Conclusion
Although stepping upgrades typically benefit the processor vendor's manufacturing site the most (and also fix known errata), the differences between two processors, which appear to be identical on paper at same clock speeds, turned out to be larger than expected. Intel’s Core i7-975 Extreme at 3.33 GHz, based on the latest D0 stepping, is clearly superior to the older C0 stepping, which was used for the Core i7-940 and the 965 Extreme.
We expected a decrease in system-wide power consumption thanks to optimizations in Intel’s D0 stepping versus the older C0 core. At idle, the system power consumption was hardly altered, but at peak load, the Core i7-975 Extreme, with its D0 stepping, managed to reduce the system-wide power consumption by 15W (or 6.9 percent). Keep in mind that all parameters including the clock rate (3.33 GHz) were identical. Thus, the efficiency benchmarks returned much better results for the newer D0 stepping.
But this isn’t all. We also found that the D0 stepping i7-975 can overclock nicely to 4.0 GHz without any voltage increase, while we had to increase the voltage to reliably run 3.74 GHz on the i7-965 processor. As a consequence, the D0 processor overclocked up to 4.27 GHz, but wasn’t stable until we reduced to 4.14 GHz. The older Core i7-965 Extreme couldn’t even do this.
Finally, we even found that the D0 stepping delivers slightly more performance. The increase is nominal, but it’s just another reason why you should only go for the latest steppings. Use Intel’s ARK to identify the model number of the processor model you want.
