The Most Common DDR DRAM Myths Debunked

All DDR Is The Same

In Part 1, we examined some basic facts about DRAM. Now, we’ll look at some topics that are often more contentious. Here's what we’ll cover in Part 2:

  1. All DDR3 Is The Same
  2. Just Add More DRAM
  3. There Are Only A Few DIMM Manufacturers
  4. 3200 MT/s Support Means You Can Use Any DRAM
  5. Mixed DRAM Runs At The Speed (Or Timings) Of The Slowest DIMM
  6. It’s Cheaper To Buy Two Sets Of DIMMs Than Larger, More Expensive Sets
  7. DRAM Will Run Faster With All Slots Filled
  8. There Are No Performance Gains With DRAM Faster Than 1600 MT/s
  9. 8GB Is All You'll Need For The Next X Years
  10. You’ll Never Use Or Need 16GB Or More
  11. I’m Not Using All My DRAM, So More DRAM Won’t Increase Speed
  12. A 64-bit OS Lets You Run All The DRAM You Want
  13. 1.65 Volt DRAM Will Damage Your Intel CPU
  14. Dual-Channel Mode Doubles The Data Rate, Or Is Twice As Fast

All DDR3 Is The Same

This topic alone could take up a lot of space, but I will try to keep it short and offer some suggestions. Here are a few examples:

  1. I’ve mentioned Kingston’s line of Fury DRAM, which doesn't come with an XMP profile but instead operates using plug and play. They look nice, come in a variety of heat sink colors, are reasonably priced and appeal to people with older systems who want to upgrade their DRAM. But since they are based off PnP, they will operate only under various motherboard chipsets: H67, P67, Z68, Z77, Z87 and H61 from Intel, along with AMD's A75, A87, A88, A89, A78 and E35. You can include Z87 and Z97. The sets I listed are direct from the company’s product page.
  2. The actual chips also differ:
    • Most of the DRAM being manufactured today uses high-density, 4Gb memory chips, whereas older DDR3 used lower-density 2Gb chips. Older memory controllers are often limited to lower densities. One of our editors recently found that none of his P55 motherboards worked with any of his 8GB modules, and if modules with different densities are mixed, a module can become undetected or unstable.
    • There is a number of different companies that make memory chips, and they make them to their own specifications. They also make multiple models of each chip. Each of those lines is split up or binned according to the strength of the chip.
  3. Most enthusiast-oriented motherboards are designed to take non-error-correcting coding (ECC), unbuffered DRAM. Usually, ECC is for servers and professional workstations where data integrity is critical, and buffered (registered) DIMMs are used exclusively in servers that require ultra-high memory capacity. Sharing technology among high-end platforms allows some enthusiasts to have the option of using ECC on their motherboards.
  4. There are other examples covered elsewhere, like DRAM with a data rate that’s too high for your CPU (but still functions at a slower default data rate).

Usually, I suggest checking with the DRAM manufacturers, which spend a lot of time testing their memory on the various motherboards out there. The motherboard manufacturers provide a qualified vendors list (QVL) of the DRAM they have tested with a given product, but it usually consists only of the small variety of DRAM they have available in their labs and isn’t as reliable as checking directly with the DRAM manufacturers. In the Tom's Hardware forums, I have found many rather knowledgeable members who provide good advice on various DRAM for specific motherboards and platforms, as well as information about what data rates the different CPUs can handle.

Just Add More DRAM

JEDEC is a council of electronic-device manufacturers and design firms that sets industry standards to be universally adapted by its members. Because some DRAM manufacturers decided to exceed the JEDEC maximum DDR3-1600 CAS 11 (and, later, CAS 9) by offering tighter timings and higher data rates, mixing DRAM has not been as easy as the council intended.

Simply put, mixing DRAM from different packages is a crapshoot, even when you have two identical packages of the same exact DRAM model. I would like to add that DIMMs that don’t appear to work well together often, but not always, can be helped with voltage and/or timing adjustments. There are a couple of examples in "DDR3 Memory: What Makes Performance Better?" where two of the companies didn’t market 32GB sets of 2400 MT/s DRAM and sent me a pair of matching 2 x 8GB sets. Neither worked initially, and it took minor adjustments for them to run smoothly.

Why is this such a problem? After all, they are the same frequency, timings and voltage.

DRAM is made up (basically) of memory chips that are soldered to a PCB (printed circuit board), driven by electricity. During the course of a DRAM production run on a given model, the manufacturer might be finishing up a large section of PCB that has been cut to the DIMM size but then might switch to a new PCB from a different production lot, which can result in slightly different properties.

The same can happen with the physical solder; the manufacturer may change to a completely different kind that has ever so slightly different conductivity properties. 

Then, there are the dies themselves. When made by the actual chip manufacturer, the chips are "binned" (sorted) according to their quality.

Let’s take a closer theoretical look at this concept. A single production lot may result in, say, 1000 memory chips, which are separated or binned. A manufacturer may classify 200 chips as entry-level, and separate 350 chips that are a little better, 300 chips that are even better and 150 chips that are the best. Then, they sell these chips to different manufacturers.

If you were to go out and buy an 1866 MT/s module from each company, you would likely be getting a different PCB in each, a different solder with various conducting qualities and quite possibly differently graded chips and/or chips from different manufacturers.

Several companies are making memory chips, which further adds to the questions about compatibility, and you might start to see why mixing DRAM can be, and often is, problematic.

We also noted earlier that most newer lines of DRAM use 4Gb densities, whereas the norm with older lines was 2Gb.

There Are Only A Few DIMM Manufacturers

This is both myth and misrepresentation. There are a number of companies that make memory chips and numerous companies that make DRAM modules. There is rebranded DRAM out there made by one or more companies for other companies. For instance, the AMD Radeon lines of DRAM are made by Patriot and VisionTek.

3200 MT/s Support Means You Can Use Any DRAM

To run pricey 3200 MT/s memory, you also need a CPU that can handle such a high data rate. Otherwise, your DRAM may only run at 1333, 1600 or 1866 MT/s, at best. 

In the days of Intel’s LGA 775, CPU and DRAM overclocking was controlled primarily by the FSB (front-side bus). Let’s say you had a Q6600, and your motherboard supported a 1066MHz FSB. Left there, your CPU would run at its native 2.4GHz and your DRAM at 1066 MT/s. If you were to overclock the CPU by raising the FSB to 1333, your CPU would run at 3GHz and you could run DRAM at 1333 MT/s. In other words, the memory was limited to the FSB's ceiling. The memory controller (MC) was in the chipset—most often, the motherboard's northbridge—and ran at FSB frequency.

Now, the MC is in the CPU. So, if you want to run at your memory's advertised specs, the CPU is the primary factor. Haswell-based CPUs are rated at 1600 MT/s DRAM, and mid- to upper-tier non-K CPUs can typically run 1866 up to 2133 MT/s fairly reliably. For higher data rate DRAM, a “K” model CPU can be overclocked, thus helping the MC support more enthusiast-oriented modules.

AMD’s current FX CPU line is rated at “up to 1866 MT/s at one DIMM per channel.” However, you may find yourself running into problems with lower-end CPUs—and, at times, even the higher-end CPUs—running 1866 MT/s. This is partly because the MC in the FX CPUs is optimized for DDR3-1333 (according to the BIOS and Kernel Programming Guide). As with any CPU, FX CPUs can be overclocked to run even higher than DDR3-1866, but mileage will vary.

Mixed DRAM Runs At The Speed (Or Timings) Of The Slowest DIMM

Let’s say you have a DDR3-1600 CAS 9 DRAM module and you add one rated at 1866 CAS 9. One outcome is that the DRAM will go to the motherboard’s default of 1333 CAS 9 or 10 (or, many AMD motherboards default to 1066). Or, both will run at 1600 CAS 9 (or 10 or even 11) if DOCP, EOCP, XMP or AMP was enabled before you added the 1866 MT/s module.

But you can also manually set them to something else. Typically, in a scenario like this, I’d try 1866 at 10-10-10-27. Add a pinch of voltage to the DRAM (e.g., +0.005V). Depending on the results, you can also tune the MC voltage.

It’s Cheaper To Buy Two Sets Of DIMMs Than Larger, More Expensive Sets

Even though you are buying two of the exact same sets, there is no guarantee that they will work together. The DRAM that goes into a package has been tested to work together. Manufacturers don’t guarantee mixing or adding one set to another, even when they are the same exact model.

Customers often try this with higher data rate sticks using XMP to set them up.  With XMP enabled, the motherboard may read the profile for two sticks of DRAM and set the secondary timings accordingly, but the tRFC timing for two modules may be 226, whereas four sticks require 314.  This can be especially hard for most users to troubleshoot/find, as few ever go into the secondary DRAM timings.

DRAM Will Run Faster With All Slots Filled

Running two sticks of DRAM puts less stress on the memory controller than four. Less electricity is needed, the memory controller needs less voltage to remain stable and, while it isn’t noticeable, the DRAM runs ever so slightly quicker (generally). The same holds true for tri- and quad-channel motherboards. Part of the misconception is that four DIMMs (often sold as a quad-channel set) will always run in full quad-channel mode, even though a dual-channel motherboard doesn’t support this feature.

There Are No Performance Gains With DRAM Faster Than 1600 MT/s

The answer to this depends on many factors. It is completely false if you are using a CPU's or APU’s on-board graphics. They use your system DRAM—and the faster, the better!

Most DRAM benchmarks measure read, write and copy performance. Many gaming benchmarks demonstrate a 3 to 5 FPS gain on DRAM between 1600 and 2133 MT/s. That’s because, in most games, DRAM is used primarily as a pipeline to feed information to the GPU and as a holding area for frequently retrieved data. The fact remains that it can increase FPS a little. Because the price of DRAM between 1600 and 2133 MT/s is only narrowly differentiated, it can sometimes still make sense to get the higher data rate DRAM.

Furthermore, the file compression program WinRAR pulls the data into DRAM and compresses it to DRAM before writing it back to the drive. Benchmarks using WinRAR can show a 25-percent gain going from 1600 MT/s to 2400. And there are plenty of other memory-intensive applications: editing video, working with images, CAD, running VMs and so on. While some of the gains might be small, the little snippets of savings add up if you use those applications.

If you are doing one task at a time—writing a memo, then browsing a Web page and then watching a video—you don’t really need faster DRAM. If you multitask, however—for instance, if you have a bunch of browser tabs open while you're working on a good-size spreadsheet, or if you're running video in a window, are working with images and maybe running a virus or malware scan in the background—then the faster memory can be far more beneficial.

You can test this out by running some applications like these with 1600 MT/s DRAM and then something faster. Once you’ve loaded up your system with several programs, use something like the SiSoftware Sandra system benchmark, and then use WinRAR on a large file. While it runs, hop around through your open Windows, and then check your scores in Sandra and the elapsed time for the WinRAR.

8GB Is All You'll Need For The Next X Years

If you don’t really multi-task, then 8GB is fine. But that isn't the case for gamers and enthusiasts. Five years ago, 2GB was thought to be enough, then 4GB and so on.

Another signal: computer vendors are typically stingy with DRAM, so when 2GB seemed like enough, they were providing 1GB, for example. Today, 6 to 8GB is the norm, and 16GB isn’t uncommon, suggesting that 8GB isn’t going to be enough for long. Games are using more DRAM. If you want to keep a rig for more than a year or two, I suggest a 16GB entry point.

You’ll Never Use Or Need 16GB Or More

This continues with the theme from Myth 9, but it's geared more to those heavy multi-taskers and users of memory-intensive software, or those who work with large data sets and files. The more DRAM you have, the more data it can hold for recurring instant access rather than having to go back to the page file on the hard drive or back to the Web to reread the data.

Many people use more than 20GB at a time almost every day, and this is also becoming the norm among those in the Tom’s Hardware forums, with members talking about maxing their 8 and 16GB kits.

Remember, too, that manufacturers are doing a ton of research and outreach to other manufacturers, software developers and users, so there are definitely reasons why motherboards are being designed to run 32GB, 64GB and 128GB (or more) of DRAM. 

I’m Not Using All My DRAM, So More DRAM Won’t Increase Speed

While more DRAM might not speed things up, in most instances, it certainly can. Many programs adjust the amount of data stored in RAM as a percentage of RAM available, so having more DRAM saves time by holding more oft-used data in RAM (rather than on the hard drive). This can be particularly beneficial when you're working on projects with multiple images or video, CAD, GIS, VMs, etc. Another benefit of having lots of DRAM is the ability to create a RAM drive for loading games, other applications or data sets. Doing so can have its pitfalls, but many swear by it.

A 64-bit OS Lets You Run All The DRAM You Want

Many people believe that, with a 64-bit OS, you can use an unlimited amount of DRAM, which isn’t true. Here are the DRAM limitations for Windows 7, for example:

Windows 7 DRAM Limits

x86 (32-bit)x64 (64-bit)
Windows 7 Ultimate4GB192GB
Windows 7 Enterprise4GB192GB
Windows 7 Professional4GB192GB
Windows 7 Home Premium4GB16GB
Windows 7 Home Basic4GB8GB
Windows 7 Starter2GBN/A

And on Windows 8:

Windows 8 DRAM Limits

x86 (32-bit)x64 (64-bit)
Windows 8 Enterprise4GB512GB
Windows 8 Professional4GB512GB
Windows 84GB128GB

1.65 Volt DRAM Will Damage Your Intel CPU

Intel recommends 1.50V for DRAM at the CPU’s specified data rate. On Haswell, that’s DDR3-1600 DRAM. But even that is a bit misleading, as Intel certifies DRAM (even DDR3-1600) that runs at 1.60 and 1.65 volts. Keep in mind that 1.60 to 1.65 volts is considered the norm for DRAM at 2133 MT/s and higher data rates.

The majority of the DRAM available in lower data rates (like DDR3-1333 and 1600) is 1.50V or less. I suggest people stay away from those data rates when voltages are 1.65, as this can mean the manufacturer used marginal memory chips. Why would DRAM with decent chips even need 1.60 to 1.65V? I generally take it a step further and stay away from 1866 MT/s DRAM that exceeds 1.50V, unless it is higher-performance (CL7 or CL8).

Dual-Channel Mode Doubles The Data Rate, Or Is Twice As Fast

This is another misconception. The DRAM itself is DDR (double data rate), so it runs at double the clock frequency (800MHz DRAM has a 1600 MT/s data rate). When you put two DIMMs in dual-channel mode, the DRAM goes from operating as an individual 64-bit device to working together and being seen by the MC as a single 128-bit device. Theoretically, that would double the bandwidth, but in actuality, it provides a performance boost of only 20 to 50 percent on Intel CPUs, and a little less on AMD rigs.

I’ve written this with input from many Tom's Hardware forum members—too many to mention individually. I’d also like to thank the fine people at Corsair, G.Skill and Team Group, whose DRAM expertise and willingness to share it are much appreciated.

As always, comments, criticism and critiques are welcome.

MORE: Best Memory
MORE: Memory in the Forums

Jim Reece is a Contributing Editor for Tom's Hardware, covering Memory.

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  • OutlawzGR
    Very nice post!
    Thank you sir!