Under Core i7's Hood: Comparing The C0 And D0 Steppings

Overclocking Core i7-975 (D0): 4.14 GHz

The processor voltage at 3.33 GHz was considerably lower on the D0 stepping processor than on the C0 model: 1.136 V versus 1.232 V.

We did not modify the BIOS voltage when we switched the multipler from 25x to 27x to reach 3.6 GHz. The effective voltage was still slightly higher at 1.232 V.

We saw the same picture at 3.74 GHz. There were no changes on the voltage side, and CPU-Z reported 1.240 V. Remember, by the time we saw this speed on the C0 sample we were already increasing the processor voltage in our board's BIOS.

Our next stop was 3.87 GHz, achieved through a multiplier increase to 28x. Once again, we didn’t have to increase the processor voltage. CPU-Z still reported 1.240 V. Might this even be the case for 4.0 GHz?

As a matter of fact, it was. Even the 4.0 GHz clock speed, which had required another voltage increase on the C0 stepping Core i7-965, was stable and reliable at the default voltage. CPU-Z still reported 1.232 V, impressively showing that the D0 stepping is capable of reaching high clock speeds at reduced voltage. But can it go much higher?

Hitting 4.14 GHz required us to adjust the voltage a bit, but then the setting was reliable again at an effective 1.312 V.

We were less successful at 32x and 4.27 GHz, despite having tried several different voltage levels up to 1.35 V. The system ran fine for most of the time, but every endurance run revealed that it was no longer reliable at peak load. Hence the maximum clock speed on air cooling was less than 4.20 GHz with our Core i7-975 Extreme D0 sample.

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