Demand Based Switching Details
When Intel introduced Enhanced Intel SpeedStep technology in its new Pentium 4 6XX processor family, the CPUs automatically acquired three new technologies, which all belong to the so-called Demand Based Switching type of technologies. We have already paid due attention to the first two technologies of the kind when we reviewed Pentium 4 570J processor based on Prescott E0 core stepping (for more details see our article called Intel Pentium 4 570J CPU Review. This new core stepping provided Pentium 4 processors with a new TM2 thermal monitoring mechanism and a new C1E power consumption reduction scheme. EIST was a nice addition to these two technologies in the new Prescott 2M core, so that now Pentium 4 6XX CPUs can also boast lower average power consumption and heat dissipation.
EIST technology is intended to manage the CPU clock frequency and its Vcore depending on the workload and level of CPU utilization, just like they do it in mobile PCs. In the desktop segment Cool’n’Quiet technology from AMD implemented in Athlon 64 processors can be regarded as an analog to EIST. We can say that this technology allows using the processor resources in a more rational way: when we work in applications that do not require 100% CPU utilization, processor doesn’t have to work at the top of its potential, so its clock frequency can be reduced, which will at the same time allow reducing the heat dissipation and power consumption of the CPU. If the application requires maximum CPU performance, the clock frequency will rise back up to its nominal value, and so will the Vcore.
EIST technology can be enabled just like Cool’n’Quiet. For example, in Windows XP there is a page called Power Option Properties. You have to change the Power Scheme from Home/Office Desk to Minimal Power Management. After that the CPU will start reducing its clock frequency when it is not fully loaded. The processor driver required for this technology is also included into Service Pack 2, which makes EIST technology compatible with Windows XP once SP2 has been installed.
At first sight, EIST seems to be pretty simple. However, I have to assure you that this pretty interesting technology is full of surprises. The thing is that all three Demand Based Switching technologies – C1E, TM2 and EIST – use the same working algorithm. That is why we decided we should once again specify how particularly they work.
The work of C1E, TM2 and EIST is based on the fact that Pentium 4 processors with Prescott E0 and Prescott 2M N0 core steppings can change their Vid multiplier coefficient “on the fly”. To be more exact, Prescott and Prescott 2M processors can reduce their clock multiplier to 14x if necessary (and this is the minimal supported value for Prescott core), and at the same time drop down the Vcore by about 0.25V. The combination of these features determines the so widely spread “power-saving mode” when the CPU starts running at 2.8GHz core clock and reduced core voltage. Here is what we get from CPU-Z utility run for the Pentium 4 660 processor (supporting 3.6GHz nominal frequency), when it works in “power-saving mode”:
Another interesting peculiarity is the fact that the power-saving mode is the same for all the Pentium 4 6XX CPUs with different nominal clock frequencies: in this mode all of them work at 2.8GHz core clock speed, independent of their initial nominal frequency.