How Does RAM Works?

[Vasyl Baran]

I have a 3 RAM sticks (3 x 4GB). So, my question is: the third RAM stick will be used only if system requires more than 8GB of RAM or they all are going to be used at the same time? (with work equally divided)

 

[WyomingKnott]

They're all going to be used; I doubt that the work will be evenly divided.

Whether or not this is a good idea depends on your motherboard - you did not include make and model. On a dual-channel board, having an odd module can decrease overall performance, since you will lose the advantage of dual-channel speed when accessing the odd 4 GB. On a three-channel board, it will work faster with all three installed. On a single-channel board, it doesn't much matter.

 

[Vasyl Baran]

Two of them works in dual-channel. My motherboard and CPU supports only dual-channel memory. So, the third RAM stick will only be used if system requires more than 8GB of RAM?

 

[WyomingKnott]

No, as I wrote above all the sticks will be used. Memory access to that last 4 GB will be slower than memory access to the other 8 GB.
1) Unless you really need more than 8 GB, if I were in your shoes I'd only use two modules.
2) Are any two modules in that set from a matched pair? Since the advent of dual-channel memory, vendors have been selling memory modules in matched pairs that have been tested together. It's better to use the two that are a matched pair.

Different people will have had different experiences and tell you that using unmatched memory makes no difference, slows your system down noticeable, or makes your system die. It depends on what experiences they had.

So if you need fastest memory performance, use two, preferably two that came together. If the extra memory is important to you, either use three or go buy a matched pair and use four. Or just buy one more and see how four totally memory modules work together.

EDIT: Depending on the motherboard, with three sticks it will either run single-channel or dual for two sticks and single for the third. See this thread: http://www.tomshardware.com/answers/id-2096208/pros-cons-sticks-ram-dual-channel-motherboard.html and Tradesman1's answer.

 

[Darkbreeze]

And just to add to that, if you have one of the few boards that supports it, and if all three modules are compatible enough for it to do so, there's a third possibility of running in triple channel. But the number of boards that support that is pretty slim. Since we don't know the model it's impossible to say but for probably 90% of cases, what WyomingKnot has outlined is the probable scenario.

 

[Tradesman1]

It all depends on the platform, if the DRAM is showing as dual channel - can check in CPU-Z (free app) in the memory tab, then you are running in Flex mode or the AMD equivalent (8GB in dual, 4GB in single). The use is spread out across all the DRAM