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