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Power consumption
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 SMPOV and the 64-bit version of the POV-Ray renderer to load up the CPUs. In all cases, we asked SMPOV to use the same number of threads as there were CPU front ends in Task Manager—so four for the dual Opteron 252, four for the Pentium XE 840, two for the Opteron 175, and so on.

The graphs below have results for "power management" and "no power management." That deserves some explanation. By "power management," we mean SpeedStep or PowerNow/Cool'n'Quiet. (In the case of the Pentium 4 600-series processors and the XE 840, the C1E halt state is always active, even in the "no power management" tests.) Sadly, the beta BIOS we used for our Tyan S2895 motherboard didn't support AMD's PowerNow, so we couldn't report scores for the Opterons with power management enabled. Similarly, the beta BIOS for our Asus A8N-SLI Deluxe mobo wouldn't support Cool'n'Quiet—which is PowerNow with a different name—on the Athlon 64 X2 processors. AMD says all of its dual-core chips will support power management once the proper BIOS support becomes available.

As advertised, both X2 models deliver power consumption comparable to their single-core predecessors, at least according to these system-level numbers. Unfortunately, the Pentium D and XE chips don't fare so well. The systems based on these chips suck up over 100W more than the systems based on the competing X2 processors.