The initial incarnation of the SpeedStep technology was code-named Geyserville and was quite simplistic compared to the power saving technologies used on the Pentium® M processor (see “Enhanced SpeedStep” on page 1433 for more information).
As implemented on the Pentium® III processor, Intel® Speedstep technology allowed the processor to switch between two core frequencies without resetting the processor or changing the FSB frequency. The processor had two FSB-to-core frequency ratios programmed instead of one (the two values were programmed into the processor when it was manufactured and could not be changed). Low Frequency Mode (LFM) maximized battery life, while High Frequency Mode (HFM) permitted the processor to operate at full speed when the laptop was connected to AC power.
The processor's GHI# input signal controlled which of the two bus ratios was used. After reset, the processor started operation at the lower of the two core frequencies (LFM). The OS transitioned the processor to HFM in the following manner (if the system was connected to AC power):
Transitioning the processor back to LFM was made by reversing these steps.
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