SpeedStep

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SpeedStep is a trademark for a series of technologies (including SpeedStep, SpeedStep II, and SpeedStep III) built into some Intel microprocessors that allows the clock speed of the processor to be dynamically changed by software.

1 Explanation
2 Operating system support
3 Versions
4 See also
5 External links
6 Notes

[edit] Explanation

Running a processor at high clock speeds allows for better performance. However, when the same processor is run at a lower frequency, it generates less heat and consumes less power. In many cases, the core voltage can also be reduced, further reducing power consumption and heat generation. This can conserve battery power in notebooks, extend processor life, and reduce noise generated by variable-speed fans. By using SpeedStep, users can select the balance of power conservation and performance that best suits them, or even change the clock speed dynamically as the processor burden changes.

The power consumed by a CPU with a capacitance C, running at frequency f and voltage V is^[1]

P = C V 2 f .

For a given processor, C is a fixed value. However, V and f can vary considerably. For example, for a 1.6 GHz Pentium M, the clock frequency of can be stepped in 200 MHz increments over the range from 1.6 to 0.6 GHz. At the same time, voltage requirement decreases from 1.484 V to 0.956 V. The result is that the power consumption theoretically goes down with a factor 6.4. In practice, the effect may be smaller because most operating systems will execute a halt instruction when the CPU is idle, which also reduces the power. A regular CPU may be working 10 percent of the time at full power, while a speed-stepped CPU would be working for 80 percent of the time at a lower power, with approximately the same total power consumption as a result.

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.

[edit] Operating system support

Under older Microsoft Windows operating systems, including Windows 2000 and previous versions, a special driver and dashboard application were needed to access the SpeedStep feature. Intel's website specifically states that such drivers must come from the computer manufacturer; there are no generic drivers supplied by Intel which will enable SpeedStep for older Windows versions if one cannot obtain a manufacturer's driver.^[2]^[3]

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.^[4]^[5]

Linux has full SpeedStep support already integrated in the kernel.

Mac OS also has SpeedStep built into the kernel, since the release of the Intel version of Mac OS X 10.4 and is controlled in the System Preference "Energy Saver"

Solaris supported SpeedStep since OpenSolaris SXDE 9/07. ^[6]

[edit] Versions

V1.1 is used by second generation Pentium III processors. It enables the CPU to switch between two modes: high and low frequency. This is done by modifying the CPU's multiplier. A 1 GHz Pentium III consuming about 20 watts could be reduced to 600 MHz which reduces the power consumption to about 6 watts.

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.