]

Originally posted by: taltamir
no heatsinks? it just... magics the head away?

Mmm... magic head...

 

[taltamir]

HEAT... magic the heat away... fruedian slip there.