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AMD's Quad FX platform

AMD decides to socket to 'em

IN THE PC REALM, when you can't win by traditional means, there may be another reliable avenue available to you: move upmarket. This form of one-upsmanship has been masking technological shortcomings in increasing measure in recent years. Intel arguably started this trend in the CPU market when, on the eve of AMD's introduction of the Athlon 64, it uncorked the first Pentium Extreme Edition processor, basically a Xeon with scads of L3 cache pulled from the server market into service as a new flagship desktop part. At the prohibitive price of just one dollar short of a grand, the Extreme Edition wasn't intended to sell at high volumes. Its job was simply to defend the performance crown to the best of its prodigious ability. That's the beauty of the ultra-high-end product: a top product can rock the benchmarks yet only ship in a few hundreds or thousands of units.

With that background, perhaps you will understand why we were skeptical when AMD unveiled its plans for a new platform, code-named "4x4", just as Intel prepared processors based on its excellent new Core microarchitecture for release. The initial concept was about as extreme as they come, with the "4x4" signifying the combination of four CPU cores (in two sockets) and four GPUs in the same system. From the sound of it, these boxes would only come from boutique PC vendors like Alienware and Voodoo, and they would cost more than a reasonably well-equipped Honda Civic. We were underwhelmed by some of these constraints, especially the initial exclusivity to PC makers, and said so at the time.

Fortunately, AMD was listening. The 4x4 concept has undergone some moderation since it was first announced, and those constraints have been eased somewhat. What's left is a new enthusiast-oriented PC platform that officially sanctions what some of us have been doing since the days of the Celeron 300A: running multiple processors in an enthusiast-class system. (By "processor," of course, I mean one of those things that you stick into a socket on a motherboard, not just another CPU core on a chip.) The first incarnations of "4x4", now known as the Quad FX platform, will deliver quad CPU cores into desktop systems starting today. You may be asking yourself a number of questions upon reading this news. Questions like: Yeah, but can it keep pace with Intel's mighty Core 2 Extreme QX6700 quad-core processor? Why would I want one? What can you really do with four cores? Will Britney and K-Fed patch things up, or is it really over? Fear not, my friend, for we have the answers to three of those four questions. Read on to find them.

Anatomy of a Quad FX
If the Quad FX scheme is borne of necessity, the cause of that necessity is undoubtedly the Core 2 Extreme QX6700 processor, which successfully shoehorns two Core 2 Duo chips into a single package for a "quad core" result—and a potent one, at that. Presumably, AMD isn't countering with two Athlon 64 X2 chips in a single package for a number of reasons—not least of which is the fact that they're still making chips on a 90nm fabrication process, and the die size of those chips probably wouldn't allow it. Instead, the Quad FX platform essentially brings a workstation-class dual-socket Opteron solution onto the desktop.


The Athlon 64 FX-74, pictured above, is a case in point. It comes in LGA-style package, just like newer Opterons, and drops into a 1207-pin socket, just like newer Opterons. Unlike Opterons, though, these new FX processors don't require pricey registered ECC memory, and they won't reside in fuddy-duddy motherboards that spoil all the fun. Instead, they use regular ol' unbuffered DDR2 DIMMs, and AMD is encouraging the development of Quad FX motherboards with tweakable BIOSes and—for shame!—robust overclocking options.

A block diagram of the Quad FX platform. Source: AMD.

Here's a look at the logical layout of a typical Quad FX system. Hanging off of each CPU socket is a pair of DDR2 memory channels, with officially supported DIMM speeds up to 800MHz. That means you're looking at up to 25.6 GB/s of memory bandwidth—far above the bandwidth available to the Core 2 Extreme QX6700, which is limited by its front-side bus. However, that AMD memory subsystem is by nature NUMA—an acronym signifying non-uniform memory access. This Opteron/K8 NUMA memory architecture is a mixed blessing. Memory bandwidth scales up linearly as more CPUs are added to the system, but memory access times rise when CPU 0 must grab data from memory controlled by CPU 1. In order to attain NUMA's benefits without stumbling on its drawbacks, software—especially the operating system—must be NUMA-aware.

If all of this sounds like a tremendous amount of complexity for a desktop system, well, you're right. It's also a tremendous amount of power for a desktop box.

The Quad FX scheme is aided and abetted by Nvidia's nForce 680a SLI core-logic chipset. Following through with the theme of doubling up for success and excess, the 680a SLI is essentially two copies of the nForce 570 SLI chip, mounted side by side together on a motherboard. The presence of both chips makes possible a total of four PCIe x16 slots (two with 16 PCIe lanes and two with eight), four Gigabit Ethernet ports, and a whopping 12 SATA ports, among other things. The two core logic chips are attached to one CPU socket via dual HyperTransport links so that the system can operate with a single processor and still provide access to all I/O capabilities.

I expect AMD, through its newly acquired ATI subsidiary, to bring its own Quad FX chipset to the market at some point in the future, but for now, Nvidia is the sole supplier of Quad FX core logic. Personally, I'd also like to see a Quad FX solution with "only" two PCIe x16 slots, six SATA ports, lower power consumption, and a more modest price, but that's not in the cards just yet.

The key to making Quad FX anything more than a marketing stunt aimed at recent lottery winners, of course, is keeping systems price-competitive with those based on Intel's quad-core parts. Since folks will have to purchase two CPUs in order to build a proper Quad FX box, that's no small concern. Happily, AMD has done its part on that front, keeping its promise to deliver pairs of FX CPUs for "well under a thousand dollars." The processors will be sold in pairs in the following configurations:


Clock speedL2 cache
(per core)
(per CPU)
(per pair)
Athlon 64 FX-702.6GHz1MB125 W$599
Athlon 64 FX-722.8GHz1MB125 W$799
Athlon 64 FX-743.0GHz1MB125 W$999

With CPU pairs priced as low as $599, Quad FX may not be cheap, but the processors are arguably affordable and maybe even a decent value, depending on how you define value.

Check the clock speed on the FX-74 once more, just to make sure you get it: a healthy 3GHz. Intel chose to back down to 2.66GHz for its top quad-core part, the QX6700, in order to meet the power and thermal requirements of a single CPU socket. With two sockets, two coolers, and more pins per socket, AMD had no such constraint, so they've actually raised clock speeds a notch beyond what's currently available in a single-socket Athlon 64 processor.

Now, we know Core 2 Duo processors typically perform better clock for clock than Athlon 64 X2s, but in this quad-core solution, AMD has vastly more memory bandwidth, a very nice system architecture, and a pronounced clock speed advantage. This could get interesting, no?

Of course, if it's low power consumption you want, Quad FX may not be your cup of tea. With a peak thermal dissipation requirement of 125W per processor, Quad FX exhibits another characteristic of a "4x4"—low gas mileage.

For those of you who are wondering how these FX processor prices will affect current Opteron prices, which are quite a bit higher, the answer seems to be: not much. AMD says FX pricing and Opteron pricing are two separate issues. FX chips won't support registered ECC memory, and AMD says FX processors aren't supposed to work on Opteron motherboards. Some folks may choose Quad FX workstations rather than Opteron ones, but AMD seems willing to accept that.

If Quad FX doesn't sound quite sweet enough to tempt you yet, AMD has one more prospect to add to the mix. Today's Quad FX systems will come out of the chute ready to accept AMD's native quad-core processors when they arrive some time next year, raising the possibility that a Quad FX box could be upgraded to eight of AMD's new-microarchitecture cores in the future. Holy moly. That one's gotta set some fanboys' hearts aflutter.

So when can you get some Quad FX action, you ask? AMD says Quad FX solutions should begin selling today, both from system builders and in the form of kits, with two CPUs and a motherboard included, from select online vendors like Newegg. (Yes, that means those of us who like to build our own systems should be able to pick up kits right away, thank goodness.) Initial quantities will be limited to these outlets, but AMD expects the CPU pairs to make it into full distribution in the first quarter of next year. The company also claims it's committed to the idea of a dual-socket enthusiast platform for the long haul.