http://download.intel.com/design/mobile/datashts/25302804.pdf

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6.3 Enhanced Intel SpeedStep Technology
The mobile Intel Pentium 4 processor, when used in conjunction with the requisite Intel SpeedStep
technology applet or its equivalent, supports Enhanced Intel SpeedStep technology. Enhanced Intel
SpeedStep technology allows the processor to switch between two core frequencies automatically
based on CPU demand, without having to reset the processor or change the FSB frequency. The
processor has two bus ratios and voltages programmed into it instead of one and the GHI# signal
controls which bus ratio and voltage is used. After reset, the processor will start in the lower of its
two core frequencies, the Battery Optimized mode.
An operating mode transition to the high core
frequency can be made by setting GHI# low,
putting the processor into the Deep Sleep state,
regulating to the new VID output, and returning to the Normal state. This puts the processor into
the high core frequency, or Maximum Performance mode. Going through these steps with GHI# set
high, transitions the processor back to the low core frequency operating mode. The processor will
drive the VID[4:0] pins with the VID of the current operating mode and the system logic is
required to regulate the core voltage within specification for the driven VID.

and the system logic is required to regulate the core voltage within specification for the driven VID