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The processors of Socket AM2
Moving to this new socket wouldn't be any fun without a welcoming party, and so AMD has uncorked a couple of new models to tempt us. The Athlon 64 X2 5000+ flips bits at a healthy 2.6 GHz, but it only has 512 KB of L2 cache for each of its two CPU cores, keeping it a half step down from AMD's previous top chip, the Athlon 64 FX-60. Here's an unnecessarily large photograph of the X2 5000+.

The Athlon 64 X2 5000+ is new for Socket AM2

The new king of the hill, meanwhile, is the Athlon 64 FX-62, which packs a meg of cache per core and a 2.8 GHz clock frequency. Like prior members of the FX line, the FX-62's upper clock multipliers are unlocked, so it's ready-made for overclocking.

The rest of the Socket AM2 family looks like so:

ModelClock speedL2 cacheTDPPrice
Athlon 64 FX-622.8 GHz2 x 1024 KB125 W$1,031
Athlon 64 X2 5000+2.6 GHz2 x 512 KB89 W$696
Athlon 64 X2 4800+2.4 GHz2 x 1024 KB89 W$645
Athlon 64 X2 4600+2.4 GHz2 x 512 KB89 W$558
Athlon 64 X2 4400+2.2 GHz2 x 1024 KB89 W$470
Athlon 64 X2 4200+2.2 GHz2 x 512 KB89 W$365
Athlon 64 X2 3800+2.0 GHz2 x 512 KB89 W$303
Athlon 64 3800+2.4 GHz512 KB62 W$290
Athlon 64 3500+2.2 GHz512 KB62 W$189
Sempron 3600+2.0 GHz256 KB62 W$123
Sempron 3500+2.0 GHz128 KB62 W$109
Sempron 3400+1.8 GHz256 KB62 W$97
Sempron 3200+1.8 GHz128 KB62 W$87
Sempron 3000+1.6 GHz256 KB62 W$77

That's a bundle of new CPUs. The first thing you'll probably want to notice in the table above is the "TDP" column. TDP stands for "thermal design power," and it's a measure of how much power and cooling a chip will need. With Socket AM2 processors, TDPs are down substantially. The Athlon 64 X2 line drops from 110 W on Socket 939 to 89 W now, and single-core Athlon 64s are down from 89 W to 62 W. The lone straggler at a higher TDP is the FX-62, but that's pretty much expected from a flagship performance part.

The prices above are correct both for Socket AM2 processors and for the corresponding Socket 939 and 754 models, where they exist. To give you some sense of the competition, Intel has positioned the Pentium Extreme Edition 965 similarly to the FX-62, with a very proud price tag of $999. The next rung down the ladder, the Pentium D 960, weighs in at $523, just below the Athlon 64 X2 4600+. The step after that is a long one, all the way down to $316 for the fire-sale-priced Pentium D 950. That would place the Pentium D 950 about 50 bucks below the X2 4200+. We will be testing some of these speed grades against one another shortly, so keep those prices in mind.

There's another new addition to AM2 CPUs not included in the table above, and that's support for AMD's virtualization technology. This feature probably won't matter much for desktop users in the near term, but these processors can execute a handful of new instructions that provide hardware assistance to virtual machine programs such as VMWare. At some point down the road, regular desktop PCs may use hardware-assisted virtualization to cordon off low-security areas, for running things like web browsers and file transfer clients, from the main program execution space. For now, virtualization will probably be more useful in servers.

That's not it for the new Socket AM2 processors, believe it or not. Think these guys aren't serious about combating Intel's Conroe? Think again. AMD has also pulled the curtains back on a whole slew of "Energy Efficient" variants of its Socket AM2 processor lineup with even lower power and cooling requirements. These processors run at the same clock speeds and deliver the same performance as the non-EE versions, but they're capable of running at lower voltages.

Energy Efficient ModelClock speedL2 cacheTDPPrice
Athlon 64 X2 4800+2.4 GHz2 x 1024 KB65 W$671
Athlon 64 X2 4600+2.4 GHz2 x 512 KB65 W$601
Athlon 64 X2 4400+2.2 GHz2 x 1024 KB65 W$514
Athlon 64 X2 4200+2.2 GHz2 x 512 KB65 W$417
Athlon 64 X2 3800+2.0 GHz2 x 512 KB65 W$353
Athlon 64 X2 3800+2.0 GHz2 x 512 KB35 W$364
Athlon 64 3500+2.2 GHz512 KB35 W$231
Sempron 3400+1.8 GHz256 KB35 W$145
Sempron 3200+1.8 GHz128 KB35 W$119
Sempron 3000+1.6 GHz256 KB35 W$101

These new Energy Efficient models come with TDPs of 65 and 35 watts, although they do carry a price premium that ranges between $24 and $52. On average, the premium for an Energy Efficient is about 40 bucks, or roughly 19% the price of the base model.

The official word on the Energy Efficient chips is that they'll be available in "May," which I suppose gives AMD a few more hours. However, we haven't yet received any of the Energy Efficient models for testing, even though we asked really nicely. They're also not yet listed on our price search engine.

Some quirks of AMD's DDR2 memory support
So. Get this. On Socket AM2 Athlon 64 systems, the clock speed of system memory is derived from the CPU's clock frequency. That makes sense, since the memory controller lives onboard the CPU. The memory clock runs at some fraction of the CPU clock speed, and the ratio of CPU clock to memory clock is determined by a divisor. Yet only whole-number divisors are available on these chips, and that limits the available memory clock speeds.

Let's illustrate. Take, for example, the Athlon 64 X2 4800+ running at 2.4 GHz—or 2400 MHz. If you want to run DDR2 memory at an effective speed of 800 MHz, you can use a divisor of six. 2400 MHz divided by six is 400 MHz, and a base clock of 400 MHz results in DDR2 memory humming along nicely at an effective 800 MHz. Similarly, a divisor of nine will produce a base memory clock of 266.6 MHz, perfect for DDR2-533 memory.

But what if you want to run DDR2-667 memory? You could use a divisor of seven to get to 343 MHz, or effectively 686 MHz once DDR2's clock-doubling does its magic, or you could try a divisor of eight, to hit 300/600 MHz memory clocks. Getting to 667 MHz is impossible, though.

AMD's answer to this problem is to get as close to the JEDEC-specified memory clock frequency without going over and call it a day. Not kidding. Our Athlon 64 X2 4800+ would run DDR2-667 memory at 600 MHz.

Try another example. The Athlon 64 X2 4200+ flips bits at 2.2 GHz. With whole-number divisors, it can set its memory clock speeds to, effectively, 489 MHz for DDR2-533, 628 MHz for DDR2-667, and 733 MHz for DDR2-800. That's it. None of 'em will run at their full potential.

I haven't consulted with the appropriate JEDEC committee, but I believe the technical term for an implementation like this one is "half-assed." Yes, it seems to work well enough, and no, underclocking memory isn't likely to be an egregious violation of the spec. We've already acknowledged that AMD's approach to memory performance focused much more on latency than bandwidth, and any overclocker knows we can probably squeeze tighter timings (and thus lower latencies) out of that underclocked RAM with a bit of tweaking. Still, I find it hard to believe AMD's new processors actually handle memory clock speeds in this way.

This quirk will likely have odd implications for the relative performance of different speed grades of Socket AM2 processors in tasks particularly sensitive to memory bandwidth. System power consumption will also be affected, I suppose.

The practical effect of this limitation on our testing was this. Two of our Socket AM2 processors, the Athlon 64 X2 4800+ and the FX-62, were able to run our DDR2-800 memory right at 800 MHz. For these chips, we used 4-4-4-12 memory timings with a 2T command rate. The Athlon 64 X2 5000+ ran the memory at 743 MHz, and the X2 4200+ was restricted to 733 MHz. We kept the same basic memory timings for those CPUs, but the command rate was cranked down to 1T. I suppose further tweaking might be possible, but there's no way to run a true apples-to-apples comparison at DDR2-800 memory speeds when the CPUs can't do it.

If you're buying memory for a Socket AM2 system, you'll want to watch out for a few things. The DDR2 memory scene to date has been dominated by DIMMs intended primarily for Intel-based systems. The Socket AM2 memory controller is a different animal, and we've found that DIMMs that work fine on an Intel system at certain memory timings may not do so on AM2. Also, because Intel systems have higher built-in access latencies thanks to their reliance on a front-side bus, makers of enthusiast DIMMs seem to have have focused more on high memory clock speeds than on tighter timings. That's beginning to change as Socket AM2 hardware hits the market. We were able to run Corsair's new 1GB TWIN2X2048-8500C5 modules at the relatively tight timings mentioned above, even though they are rated for 1066 MHz clock speeds with much looser timings.

These puppies did 4-4-4-12 with a 2T command rate at 800 MHz on AM2

As for higher memory clock speeds on the new Socket AM2 processors, they ought to be possible given how AMD's memory divisor system works, but the BIOS of our Asus M2N32-SLI Deluxe mobo would only allow us to choose from DDR2 400, 533, 667, and 800 memory speeds. The BIOS has tweaking options galore, but I saw no way to directly access the memory divisor. In fact, the BIOS obscures the true memory clock speed from the user. One would presumably have to overclock the base system clock/HyperTransport clock in order to get to 1066 MHz memory frequencies with this mobo.