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AMD's Phenom II processors

The underdog aims for a comeback
— 11:00 PM on January 7, 2009

One of the great privileges of this job is having my own test lab full of the very latest PC hardware for testing and comparison. At least, it's a privilege if you're a huge geek who's into such things. For years now, the prime slot on my CPU test bench, closest to the monitor and keyboard and associated with port one on my KVM switch, has been occupied by AMD-based systems. The positioning doesn't mean much of anything, really, but I figure an underdog like AMD ought to be first at something, so why not?

Port one has fallen on hard times lately, though. AMD's Phenom processors haven't quite been good enough to keep pace with Intel's latest CPUs, for a variety of reasons. They were late to market, tripped up early on by a show-stopping bug, and couldn't reach the right clock speeds within the power and thermal limits common to PC processors. AMD has remained fairly competitive by keeping Phenom prices low, but a great many PC enthusiasts have been wooed by the Core 2 processors' combination of strong performance, low power consumption, and considerable overclocking headroom. The picture only has grown more difficult for AMD with the arrival of the Core i7 and its occasionally heart-stopping speed.

AMD has a potential remedy for the port one blues, though, in the form of the Phenom II, a revised Phenom processor that has been moved to a new, smaller chip fabrication process and tweaked in a variety of ways to achieve higher clock speeds and to wring more performance from every tick of the clock. As a result, port one has been producing some very respectable benchmark scores of late. Could it be that, in the midst of Intel's ongoing resurgence—nay, dominance—AMD somehow has its swagger back?

The Phenom II X4 940 Black Edition WRX STi LX40 Shazbot 2400XL FTW
Ok, so I added a few extra terms at the end of its name in the subhead above, but the new top Phenom really is named the "Phenom II X4 940 Black Edition." I kid you not. We've come quite a ways from the days, just a year ago, when AMD introduced the Phenom 9600 under a new, simplified naming scheme and extolled the virtues of concise labeling. Since then, we've added back the "X4" designator for quad-core processors, even though the model number alone is sufficient to specify the core count, and we've now picked up a "II," courtesy of the die shrink. Oh, and the "Black Edition" thing specifies a CPU with an unlocked upper multiplier. All of these extra letters denote additional goodness, but we're on goodness overload here, folks.

The more intriguing thing about the Phenom II's naming scheme may be the model numbers themselves. AMD is introducing a pair of Phenom II X4 products today, the 920 and the 940, and those model numbers sure do ring familiar, what with the Core i7-920 and -940 kicking around out there. I could swear I heard someone from AMD claiming that the whole thing was a big coincidence, but wow. This particular coincidence seems to have made it past the "Whoops" stage, through several months of vetting, and into the "that will sure make for an interesting comparison on the shelves at Best Buy" stage.

Not that we really give a flip about what AMD has decided to call it. What we care about most is the technology. Of course, like the Phenom before it, the Phenom II is a native quad-core design with a very nice system layout that includes an integrated, dual-channel memory controller and dedicated HyperTransport links to and from the rest of the system. AMD's system architecture has long been like this, and Intel has only just recently delivered a similar infrastructure alongside the Core i7.

The big changes with the Phenom II come in the chip itself, and those changes start with the conversion to a 45nm fabrication process. Because the basic building blocks are smaller than the 65nm process used to build the original Phenom, the Phenom II can pack more transistors into a smaller space while drawing less power and, potentially, operating at higher clock speeds. Intel has been at the 45nm process node for quite a while. AMD may seem a little late to the game, but the underdog brings its own particular spin by employing silicon-on-insulator technology and a brand-new technique called immersion lithography, in which a layer of water is used to focus light.

This new fab process has allowed AMD to fit many more transistors into a Phenom II die—an estimated 758 million, versus 463 million for the Phenom—while reducing the die size from Phenom's 283 mm² to just 258 mm². Interestingly enough, the Phenom II's basic specs sound remarkably similar to those of the Core i7, which weighs in at roughly 731 million transistors and 263 mm².

If you're wondering where the Phenom II's additional transistors come from, look no further than its L3 cache, which has grown in size from 2MB to 6MB. This larger L3 cache is the centerpiece of AMD's effort to improve the clock-for-clock performance of its quad-core processor architecture. This cache isn't just larger, though. It's also faster, with what AMD claims is a two-cycle improvement in access latencies versus the 65nm Phenom's L3 cache. Since the L3 cache in this architecture runs at a lower clock frequency than the CPU cores themselves, the improvement in access times may be more substantial than this claim might first seem to suggest. The cache hierarchy is smarter in various ways, too, with more aggressive data prefetch algorithms, twice the bandwidth for L1/L2 coherency probes, and 48-way set associativity for the L3 cache. AMD has made quite a few changes to improve per-clock performance. If you'd like to read about them in more detail, I suggest checking out my review of the 45nm Opterons, which discusses this same silicon in more depth.

One specific I should mention here, though, is a nifty new power-saving feature. The cores on the 65nm Phenom were clocked independently of one another, so that any core could enter a lower-frequency, lower-power state when not in use, but they couldn't shut down entirely because the contents of their L2 caches needed to be kept available for other cores to check and possibly access. The Phenom II introduces another possibility: the contents of a core's L2 cache can be transferred into the L3 cache, and the core may then shut down entirely. AMD claims this feature can enable the Phenom II to achieve much lower idle power usage.

There's some tension, though, between this feature and another change AMD has made in the Phenom II. The firm found that the varying power states (or P-states) on the Phenom could prove to be confusing to the Windows Scheduler, which wouldn't necessarily choose wisely when deciding whether to schedule a thread on a core with a low P-state or a high one. As a result, enabling the Cool'n'Quiet dynamic power saving feature could lead to unintended performance degradation. To work around this problem, AMD has decided to link together the P-states of the Phenom II's cores, via some BIOS-level changes. Obviously, this is not the ideal solution, and AMD says it is working with Microsoft to ensure such things work properly in the future.

We don't yet understand entirely how these linked P-states affect the Phenom II's ability to put a core into a deep idle state where its L2 cache is flushed into the L3. We've heard rumblings from AMD to suggest these two attributes can coexist peacefully, but we don't yet have an entirely clear sense how they interact.

So here's the plan
Right now, AMD's plan for the Phenom II is simple. There will be two models. The Phenom II X4 940 will run at 3GHz and will be priced at $275. The 920 will run at 2.8GHz and list for $235. Both chips will have a 125W TDP (thermal design power) rating, and both will have a north bridge clock of 1.8GHz. (That means their L3 caches and memory controllers will operate at 1.8GHz, and their HyperTransport links will be capable of 3.6 GT/s.) Both the 920 and 940 will be compatible with existing Socket AM2+ motherboards and will support DDR2 memory at up to 1066MHz. As a Black Edition CPU, the 940 will have an unlocked upper multiplier to facilitate easy overclocking. Both processors should be available immediately—if not sooner, considering the spate of early listings at online retailers.

That pricing sets up the Phenom II X4 940 as a direct rival to Intel's Core 2 Quad Q9400, a 2.66GHz processor whose street price is about $269 right now. That's not far from the $284 list price of the Core i7-920, but AMD rightly argues that the additional cost of an X58 motherboard and DDR3 memory puts the Core i7-920 in a different price category. Meanwhile, the 920's closest competition may be the Core 2 Quad Q9300, which sells for around $240-250, although one could make a case for the similar but slightly lower spec Q8300.

The Phenom II's fairly near-term future, however, will become considerably less simple. Not too terribly long from now—probably weeks rather than months—AMD will introduce a new version of the Phenom II with a memory controller capable of working with DDR3 memory at up to 1333MHz, and it will introduce a new socket type, Socket AM3, to go along with it. In one of the neater tricks we've seen along these lines, Socket AM3-capable Phenom II processors will, happily, be backward compatible with current Socket AM2+ motherboards and DDR2 memory.

I'd expect the Socket AM3 versions of the Phenom II to completely supplant the products being introduced today, because they will offer similar functionality along with an upgrade path. I'd also expect these new chips to be where Phenom II really flourishes, with a fuller lineup of products extending to mainstream (~65W) TDP ratings, triple-core budget chips, and probably a higher-TDP flagship FX processor. In addition to all of that, I suspect this newer silicon rev will bring higher clock speeds for the L3 cache, memory controller, and HyperTransport links.

Meanwhile, the best these Socket AM2+ Phenom II processors can hope for is probably a James Dean-style run in which they burn brightly for a short period and then go out in spectacular fashion. Those folks who have already invested in Socket AM2+ motherboards and wish to upgrade may find these first Phenom IIs compelling, but most others will probably want to wait for the Socket AM3 version before taking the plunge.