SpeedStep is a trademark for a series of technologies (including SpeedStep, SpeedStep II, and SpeedStep III) built into some Intel microprocessors that allow the clock speed of the processor to be dynamically changed by software. This allows the processor to meet the instantaneous performance needs of the operation being performed, while minimizing power draw and heat dissipation.
The power consumed by a CPU with a capacitance C, running at frequency f and voltage V is approximately
Older processors, using older versions of the SpeedStep technology, have fewer increments, such as the Pentium 4-M. For example, a 1.7 GHz Pentium 4M can run at 1.6 GHz, at 1.2 GHz, and at 786 MHz.
SpeedStep technology is partly responsible for the reduced power consumption of Intel’s Pentium M processor, part of the Centrino brand.
Under Microsoft Windows XP, SpeedStep support is built into the power management console under the control panel. In Windows XP a user can regulate the processor's speed indirectly by changing power schemes. The "Home/Office Desk" disables SpeedStep, the "Portable/Laptop" power scheme enables SpeedStep, and the "Max Battery" uses SpeedStep to slow the processor to minimal power levels as the battery weakens.
In contrast, AMD continues to supply and support drivers for its competing PowerNow! technology that will work on Windows 2000, ME, 98, and NT.
Linux has full SpeedStep support integrated into the kernel version 2.6.
Mac OS also has SpeedStep built into the kernel, since the release of the Intel version of Mac OS X 10.4 and is already enabled. It cannot be controlled in the System Preference "Energy Saver." To disable this feature, and set a specific clock speed (full speed or reduced) requires a third party application, such as coolbook
The BSD kernels also have full SpeedStep support integration.
V2.1 (Enhanced SpeedStep) is used in Pentium III-Mobile processors and is similar to the previous version, but in the low frequency mode the CPU also uses a different voltage than the high frequency mode.
V2.2 is adapted for Pentium 4-Mobile processors. With this, a 1.8 GHz Pentium 4-M consuming about 30 watts can lower its frequency to 1.2 GHz, thus reducing power consumption to about 20 watts.
V3.1 (EIST) is used with the first and second generation of Pentium M processors (Banias and Dothan cores, used in Centrino platforms). With this technology, the CPU varies its frequency (and voltage) between about 40% and 100% of its base frequency in increments of 100 MHz (for Banias core) or 133 MHz (for Dothan core). With this technology, Intel also introduces realtime Level 2 cache capacity variation, further improving power savings.
V3.2 (Enhanced EIST) is adapted for multi-core processors with unified Level 2 cache.
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