Dual channel mode?

So I read a few comments on a youtube video instructing how to replace RAM. A person commented asking if he could use 2 2GB modules in one bank and one 4GB module in the other and the response was no because he would lose the dual channel advantage.

On wikipedia, it says dual channel works provided you have the same amount of memory in each bank and they gave an example where they said 1 1GB module and 2 512MB modules would enable dual channel support.

Even further, Corsair's webpage on the concept just confused me more because they didn't really state this specifically, except saying the two different sets of modules needed to be the EXACT SAME.

Can anyone help me with this concept? Would I need the exact same amount of memory in the two banks or would I need the same amount of SODIMMS? Both? Same size of SODIMMS?

Also, does anyone know if the laptop slots are color coded? I know my desktop mobo has I think red and orange colors to distinguish the two banks. I know that generally one set is located under the back panel and one under the keyboard. Does anyone know if these are the two separate banks or does each have two separate slots?

 

I believe that equivalent stick pairs (speed and size) need to be used for dual channel to work but only for those paired. The speed can actually be different but the speeds will all default to the slowest module. I thought that if you had 4 sodimms they'd be divided into two groups (I'll just call them A and B) and for dual channel to work, both of the sticks in A had to be the same and both in B had to be the same, but the sticks used in A could be a different size (but still the same speed) than the sticks used in B. I also thought that the manufacturer could be different as long as the size and speed were the same. You could have 6 gb of ram running in dual channel if you had 2x2gb in the A slots and 2x1gb in the B slots. It's recommended, but not necessary, to use the same speed and manufacturer to reduce any incompatibility issues.

As far as the different color scheme, I think it's there to represent that you can use sticks of different capacity. So would it mean if they are one color you'd have to use all the same size?

Edit: I'm researching now to make sure what I wrote is correct. I'm not an expert so I could very well be wrong and I just want to make sure.

 

Not even the speed, as far as I know. I'm pretty sure RAM will downclock to the slowest you have installed. As long as your sticks are the same size everything should play nice.

 

Regarding dual channel RAM sockets are paired in that if you wan to run in true dual channel mode, you'll need to have sticks of matching capacity in paired slots.

I'll use a desktop motherboard as an example since it's easy, if you look at a desktop motherboard with 4 sockets, you'll see they are paired by color. Usually, 1 and 3 are paired and 2 and 4 are paired. Sockets 1 and 3 are blue and 2 and 4 are black for example. As long as you have matching RAM for each pair, you'll run in dual channel. For example, you could have a 2x4GB pair in sockets 1 & 3 and a pair of 2x2GB in sockets 2&4, both will run in dual channel.

It works the same for laptops, it's just harder to tell which sockets are paired and it depends on the notebook.

Regarding speed, the RAM will run at the speed of the slowest stick installed. Usually this doesn't pose a problem, but in some rare cases the RAM decides to be picky and not run properly with the other sticks. I personally would make sure to get RAM with the same clock speeds and timings, for example DDR3-1600MHz CL9. This shouldn't affect dual channel, but as i said there's always the rare random incompatibility issue so some of us get RAM with he same speed and timings to avoid potential issues. I happen to be running 4x4GB 1333MHz CL9 Kingston in my laptop, i got it because it was the cheapest available at the time. I used to run 2x2GB + 2x4GB, same speed and timings as the 4GB, never got any problems.

 

Aight thanks tijo. That cleared it up. So basically each SET (bank 0, bank A whatever you wanna call them) should have similar RAM but they can differ to bank 1 or bank B or whatever?

 

Yes, you need a matching pair for each paired sockets, example of a not so simple laptop setup from the G73JH manual. I ran 2x4GB in 1&3 and 2x2GB in 2&4 full dual channel. You should look into asynchronous dual channel too, it complicates understanding things a bit, but basically, you can still run some of your RAM if it meets dual channel criteria while the other doesn't. By the way, the difference between dual channel and single isn't all that apparent, kinda like DDR3-1066MHz vs DDR3-1600MHz. You'll notice a little more snap to the system, but nothing game changing like SSD vs HDD.

 

The pairs can differ in capacities to eachother, but they must have the same frequency and each pair bust be matched with the same frequency & capacity. If you use 2 pairs of different frequency, then it will run at the lowest speed and throttle the other set.

Intel has some new tech which basically matches any sticks of RAM together into an emulated dual-channel but i'm really not sure how the performance fairs.

 

Thanks. Yeah I know. I just wanted to understand what it meant coz I would wonder when people would talk about dual, triple and last year even quad channel.

Ok now that confused me again . Tijo said that 2x4GB and 2x2GB would work in dual channel. Do you mean to say that 1x4GB and 1x2GB in one bank and 2x2GB in another bank would also work as dual channel?

 

You might want to read this: http://www.hardwaresecrets.com/article/133, it's a pretty good read and should answer all of your questions. Gotta love hardwaresecrets for those kind of tutorials. They have a pretty neat one on RAM timings as well.

The 2x2GB pair will run in true dual channel mode, the other will not.

 

That's exactly what I had thought. Thanks a lot for your help, all of you, especially tijo. Also thanks for the hardwaresecrets link.

 

That's exactly what i said - The capacities can be different between the pairs (ie. 2x4GB [pair #1] & 2x2GB [pair #2] would work), just as long as the sticks in each pair are identical to eachother (ie. 2x4GB [pair #1] must both be 1333MHz & 2x2GB [pair #2] must both be 1333MHz).

However, if one of the pairs are at a different frequency (ie. 2x4GB [pair #1] both at 1333MHz & 2x2GB [pair #2] both at 1066MHz) then all of the RAM will run at the lowest speed (@ the 1066MHz speed of pair #2 - It can not run faster to match the other pair, but the other pair can drop its speed to match this pair).