Athlon 64 for Quiet Power

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Another feature which may be beneficial to some is AMD’s Cool’n’Quiet (CnQ). This feature automatically underclocks and undervolts your CPU when it is not under heavy load. The exact frequencies and voltages that are used are contained in the previously referenced AMD Athlon 64 Processor Power and Thermal Data Sheet. It is essentially the same technology that has been used in both AMD and Intel mobile processors to maximize battery life in laptops.

When discussing CnQ I'm frequently asked, “So, the Athlon 64 runs cooler with Cool’n’Quiet enabled then?”

  • First of all, that’s not a question ;)
  • Secondly, that all depends on how you use your CPU.

The most important thing to remember with CnQ is that it makes absolutely no difference when your CPU is under heavy load. As soon as the CPU is taxed, it will revert to its stock speed/voltage settings. This means that CnQ is of no benefit to all the diehard members of SPCR’s folding@home team. If, however, your computer is rarely running heavy loads, CnQ can indeed bring significant power reductions.

Many Athlon 64 users have found that combining CnQ with their motherboard’s automatic CPU fan controller can bring good results. In this case, the motherboard will slow down your CPU fan when the CPU temperature is low, and speed it up if the CPU heats up. With CnQ, under light load, the CPU will remain very cool, and some motherboards may even turn the CPU fan completely off. This behavior is highly dependant on the specific motherboard, however, and results are somewhat varied. The fan/thermal control utility SpeedFan can also work in conjunction with CnQ on many motherboards; you will have to experiment to find out how the combination works for you.

Other Athlon 64 users have had great success when combining CnQ with custom under-volting. This SPCR Forum thread details a few of those experiences. One Athlon 64 user was running his CPU at only 0.85V at 1 GHz!


Recently, AMD released a series of Athlon 64 Mobile CPUs that run on the same socket 754 platform as their desktop counterparts, but at a lower default voltage. The details of these CPUs can be found in the previously referenced AMD Athlon 64 Processor Power and Thermal Data Sheet. There are basically two lines of Athlon 64 mobile CPUs:

  • the 62W mobile which run at a default voltage of 1.4V
  • the 35W mobile which run at a default voltage of 1.2V

The few reports from users of these CPUs have been positive. It would seem, though, that there are still a few kinks to work out with regard to motherboard support. Depending on your motherboard, these CPUs might not “just work”. There is a report in this forum by a user who is running his 35W 2800+ at 2.25 GHz and 1.25V. He can’t go higher at the moment due to the limitations of his motherboard. Still, 2.25 GHz at 1.25V is VERY impressive. I fully expect that as soon as BIOS updates work out any motherboard kinks, these CPUs will become the undisputed champions of powerful, quiet computing.


One issue with the Athlon 64 right now is that AMD seems to be having a bit of an identity crisis with sockets. Currently, there are 3 different sockets for Athlon 64 CPUs:

Socket 754 – Until recently, this was the only socket for non-FX, non-Opteron Athlon 64 CPUs. It supports single channel unbuffered memory, and currently has the largest range of processors available for it.
Socket 939 – This is supposedly socket 754's replacement for higher end Athlon 64 CPUs. It supports dual channel unbuffered memory, and currently only has 3 processors available for it—the 3500+, 3800+, and FX-53.
Socket 940 – This is the only socket that Opteron CPUs are available on. It supports dual channel registered memory. The FX-51 and FX-53 were made in socket 940, but future FX CPUs will only be available for socket 939.

Since socket 754 is already slated to be discontinued, many are wondering whether it is better to get a system based on socket 939 or socket 754. The answer will vary on a case-by-case basis, but these are the major things to consider:

• Dual channel memory makes little performance difference with the Athlon 64. The only place you will really see a performance difference is in synthetic memory benchmarks. Most real applications just do not use memory in a way that can benefit greatly from dual channel memory.

• Socket 939 CPUs have half the L2 cache of their socket 754 counterparts (512k vs. 1mb). The dual channel memory supposedly makes up for the performance difference and then some, giving socket 939 CPUs a slightly higher performance rating at the same clock speed.

• AMD’s top of the line Athlon 64 will probably not be available in socket 754 a year from now (though nobody knows for sure). For those of you who plan on upgrading only the CPU more than a year from now, this could be an issue. For now, however, AMD offers its top of the line CPUs (except for the FX line) in both 939 and 754 sockets.

• You cannot buy a socket 939 CPU right now [June 14, 2004] for less than ~US$480. The 3500+ is the cheapest CPU offered for socket 939. In contrast, the Athlon 64 2800+ for socket 754 is available for as little as ~US$170.

• Athlon 64 mobile CPUs are only available in socket 754.

• Socket 939 CPUs cost more at the same clock speed than their socket 754 counterparts. The performance differences between the 3400+/3500+ and 3700+/3800+ are negligible, yet the 3500+ costs ~US$80 more than the 3400+ and the 3800+ costs ~US$10 more than the 3700+. [June 14, 2004 pricing]

• Initially, socket 939 motherboards will be more expensive than socket 754 boards, and there will be few value-oriented socket 939 motherboards.

Having built a socket 754 Athlon 64 machine over 6 months ago (well before socket 939 was released), I was recently asked the following question: “If you were to build an Athlon 64 machine now, would you do it differently than the one you already have?”

I can honestly say NO. I would definitely go with socket 754 over socket 939.

I can only remember one time in my life that I have ever upgraded a CPU without upgrading the motherboard as well—I went from a Pentium 133 to a Pentium 200. Since then my CPU upgrades (which have been numerous) have always involved a new motherboard. If I were to build a system now I would choose the system that offers the best value NOW, not worrying that certain CPUs might or might not be released for that socket in the future. The only thing I might consider doing differently is using an Athlon 64 mobile CPU. The mobile CPUs offer the same performance as their desktop counterparts, but at astoundingly low power levels. The less adventurous, though, would be advised to wait until motherboard compatibility issues settle down.

My Athlon 64 machine has been both the most powerful and the quietest system I have ever used. Where AMD has faltered with marketing, they have triumphed with technology. Hopefully this article has provided some valuable information for those looking for a computer with top performance and minimal noise.

Russ Kinder says

Most of the points I had planned on writing, Bryan already covered very nicely in his portion. So... how about a few semi-random thoughts?


One aspect not touched upon in Bryan's description of the A64's on-die memory controller is its impact on motherboard design. Because the memory controller requires equal length traces to each of the RAM pins, and the traces need to be as short as possible to reduce latency, there is much less variety in motherboard layout than with designs for previous processors. These requirements make it unlikely that you will ever see a BTX form-factor A64 motherboard. BTX shoves the CPU up to the edge of the board, and turns the RAM at 90° to it, along the other side of the motherboard. Not an impossible arrangment, but nowhere close to ideal.

Some AMD engineers have been quoted as saying they will not be supporting BTX. If Intel does manage to bully BTX onto the Intel motherboard market, you may be forced to buy a completely new case and PSU if you want to switch from an Intel to an AMD product.


While current 32bit benchmarks show the A64 to be performing on par with the equivalent P4, the efficiency increases gained by using the A64 with a 64bit OS and apps will undoubtedly increase the work/clock-cycle advantage that the A64 already holds. It's really no different from the performance that the P4 gained once SSE/SSE2 implementation began to appear.


One other method for ballparking the TPDs of AMD and Intel is to look at the information that they give to motherboard makers, specifically the Core Voltage (Vcore) and the Maximum Core Amperage (Acore). Motherboard makers need accurate data for these limits to design the voltage regulation circuitry of the board. By multiplying the core voltage by the maximum amperage you can get a pretty good guess at what the Maximum Power (Wmax) that the CPU could draw from the motherboard:

XP2500 Barton
A64 2800-3800+
Intel P4 3.4

*Barton values included for comparison

This table confirms Bryan's comments: Intel underestimates maximum power dissipation with their TDP for P4 while AMD gives the maximum power for the fastest processor for their Athlon 64 TDP.


AMD has stated pretty clearly that 939 is the desktop socket "for the foreseeable future". Socket 754 will continue on, but not indefinitely. Even the new value Sempron line will eventually transition to 939.

Mike Chin says

One of my first SPCR articles was Silencing a P4-1.6, a real project to make a very quiet and powerful (for its time) work PC. It was a great overclocker even at or slightly less than standard Vcore. I've liked Northwood P4s since, and have had good experiences with 1.8, 2.0, 2.6, and 2.8GHz versions. Despite the misnomer, the "heat spreader" made the P4 highly resistant to damage caused by user errors with enthusiast heatsinks. Up to the P4-2.8, I've have little trouble making nearly silent, powerful PCs with them.

The Prescott and the >3Ghz Northwoods change everything. A brief experience with a P4-2.8 Prescott made it very clear that silencing P4s based on this core is a serious and probably expensive challenge. They run much hotter than comparable Northwood cores, and as Bryan's research bears out, much hotter than equivalent speed Athlon 64s as well.

In contrast, the Athlon 64-3200+ I've been playing with is a very cool character. Casual experiments told me this A64 system ran cooler than my P4 main rig, but it was not until I did A/B comparison on AC power dissipation that the difference hit home.

The ARM Systems StealthPC Powerhouse P4-3.2 system I reviewed had a max AC power draw of 235W under load. Because the efficiency of the Zalman PSU in that system is ~75%, I know the DC power draw is ~176W.

My A64-3200+ system has almost identical components except for 512mb more RAM, one less drive, and an ATI 9800-256 Pro instead of 9800XT VGA. I'd say the RAM & drive balance each other off; the 9800XT represents ~15W more power.

This system draws max 168W AC under the same load used for the ARM System P4 PC. With the Zalman PSU efficiency at ~73% at that power level, the DC power draw is 123W. Add 15W to that to compensate for the VGA card. We're at 138W. This is nearly 40W lower than the DC power draw of the P4-3.2 system

The AC power draw of the system is a very good indication of the total heat in a PC. Just add 20W to the 168W of the A64 system to compensate for the XT. Now you have 188W vs 235W; the heat difference in those two PCs is 47W.

How does this translate in terms of noise? The P4-3.2 system referenced above was judged as borderline quiet when at max load, right around the 30 dBA/1m mark. My A64-3200+, in a virtually identical ARM System Foundation Case / PSU Kit, runs no higher than 25 dBA/1m, and the temperature of the CPU never exceeds 55C (in a room with ambient up to ~26C this far.)

As for performance, it's significant to note that in the mainstream PCWorld magazine's July 2004 system rankings, of the seven top performance desktops, the four Athlon 64 machines scored 141-150 on the magazine's Worldbench 4 benchmark. The three P4 systems trailed at just 127-130.

For enthusiasts, perhaps nicest of all is the improved heatsink retention bracket on the motherboards and the protective "heat spreader" on the processor itself. It took a long time for AMD to follow Intel's lead on this, but it's better late than never.


Discuss this article in the SPCR Forum.

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