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New chips, new chipsets
The new dual-core processors will require the support of new core logic, and for our testing, Intel supplied us with an early version of its own D955XBK motherboard based on the new 955X chipset. This new chipset is more than just a tweak for dual-core capabilities, however; it includes several other improvements over its predecessor, the 925XE. Let's review them quickly:

  • An enhanced memory controller — The 955X officially supports DDR2 memory clocked at 667MHz, up from 533MHz in the 925XE. Heck, the Intel 955X motherboard actually has an option listed for DDR2 800, although I haven't tested to see if it works yet. The 955X will also support up to 8GB of RAM (optionally with ECC support) so users of 64-bit operating systems can cram in lots of memory.

  • Improved Matrix Storage — The 955X north bridge will pair up with Intel's new ICH7 south bridge I/O chip, and the ICH7 adds even more server-class features to Intel's Matrix Storage Technology with the addition of RAID levels 5 and 10. Both of these RAID flavors offer a combination of better performance, higher capacity, and better data integrity than a single drive or the more common RAID levels 0 or 1. RAID 5 allows for lots of storage with redundancy on a three-drive array, and RAID 10 offers better fault tolerance in a four-drive array than RAID 0+1. Of course, the ICH7 will continue to support Native Command Queuing for SATA drives, and it adds 3Gbps SATA-II transfer rates. The addition of these new RAID levels and SATA-II transfer rates should keep Intel at least half a step ahead of NVIDIA's nForce4 SLI Intel Edition on the storage front.

  • More PCI Express lanes — The ICH7 south bridge also has two more PCI Express lanes than the ICH6, for a total of six. Intel has attached four of those lanes on its D955XBK motherboard to a PCI Express x16 physical slot that promises the possibility of a dual-graphics configuration, like so:


    Intel's D955XBK motherboard packs dual PCI-E graphics slots

    Intel is quick to caution, though, that this slot hasn't been validated for use with graphics cards. Yet.

Beyond those changes, the 955X chipset is fairly similar to the 925XE. Intel will be bringing out lower end versions of this chipset in the 945 lineup, and those won't feature internal memory controller timings as aggressive as the 955X's.

Dual-core approaches compared
Intel's approach to dual-core processors is somewhat different from the approach taken by its primary competitor, AMD. The Pentium Extreme Edition 840 offers essentially the same thing as a pair of Xeon 3.2GHz processors, but in a single CPU socket with a faster memory subsystem. Although its two cores are on the same chip, they communicate with one another and with the rest of the system by means of a shared 800MHz front-side bus. All memory accesses, I/O, and cache coherency updates between processors must traverse this shared bus, which has a peak throughput of 6.4GB/s. That's less bandwidth than the 10.7GB/s theoretical peak transfer rate of the 955X chipset's dual channels of DDR2 667 alone.

AMD's dual-core processors, including the newest Opterons and the upcoming Athlon 64 X2, have a design modified specifically for dual-core implementations. You can read my review of the dual-core Opterons for a more detailed discussion of AMD's design, but I'll summarize here. AMD's dual-core chips share some common resources between cores, including a single system request queue, a single on-die memory controller, and a single set of HyperTransport links for external I/O and cache coherency updates. The two cores can share data with one another via the very high speed on-chip system request interface, which is how cache coherency updates (and cache-to-cache data transfers) are passed. Overall, this arrangement gives the Athlon 64 more paths for critical data with higher bandwidth and lower latencies than Intel's shared bus approach. In short, it's more elegant.

That's not the whole story, though. If the Pentium Extreme Edition 840 performs well, we can forgive some technical inelegance. And Intel's plans for dual-core processors extend well beyond this first implementation or its technical merits. Intel has made public its ambitions for dual-core products, and those plans rely heavily on Intel's traditional strengths: manufacturing and selling chips in volume. While AMD is focusing primarily on servers for its dual-core parts, Intel has committed to bringing dual-core processors to desktop PCs sooner, and at lower prices, than the relatively expensive Athlon 64 X2. The X2's availability will likely be rather limited until late 2005, too. As a result, the value proposition for Intel's dual-core processors may be much more tempting.

Another key to success in dual-core chips for Intel will likely be its transition to a new 65nm fab process, which is coming soon. The 65nm dual-core desktop processor, code-named Presler and due in early 2006, will pack 2MB of L2 cache but should be much cheaper to manufacture. Not only will the chips be much smaller thanks to the die shrink, but Intel also intends to package together two separate pieces of silicon to make one dual-core processor. This approach should bring much higher yields, because two "good" cores need not be cut from the same place in the wafer, and a "bad" core doesn't necessarily torpedo the one next door. As much as we dislike Intel's shared system bus, this is a smart way to make chips, and it probably wouldn't be possible with AMD's dual-core design.