Power consumption
In order to test the power management features of the P4 600 series, we decided to throw out all of our old results and start over with fresh test systems. There were a number of reasons for this decision, including the fact that our prior Athlon 64 motherboard didn't properly support Cool'n'Quiet, AMD's power management technology. Below you'll find the test configurations we used for power consumption testing.

Please note that some of the CPUs in the spec table above and on the graphs below are marked with an asterisk. That's because they are not true-blue versions of the chips in question. They are, instead, CPUs rated for higher speed grades that have been underclocked to match the model in question. For instance, Intel only supplied us with a Pentium 4 660 review sample, so we reduced its CPU multiplier to simulate a 650 and 640. I mention this fact because every chip is different, and generally, the ones that are chosen to run at higher speed grades exhibit better power and heat characteristics, as well.

That said, clock speed and voltage are still major determinants of overall power consumption, so we decided to include these "simulated" CPU models in our results. In all cases, we allowed the motherboard to determine the appropriate CPU voltage levels. That's handled dynamically with SpeedStep and Cool'n'Quiet, anyway.

Speaking of simulation, because the DFI desktop motherboard for the Pentium M doesn't support SpeedStep, we used the RMClock utility to simulate SpeedStep in our testing. The comparison won't be exact, because RMClock uses its own algorithm to scale clock frequencies, but we did set the RMClock profile to match the Pentium M 755's minimum clock speed of 600MHz and minimum voltage of 0.988V.

We measured the power consumption of our entire test systems, except for the monitor, at the wall outlet using a Watts Up PRO watt meter. The test rigs were all equipped with OCZ PowerStream 520W power supply units. The idle results were measured at the Windows desktop, and we used Cinebench 2003's rendering test to load up the CPUs. For P4s, we used the multithreaded version of the test to take advantage of Hyper-Threading.

Finally, the graphs below have results for "power management" and "no power management." That deserves some explanation. By "power management," we mean SpeedStep or Cool'n'Quiet. In the case of the Pentium 4 600-series and 500J-series processors, the C1E halt state is always available, even in the "no power management" tests.

The first thing one notices about power consumption at idle is that there's practically no difference between the Pentium 4 600-series chips with SpeedStep enabled or disabled. That's because the C1E halt state accomplishes essentially the same thing. Either way, though, the 600-series and 500J-series CPUs both consume quite a bit less power at idle than the Prescott chips that don't support C1E or SpeedStep, like the P4 560 or the new Extreme Edition 3.73GHz.

Because current Athlon 64 processors don't have anything comparable to the C1E halt state, they pull more juice at idle than the newer Pentium 4 chips. With Cool'n'Quiet enabled, though, all of the Athlon 64 processors consume even less power than the C1E and SpeedStep-enabled P4s. There's no doubt, though, that Intel has made great strides.

Under load, SpeedStep and Cool'n'Quiet don't have any significant impact, but we do see that the P4 600 series manages to consume less power than the older Prescott-based processors we're testing. I wouldn't attribute that difference to the presence of the C1E halt state, in part because Cinebench is a very full CPU load, and in part because the P4 500 and 500J models consume roughly comparable amounts of power at the same clock speed. The 600-series processors, and even the new P4 3.73GHz Extreme Edition, are relatively more efficient under load—despite the fact that they're packing another 1MB of L2 cache.

I asked Intel what to make of these results, but unfortunately, they weren't able to give me an answer before this article went online. I hate to speculate about why the newer P4s with 2MB of L2 cache aren't drawing as much power as the older models with 1MB of L2. Is it just better properties of newer chips, or has the new CPU core been otherwise tweaked? Perhaps we'll get some answers from Intel before too long.

Whatever the reason, the new P4 core does require relatively less power under load than the older chips that we tested. These things do vary from chip to chip, so I don't want to make too much of these results from just a few processors. Indications are certainly good, though. That said, the 90nm version of the Athlon 64 3500+ still pulls about 60W less under load than the P4 650 does, and the Pentium M is even more efficient.