Not All Core i7 CPUs Are Created Equal
Whether you're looking at the IT hardware business or automobile industry, products often receive bug fixes, improvements, or upgrades during their active life cycle. Cars get face-lifted once they’ve been on the market for a while, usually at that halfway point between launch and the introduction of a successor.
In terms of silicon devices, we’re talking about steppings, which often incorporate a number of different improvements. When Intel introduced its Core i7-975 Extreme, it started to replace the 965 Extreme's C0 stepping with the more advanced D0 silicon. We found some significant differences between the two.
New Steppings Don’t Mean More Performance
First of all, we’d like to make clear that modified processor steppings eliminate known bugs, while also improving the production process. Typically, this includes optimization on a transistor level, which may result in extended clock speed margins, lower voltage requirements for standard operation, or even the addition of features.
As a result, the latest processors normally show better energy efficiency, as they may require slightly less power than previous steppings to deliver the same performance results. However, stepping updates usually do not introduce any performance increase, as the processor vendor would rather have you pay for that.
New Stepping Mean More Possibilities
Other publications, such as AnandTech (Core i7-920 D0 Preview) and X-bit labs (D0 Processor Stepping in Action), already reported impressive results with D0 parts over the previous, first-generation Core i7 C0 stepping. Anand looked at a model 920 while X-bit labs tested a 965-Extreme.
We decided to do an apples-to-apples comparison with both Core i7 Extreme editions: the 3.2 GHz 965 and the 3.33 GHz 975. We set both to run at 3.33 GHz in order to compare performance and, surprisingly, we did find differences. Then we tried to overclock as far as possible on air cooling using a Zalman CPNS 10X cooler. Finally, we performed power comparison measurements at the highest clock speed, which both the C0-based 965 and the D0-based 975 processor would support reliably.