Enhanced Turbo Mode

There was wailing and gnashing of teeth among hardware enthusiasts when Intel announced that some Lynnfield CPUs would be named Core i7. All along everyone just assumed Lynnfield CPUs would be called Core i5.

With Lynnfield relying on a totally different 1156-pin socket than Bloomfield, critics pointed out giving two different cores the same name is confusing, possibly leading to uninformed users accidentally purchasing the wrong motherboard for their CPU. Many just yearned for the days when you could tell what features a CPU supported based on its name: buy a Core 2 Quad for example, and you know you’re getting a 4-core processor; AMD uses the X2, X3, X4 designation to denote the number of processing cores.

So why are we harping on Lynnfield’s Core i7 designation on a page that’s supposed to be dedicated to Turbo Mode? Because as you’ll see in the benchmarks later in this article, Lynnfield’s new Turbo Mode allows it to keep up with Bloomfield CPUs in many benchmarks despite its disadvantage in peak bandwidth.

How is Lynnfield able to do this? Simple. Lynnfield’s Turbo Mode sports more aggressive scaling.

As we mentioned at the outset of this article, Nehalem’s Turbo Mode can dynamically OC the processing cores based on factors like usage, power, and temperature. In the Core i7-920 for example, if the CPU is running a single-threaded app that’s only pushing 1 of the CPU’s 4 cores, and the processor is coming nowhere near its temp and power thresholds, it can OC that 1 active core by up to two speed bins (+266MHz). Your 2.66GHz Core i7-920 automatically becomes a 2.93GHz CPU.

If only two cores are being used, and the temp and power conditions are met, it will OC the two active cores by one speed bin (+133MHz), 2.80GHz.

Turbo Mode does this by upping the clock multiplier, which is normally locked at 20 in the case of the Core i7-920 (20.0x multiplier x 133MHz base clock (bclk) = 2.67GHz). Under the max Turbo Mode scenario with only one core being used, the Core i7-920’s multiplier is increased to 22x, yielding 2.93GHz. If 2 or more cores are being taxed, Turbo Mode increases the multiplier to 21x, giving you a final clock speed of 2.80GHz.

The following chart outlines the maximum Turbo Mode speeds for Intel’s Bloomfield-based Core i7 CPUs:

Bloomfield Core i7 - Max Turbo Frequency
CPU	Stock Clock Speed	Max Speed w/4 Cores Active	Max Speed w/3 Cores Active	Max Speed w/2 Cores Active	Max Speed w/1-Core Active
Core i7-975 Extreme	3.33GHz	3.46GHz	3.46GHz	3.46GHz	3.6GHz
Core i7-965 Extreme	3.2GHz	3.33GHz	3.33GHz	3.33GHz	3.46GHz
Core i7-950	3.06GHz	3.2GHz	3.2GHz	3.2GHz	3.33GHz
Core i7-940	2.93GHz	3.06GHz	3.06GHz	3.06GHz	3.2GHz
Core i7-920	2.66GHz	2.8GHz	2.8GHz	2.8GHz	2.93GHz

As you can see in the chart above, Intel’s flagship Core i7-975 Extreme Edition can use Turbo Mode to clock itself up to 3.6GHz max. The Core i7-940 uses Turbo Mode to clock up to 3.2GHz. It’s important to note that this is the best case scenario when all the temp and power conditions are met, and only one core is being used. If you’re running a dual or multi-threaded app that can take advantage of 2 or more cores, you’re limited to just one speed bump of 133MHz, with the i7-940 clocking itself up to 3.06GHz.

For Lynnfield, Turbo Mode is much more aggressive. It can bump the clock multiplier up to five speed bins. In the case of the Core i7-870 for example, it features a clock multiplier of 22.0, with a 133MHz bclk (22x133=2.93GHz). With Turbo Mode enabled, it can up the clock multiplier to 27.0, yielding a final clock speed of 3.6GHz. That’s the same max turbo frequency as Intel’s Core i7-975 Extreme Edition!

Lynnfield Core Max Turbo Frequency
CPU	Stock Clock Speed	Max Speed w/4 Cores Active	Max Speed w/3 Cores Active	Max Speed w/2 Cores Active	Max Speed w/1-Core Active
Core i7-870	2.93GHz	3.2GHz	3.2GHz	3.46GHz	3.6GHz
Core i7-860	2.80GHz	2.93GHz	2.93GHz	3.33GHz	3.46GHz
Core i5-750	2.66GHz	2.8GHz	2.8GHz	3.2GHz	3.2GHz

If you’re only pushing two cores, and your CPU temps and power consumption are within tolerances, Turbo Mode will increase the clock multiplier on the 870 to 26, yielding a final clock speed of 3.46GHz. If you’re using three or all four cores, Turbo Mode will increase the clock multiplier by two speed bins, to 24.0x (+266MHz) resulting in a final clock speed of 3.2GHz.

Like Bloomfield, overclocking won’t automatically disable Turbo Mode either. So if you can keep your CPU cool enough and power consumption remains in check, you can mix the two together for max performance.