DDR DRAM FAQs And Troubleshooting Guide

By Jim Reece
published

DRAM is probably one of least understood components in a computer. Here are answers to our readers’ most common questions about memory.

What Is Flex Mode?

Intel Flex Memory technology dates back to 2004. It lets you use DRAM of different capacities to enable the exploitation of the motherboard’s multi-channel architecture whenever possible. Maybe you already have two 2GB DRAM modules and want to add a 4GB module. Generally, you would be running the 2x 2GB set in dual-channel mode (normally, slots 1/3 or 2/4). With flex mode, you can put the 4GB one in slot 1 and the two 2GB modules in slots 3/4 so that you have 4GB in each channel.

Flex mode can also take into account uneven amounts in the channels — if, say, that 4GB module were 8GB, then you would have 8GB running in dual-channel mode and the leftover 4GB as added capacity in single-channel mode. Another possibility would be to have 8GB in slot one, 2GB in slot two, 4GB in slot three and 4GB in slot four, giving you 18GB total or 16GB in dual-channel mode and 2GB in single-channel mode. This is sort of an odd setup, but I have seen it used. It is more common to have 2 x 8GB and 2 x 4GB where, ideally, it would be placed 8-4-8-4 so that 12GB is in each channel.

While this is a nice feature, I don’t suggest planning to buy DRAM to run in this manner. There are no guarantees that any DRAM you buy will play well with what you already have. We will expand on this in Part 2.

Jim Reece
Topics
DDR3
Memory
47 Comments Comment from the forums
  • Nuckles_56
    Thank you for this useful and informative article
    Reply
  • das_stig
    With AMD motherboards, you should set the memory to unganged mode for a tiny performance improvement unless you're running a webserver.
    Reply
  • vicstead
    Thank you Jim for this very informative article, an enjoyable read.
    Reply
  • Mahruay
    Nice read although could you explain what real world improvements can be seen in faster RAM.
    Reply
  • boju
    Nice article, answers a lot for people and definitely will link for future references. I need to ask though, is there any reason to discriminate DDR3 as per title?
    Reply
  • TechyInAZ
    Wow this is very informative thank you for this!!
    Reply
  • MPA2000
    Lost me after Virtual v Physical.
    Reply
  • clonazepam
    I hope the next part covers performance when using say 3 sticks, where 2 are dual channel, 1 is single channel. Some real world results would be stellar (maybe as a follow-up?)

    I'd also like to see RAM drives covered. Suppose you allocate 4GB out of 16 for a RAM drive. How does the software create the 4GB? Is it using a single chunk of memory, is it taking 1GB from each of the 4 sticks? Is it from the beginning, middle or end of the 16GB of memory?

    Covering how to identify true "memory leaks" versus a more common scenario where RAM usage grows intentionally from the caching of more and more assets.
    Reply
  • damric
    Great article, Tradesman! I give it Two thumbs up and two big toes up too!

    Only 1 issue:

    Ganged vs Unganged: that actually doesn't have to do with single or dual channel.

    Quote AMD:
    Ganged mode means that there is a single 128bit wide dual-channel DRAM Controller (DCT)
    enabled. Unganged mode enables two 64bit wide DRAM Controllers (DCT0 and DCT1).
    The recommended setting in most cases is the Unganged memory mode. Ganged mode may allow slightly
    higher Memory performance tuning and performs well in single-threaded benchmarks.
    Depending on the motherboard and BIOS, it may be required manually setting the timing parameters for each
    DCT (in Unganged mode) when performance tuning the memory or fine tuning the timings. Some BIOS
    versions apply the same timings automatically for both DCTs in an Unganged mode.
    Unganged is like a normal divided highway with two directions. Ganged let's traffic use all of the lanes in one direction at a time. Unganged is said to be more efficient but no one really ever tested this thoroughly to see if any applications would be better served in ganged instead. You could still have unganged single channel or dual channel, and ganged single channel or dual channel. If that's confusing I'll try to explain with more complicated interstate highway anecdote.

    Lastly, I see you have a new AMD rig. Did your head explode when you saw how much more difficult it is to tune memory on that platform than on your past intel rigs?


    Reply
  • Shankovich
    Awesome article! These kinds of articles is what brought me to Tom's in the first place years ago!
    Reply