Platform Trends: AMD's Phenom-enal New Athlon

Just as we're all waiting for AMD to launch its much-anticipated Phenom II processors, the company has caught the market unawares with a new Athlon X2 that actually uses a Phenom (I) core architecture. If this sounds confusing, it is. And for reasons of its own, AMD has chosen the name Athlon X2 7000 Series for what might reasonably be referred to as a Phenom X2.

A Phenom by any Other Name ...

The first sign that we're not dealing with a traditional Athlon 64 X2 is the product name (which loses the "64") and the lower clock speed. At only 2.7GHz, the flagship Athlon X2 7750 Black Edition would not come close to the performance of a 3.1GHz Athlon 64 X2 6000+ had it sported the older CPU's "Brisbane" core design. The base architecture of the Athlon X2 7000 series is pure K10, and features a design similar to a 65nm Phenom X3 or X4, but with only two operational cores.

This is AMD's new "Kuma" core architecture, and it follows along with what we imagine a Phenom X2 would/will look like. This translates into a 65-nanometer-process B3 revision, with 64K apiece of Level 1 instruction and data cache per core, along with 512K of Level 2 cache per core. A shared Level 3 cache also makes it way over from the Phenom family, with the Athlon X2 7000 series sporting a full 2MB. The onboard memory controller is a dual-channel unit that supports up to DDR2/1066 speeds.

As mentioned, the top-of-the-line Athlon X2 7750 Black Edition combines a 2.7GHz clock speed with a fully unlocked multiplier for avid overclockers. The other models -- Athlon X2 7450 and 7550 processors clocked at 2.4GHz and 2.5GHz, respectively -- are conventional CPUs with locked clock multipliers. AMD has priced the Athlon X2 7750 at $79 in quantity, with the lower-clocked models aimed mostly at desktop OEMs.

The Good, the Bad, and the Ugly

So what's the big deal about The Last of the Athlons? Power users will almost certainly stick with a triple- or quad-core Phenom, or wait for the upcoming Phenom II. The loss of physical cores cuts multithreading performance quite noticeably; regardless of clock speed, tasks such as audio or video encoding will be significantly slower on an X2 7000 series processor. Multitasking will not be as smooth with two cores as with three or four. In operating environments such as Windows Vista with multiple programs loaded and fighting for CPU resources, the difference between a dual- and a quad-core system is palpable.

But that doesn't mean that a dual-core Phenom is a bad thing. In fact, there are just as many advantages to the new CPU as there are disadvantages. It all depends on what you're going to do with it.

For example, all things being equal, a dual-core processor consumes less power and generates less heat than a triple- or quad-core. Running at 2.7GHz, the Athlon X2 7750 has a thermal design power (TDP) of 95 watts, which puts it on par with a 2.4GHz Phenom X4 9750.

These lower power and heat requirements pique the interest of silent-PC aficionados, while you can almost feel the excitement of the overclocking community as it weighs the envelope-pushing potential of a dual-core Phenom. It's no surprise that the first Athlon X2 model off the line is a Black Edition, which is AMD's label for an unlocked processor. This makes overclocking a very simple proposition, without many of the arcane BIOS adjustments often required, and early results have been good: So far, the Athlon X2 7750 Black Edition has routinely reached speeds greater than 3.0GHz.

The upgrade potential of the Athlon X2 7000 series is much better than that of a Phenom X3 or X4; due to its lower power consumption, it's a more suitable match for older AM2 platforms. The lack of a HyperTransport 3 link on older AM2 motherboards also incurs less of a performance penalty with a dual-core CPU than it would with a triple or quad.

Four Cores Good, Two Cores Bad

There is also a question of exactly how AMD is allocating these dual-core Kuma processors. The Phenom X3 was openly designed as a fallback option for Phenom X4 production -- if one of the cores in a given chip had a problem running at the stated speed, AMD could simply disable it and sell the processor as an X3. (It's not as glamorous as multithreading performance, but the flexibility to disable physical cores one at a time is one of the real benefits of a true multicore design.)

Conventional wisdom, therefore, tells us that these initial 7000 series CPUs are simply Phenom X3s that couldn't make the grade and have had yet another core switched off. How users will respond to a CPU that is only half-functional is anyone's guess, but at sub-$80 prices, it's hard to complain. Another positive is the fact that the 2MB of shared L3 cache remains consistent no matter the number of active cores.

The Proof is in the Performance

As far as benchmarks are concerned, the Phenom X3 and X4 didn't exactly make mincemeat out of Intel's Core 2 family, so it's hardly a surprise that the Athlon X2 7750 isn't a Core 2 Duo killer. It performs as you might expect a dual-core Phenom to, i.e., lagging behind comparable Intel processors. On the other hand, it's less expensive and really competes price-wise against the Celeron dual-core models.

Nor do the new Athlon X2s fare that well against the top-end Athlon 64 X2 models, as the 2.7GHz Athlon X2 7750 simply can't make up the necessary ground against a 3.0GHz or 3.1GHz Brisbane core. Overall performance per clock might be better on a Phenom-based architecture, but it's still tough to make up a 400MHz gap.

Gaming tests bring some interesting results, especially in the majority of PC games that are single-threaded. With some noted multithreaded exceptions, game performance within a given processor architecture tends to depend on clock speed, not the number of cores. At 2.7GHz, the Athlon X2 7750 is actually clocked higher than the 2.6GHz Phenom X4 9950 Black Edition, and can yield similar or even slightly faster single-threaded gaming action.

Why AMD chose to use the Athlon X2 branding for what is really a Phenom X2 is anyone's guess, but it likely concerns product segmentation. To compete against Intel, AMD needs a current-generation dual-core processor in its fleet, but it also doesn't want a low-cost Phenom X2 taking business away from higher-priced Phenom X3 and X4 models.

Of course, this choice is a double-edged sword. What happens if buyers simply assume that AMD doesn't offer a mainstream, dual-core Phenom, and move over to the Intel Core 2 Duo instead?