XMP and the Real World
First off, a little bit about what XMP is:
XMP stands for Intel eXtreme Memory Profile, it’s a standard that Intel came up with to supplement JEDEC’s ram SPD profiles. What, that wasn’t a useful description? Ok, well here comes the long version!
JEDEC is the outfit that defines memory types. DDR3? Designed by JEDEC. DDR2? Deisgned by JEDEC. They come up with the basic design and protocal for a type of ram, and set some basic speeds and timings and voltages that can be used. For DDR3 you can have 800mhz, 1066mhz, or 1333mhz. That’s it, no more. At the same time you’re limited to 1.5 volts.
Now some ram can go pretty fast on 1.5 volts, but not much. Most requires more to really get flying, but if it isn’t 1.5v they manufacturers can’t save a profile in the ram for the BIOS to automatically use, it’s simply impossible.
This is where XMP (and EPP, the older DDR2 version from nVidia) comes in. It’s published by Intel and allows plenty of voltage as well as speeds that aren’t officially JEDEC DDR3. Like this kit’s 1.65v 2000mhz settings.
This sounds great, but it has a major drawback, namely that what is stored in an XMP profile has to work on every system it could possibly be plugged into. This is an issue for two reasons, the first is that ram manufacturers simply don’t have the time to test every single kit of ram to see how fast it will really go. The second is that ram manufacturers can’t test every cpu the ram will be used on, and every single CPU ever made requires slightly different amounts of voltage to run a given ram speed.
Why is this an issue? Because it means that the voltages in the XMP profile are generally set so that they’ll work on all cpus, in the case of very fast ram (like this kit) this means a lot of voltage, probably more then the CPU needs, and someitmes enough that it will do damage to the CPU over time. That’s a problem!
The only other option is for them to leave cpu voltage out of the XMP profile entirely and let the motherboard BIOS try to figure out what is needed. Then it’s the motherboard that risks frying the CPU and getting it’s manufacturer in trouble. On the plus side to that approach, motherboard manufacturers have a much better idea on voltage then ram manufacturers do, they’ve already tested a lot of cpus with their motherboards.
On the minus side, some motherboards don’t raise the Uncore voltage when you leave it on AUTO, so unless you have a “Golden” CPU that can run very fast ram on the default Uncore voltage it won’t work unless you set the voltages manually. Really you should do it manually anyway though.
This kit of Adata ram went with the second option, the profile doesn’t specify a voltage for the “Uncore” (or “IMC”, the memory controller built into the cpu), so the motherboard does.
This Rampage II Extreme is built mostly for benching, where lots of voltage is the normal. You can tell too, for 2000mhz ram it gives the Uncore 1.6volts! Substantially more then the generally recommended 1.35 to 1.45 maximum.
Now the next issue…
Here’s an example of how to enable XMP in the BIOS (for motherboards that support XMP).
2000mhz Ram and Memory Controllers
The second issue that can arise with very fast ram is that the memory controller in core i7 9xx series quad cores must run at twice the rated ram frequency. Default stock clocks for the Uncore (IMC/memory controller, also the L3 cache) are a bit over 2ghz, for this Adata ram to run at full spec the Uncore has to run at 4ghz, twice as fast!
Some chips flat out cannot do this, others require a lot of uncore voltage, more then is safe for long term use. Still other chips do it without even getting close to the “safe” voltage limits, some can run 2000mhz ram at only a tiny bit over the stock Uncore voltage! What yours will do is totally unknown, it’s not a bad idea in my opinion to test your uncore before you buy high speed ram. Testing is fairly simple, but is beyond the scope of this article. If you’d like a detailed walkthrough register for the FunkyKit forums and newsletter and make a post in the overclocking section of the forums. I’m happy to walk people through overclocking.
My i7 950 is of the second type of chip, it requires around 1.55 volts to the Uncore to run at the 4ghz needed for 2000mhz ram. This is fine for my purposes as I won’t be running at this speed 24/7, but it’s more then I recommend for daily use.
For this reason I also included a set of benchmarks using the XMP timings but at 1866mhz, this only requires the Uncore to run at 3.7ghz or so and is something that almost all CPUs should be able to do without being forced to run high voltage.
If you’re lucky enough to have a Gulftown cpu (six core, i7 970, 980x, 990x) you can run the uncore at 1.5x the ram speed and shouldn’t have any issues at all.
Now onward to the XMP and 1866 benchmarks!
