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Remember to Budget for Memory: Triple Channel DDR3 Kits

written by: •edited by: Lamar Stonecypher•updated: 5/23/2011

Unless you already own DDR3, you’ll need new memory to run a Core i7. Even if you already have a pair of DDR3 sticks, to get the benefits of triple-channel, you will obviously need a third. Don’t forget the voltage and speed limitations imposed by the i7’s on-board memory controller.

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    DDR2 to DDR3: Has to Happen Sooner or Late

    Though for the overwhelming majority of users, and even many enthusiasts, the move to DDR3 still doesn’t make financial sense, it will become more popular as people step into Core CPUs. Thankfully, this already appears to be making a dent in DDR3 prices. A 3 x 1 gig kit of PC3 8500 (runs at 1066Mhz) from Kingston is only 125 USD, while 3 x 2 kits start around $220.

    If the 1066 is that cheap, should you lay out for some faster DDR3? Now that the memory controller is on the CPU, it depends on which i7 you are buying.

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    It’s in the Fine Print

    The first things one looks at in comparing chips from the same family are price and clock, and indeed we did that in the first article. There are other very important things to consider, though. Intel buyers are used to seeing which instruction sets are supported and how big the caches are to help them make buying decisions, and for i7 these are the same in all three chips.

    What might sneak up on them is that two of the big new features of the Core architecture, QPI and the on CPU memory controller, are implemented at different speeds on the different CPUs. The Quick Path Interconnect runs at full speed (6.4 GT/s) on the top-end 965 XE, but the 920 and 940 run at 4.8 GT/s.

    This didn’t have a significant effect on overall performance in the myriad tests we found published. Plus, next year’s Ibex Peak platform for mainstream and performance Core CPUs will use the much slower Digital Media Interface instead of QPI, so the difference between the low and high QPI speeds almost certainly isn’t an issue. We have more on Ibex Peak in the next article.

    The different memory controller speeds, however, do impact performance; not tremendously, but notably. The 965XE will run memory up to 1600 MHz , while the 920 and 940 can only handle 1066.

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    Isn’t that Pretty Slow for DDR3?

    Absolutely. Intel may not have made the decision freely though. It is possible Intel is actually having trouble getting enough chips with fast controllers out the door. It is a new architecture and the memory controller is a new feature, so manufacturing yields may not be all that good yet.

    Also, if Intel was using the different memory controller speeds as deliberate differentiation points (i.e. rating the part lower than it can actually go for supply and demand reasons), the 940 could be made more attractive if it was rated for 1333 MHz memory. Right now, it is kind of a lame duck between the excellently priced 920 and the king of the hill 965 XE.

    If you already own some sweet DDR3 memory you were hoping to run with a 920 or 940, you’ll have to run it under clock. Then again, the memory controller can only handle memory voltages up to 1.65V, so if your memory was using more juice than that to hit its clock, you were going to have to turn it down anyways. Note that this limitation affects all three Core i7s, even the 965XE.

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    1.65 Volts, Tops

    No matter which i7 you choose, Intel doesn’t want you using it with memory that takes over 1.65V. The system might boot and run just fine, but you could be seriously reducing the lifespan of your expensive new chip. With stock DDR3 set to 1.5V, and the new triple channel kits hitting factory rated speeds of 2000 MHz without breaking the 1.65V limit, that isn’t exactly a deal breaker if you were looking to upgrade your memory anyways.

    But what about the early adopters of DDR3? If you have a beautiful pair of super-fast sticks you were hoping to round out with a third stick and run in triple-channel, you’ll have to see how fast they run at 1.65V. Again, no problem for the new kits, but the older pairs are often factory specified to run on as much as 2V. Set them to run at 1.65V, and start looking for the best timings and speed at which they are stable.

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    A Lot to Keep in Mind

    With all the new memory and motherboard considerations, along with those around the CPUs themselves, your upgrade and new computer choices are less than clear. The next article will explore which users should choose what options when deciding whether to get a Core CPU, and, if yes, which one.

    Nehalem, CPU, Core, i7, performance