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AMD's Socket AM2 processors

With DDR2 and a few new wrinkles

AMD HAS REAPED tremendous benefits from integrating a memory controller into the Athlon 64. Bringing the memory interface one step closer to the CPU has minimized a persistent performance bottleneck in modern microprocessors, and that's part of the reason the Athlon 64 has been performing backboard-shattering dunks over the heads of Pentiums with regularity since late 2003. The quickness and towering vertical leap came at a cost, however: AMD gained responsibility for managing the memory interface for its processors. That responsibility is why we are gathered here today—to take a look at AMD's new Socket AM2 processors that incorporate a revamped memory controller capable of talking with the industry's emerging PC memory standard, DDR2.

AMD's move to DDR2 memory is a way more complex beast than past transitions to new memory technologies precisely because of that integrated memory controller. We're not just talking about some new chipsets and motherboards here. Oh, no, old timer; this here newfangled technology is different. AMD has cooked up a new CPU socket and an entire top-to-bottom lineup of new CPUs in order to make DDR2 support possible, from lowly Semprons to the fire-breathing Athlon 64 FX-62.

These new processors face some daunting challenges. Intel is well ahead of AMD in manufacturing technology right now, and the latest Pentiums manage to bring higher performance, lower power use, and smaller thermal envelopes than the "Hot'n'Loud" Pentiums of the 90-nanometer era. At the same time, Intel has slashed prices like a used car salesman after a hail storm, making its Pentium D series a better value than ever. Not only that, but these Socket AM2 processors will have to fend for themselves against Intel's upcoming Conroe processor, all but assured to set new standards for high performance and low power consumption when it hits the market in a month or two.

AMD has equipped the CPUs in its new lineup with a number of new tools, so that they may face these difficult trials. But do the Socket AM2 processors bring the kinds of advances in performance and energy efficiency necessary to keep AMD in the lead? We've tested 10 different CPU configurations in a battery of benchmarks in order to find out.

AMD goes for DDR2 memory
Socket AM2 brings a cluster of new things from AMD, some related to the main goal and some not so much. The main goal, of course, is support for DDR2 memory. The industry created this memory standard in order to achieve higher clock speeds and more bandwidth than the first wave of DDR memory, and after a slow start, DDR2 has delivered. Currently, DDR2 modules run at clock speeds up to 800 MHz and can thus sustain up to twice the peak theoretical bandwidth of DDR400. Some enthusiast-class RAM runs at even higher speeds, although those haven't yet been sanctioned by standards body JEDEC. DDR2 memory also has the advantage of using less power than DDR, because DDR2 memory chips operate at relatively lower internal clock speeds and require less voltage, so DDR2 is potentially more attractive for laptops... and servers, and desktops, and all the rest.

AMD needs to move to DDR2 not just because of its positives attributes, but also because the rest of the industry is making this same transition. DDR memory will become scarcer and probably more expensive with time, while DDR2 becomes cheaper and more plentiful.

Thus, the Athlon 64 and gang are getting a new memory controller. AMD's single-core Socket AM2 CPUs can handle DDR2 memory at speeds up to 667 MHz, while the dual-core models like the Athlon 64 X2 and FX-62 can accommodate DDR2 DIMMs up to 800 MHz. Like Socket 939 before it, Socket AM2 has the ability to host dual memory channels so that memory accesses are effectively 128 bits wide. Poke around on the calculator a little bit, and you'll find that combination gives single-core Semprons and Athlon 64s over 10.6 GB/s of total memory bandwidth with DDR2-667, up from 6.4GB/s with a dual-channel DDR400 config. With dual channels of 800 MHz DDR2, the dual-core chips have as much as 12.8 GB/s of memory bandwidth, fully twice that of the previous generation.

Those are some mighty fancy numbers, but the big question about them is simple: will DDR2's higher potential bandwidth lead to higher performance? We are, of course, about to put that question to the test in a multitude of ways, but don't get your hopes too high. AMD says to expect about a 1% performance gain on average from the transition to Socket AM2.

Jump back! A whole percent!?

That may help explain why AMD has taken until now to make the DDR2 transition. The Athlon 64's approach to memory performance has been more about quickness than top speed—about minimizing access latencies rather than pumping through lots of data—and DDR2 has less to offer on the latency front. So we're not talking about the kinds of real-world performance gains one might expect after hearing those crazy-high bandwidth numbers.

The Socket AM2 infrastructure

Socket AM2 (left) is keyed differently than Socket 939 (right)

As any good geek knows, there's really nothing more exciting than a CPU socket. It has a whole bunch of small holes, and tiny pins go into them. Don't hyperventilate, but above you can see the pinouts from processors intended for sockets AM2 and 939. The layouts are similar, but the pins are arranged in different patterns. The hope here is that you won't succeed in putting an AM2 CPU into a Socket 939 motherboard. If you do, you'll probably wind up with an interesting conglomeration of bent pins and spent expletives. AMD has done its best, though, to avoid any confusion between the two sockets. ('tis interesting to note, incidentally, that AM2 has 940 pins, only one more than Socket 939. DDR DIMMs have only 184 pins along their edges, while DDR2 modules have 240. Yet AMD has basically had a sufficient number of pins in its sockets to support dual channels of DDR2 since the K8's inception.)

Socket 939 had a two-post mounting system...

...while Socket AM2 has a four-post arrangement.

The other change with Socket AM2 itself is the heatsink retention mechanism. There are now four holes in the mobo around the socket, and the plastic mounting bracket for standard coolers is now secured by four screws rather than two. The standard retention bracket has changed, as well, with only two tabs available to keep coolers tensioned, where Socket 939's bracket had six. Some current Socket 939 coolers only use two tabs and will adapt well to Socket AM2. Others will be consigned to wandering the streets aimlessly, muttering to themselves about where the tabs or holes went. My personal favorite cooler, the wondrous Zalman CNPS9500 LED, won't work with Socket AM2 unless someone comes up with a new retention kit for it.