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AMD's Phenom X3 processors


Here come the, er, Musketeers
— 12:08 AM on April 23, 2008

Good things often come in powers of two, especially in computers. Two, four, eight, or sixteen copies of a common resource—rendering pipelines, megabytes of memory, processors, what have you—are instantly recognizable quantities that will most assuredly lead to additional goodness.

But three? Not so much.

Oh, sure, you have the odd exceptions, like a three-disk RAID 5 array or three-way SLI, but these are exceptions, and they are quite literally odd. Even less common is the case of three CPUs. I've been racking my brains for a few days trying to come up with past examples of three-way multiprocessor configurations in PC history, and I've been coming up blank. Now that I've said that, some old-timer will post in the comments about the Univac EP-3333, to which he fed punch cards back in the day. Bully for you, Methuselah, but my point remains: triple-processor configurations are exceptionally rare in the PC world.

They are, however, about to get a whole heckuva lot more common thanks to AMD's new triple-core Phenom X3 processors. These are essentially just quad-core chips with one core disabled, sacrificed for the cause of product segmentation. Can't you just hear millions of tiny transistors screaming out in pain and then going silent? The core-botomy has happy side-effects, though, not least of which is extending the Phenom lineup to under 150 bucks.

The advent of these triple-core specimens raises some intriguing questions. Can AMD gain ground on Intel's very potent dual-core CPUs by disabling a core and slashing its prices? Will the Phenom's relatively low per-core performance be offset by the presence of a third core? What's the right tradeoff here? We've taken these questions as an excuse to run way too many benchmarks on the new Phenom X3 chips. Then we made up some answers. Keep reading to see what we found.

The powers of three
Since the Phenom X3 really is a quad-core Phenom with one core disabled, there's really not much new to know about it. (You can find out more about the Phenom itself in our original review of it.)

We should talk about cache briefly, though. Each core on a Phenom has 64K of L1 cache and 512KB of L2 cache associated with it, so Phenom X3s have a total of 1.5MB of L2 cache onboard. The L2 caches are augmented by a larger 2MB level-3 cache designed to assist in data sharing between the cores. CPU geeks take note: because the Phenom shares its L3 cache between all of its cores in a round-robin fashion, we had suspected the deletion of one core might reduce L3 cache access latencies, an Achilles' heel of the Phenom architecture. Unfortunately, we weren't able to measure this effect in our testing. At 2.4GHz, the Phenom X3 and X4 have more or less identical access latencies.

Anyhoo, that's the cache picture. Here's how the Phenom lineup looks with the addition of the triplets:

Model Clock speed North bridge
speed
Cores TDP Price
Phenom X3 8450 2.1GHz 1.8GHz 3 95W $145
Phenom X3 8650 2.3GHz 1.8GHz 3 95W $165
Phenom X3 8750 2.4GHz 1.8GHz 3 95W $195
Phenom X4 9550 2.2GHz 1.8GHz 4 95W $195
Phenom X4 9750 2.4GHz 1.8GHz 4 125W $215
Phenom X4 9850 Black Edition 2.5GHz 2.0GHz 4 125W $235

Notice, first, that all of the Phenoms in the table above come with model numbers ending in -50. That means they're all based on B3-revision silicon, which squashes the unfortunate TLB bug. AMD has been selling tri-core Phenoms based on older silicon with -00 model numbers through PC vendors, but only these newer chips should make it into regular distribution channels.

Step past that issue, and your eye will probably focus on the $195 price point, where AMD presents us with a perplexing choice. You can buy a Phenom X4 9550 with four cores and a 2.2GHz clock speed, or you may choose a Phenom X3 8750 with three cores at 2.4GHz for the same price. On the face of it, giving up a CPU core in order to gain 200MHz seems like a bad bargain to me. Then again, Phenoms are currently a little low on single-threaded performance, so perhaps the compromise works. The 8750's closest competition from Intel is probably the Core 2 Duo E8400, which lists at $183.

Things become infinitely simpler as we move down the price ladder and the quad-core options become more distant. At $145, the Phenom X3 8450 brings AMD's new microarchitecture into territory formerly occupied by the Athlon 64 X2. The X3 8450 has a relatively low 2.1GHz clock frequency, but packs a third core; the tradeoff here is clear. This product will face off against Intel's Core 2 Duo E7200, similarly priced at $133.

Three cores is weird
There, I've said it. You know you were thinking it. We're modern folks, open to many possibilities in life, including this one. But three cores is just plain weird. You will need to know this before making the decision to drop a Phenom X3 into your own computer.


Dude. Three.

This weirdness manifests itself in several ways. Although many of the applications we use for CPU testing had no trouble recognizing the X3's triple cores and putting them to good use, some did. Several SiSoft Sandra modules lost bladder control when asked to quantify the performance of a tri-core processor and simply refused to run. Microsoft's Windows Media Encoder pegged the X3 at 67% utilization and would go no further; two cores were all it would use. Even the 32-bit versions of Windows Vista apparently have trouble recognizing odd numbers of CPU cores. Already, updates are becoming available to fix some of these problems, but owners of Phenom X3s are bound to run into such issues over the next little while as software developers adjust to unconventional core counts.