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Features

6.2.4 Enhanced HALT Snoop or HALT Snoop State, Grant Snoop

State

The Enhanced HALT Snoop State is used in conjunction with the new Enhanced HALT state. If

Enhanced HALT state is not enabled in the BIOS, the default Snoop State entered will be the

HALT Snoop State. Refer to the sections below for details on HALT Snoop State, Grant Snoop

State and Enhanced HALT Snoop State.

6.2.4.1 HALT Snoop State, Grant Snoop State

The processor will respond to snoop transactions on the FSB while in Stop-Grant state or in HALT

Power Down state. During a snoop transaction, the processor enters the HALT:Grant Snoop state.

The processor will stay in this state until the snoop on the FSB has been serviced (whether by the

processor or another agent on the FSB). After the snoop is serviced, the processor will return to the

Stop-Grant state or HALT Power Down state, as appropriate.

6.2.4.2 Enhanced HALT Snoop State

The Enhanced HALT Snoop State is the default Snoop State when the Enhanced HALT state is

enabled via the BIOS. The processor will remain in the lower bus ratio and VID operating point of

the Enhanced HALT state.

While in the Enhanced HALT Snoop State, snoops are handled the same way as in the HALT

Snoop State. After the snoop is serviced the processor will return to the Enhanced HALT Power

Down state.

6.2.5 Enhanced Intel SpeedStep

Technology

Enhanced Intel SpeedStep Technology enables the processor to switch between frequency and

voltage points, which may result in platform power savings. To support this technology, the system

must support dynamic VID transitions. Switching between voltage/frequency states is software

controlled.

Note: Not all processors are capable of supporting Enhanced Intel SpeedStep Technology. More details

on which processor frequencies support this feature will be provided in future releases of the Intel

Pentium

Processor Extreme Edition and Intel

Pentium D Processor Specification Update.

Enhanced Intel SpeedStep Technology is a technology that creates processor performance states

(P states). P states are power consumption and capability states within the Normal state as shown in

Figure 6-1. Enhanced Intel SpeedStep Technology enables real-time dynamic switching between

frequency and voltage points. It alters the performance of the processor by changing the bus to core

frequency ratio and voltage. This allows the processor to run at different core frequencies and

voltages to best serve the performance and power requirements of the processor and system. Note

that the front side bus is not altered; only the internal core frequency is changed. To run at reduced

power consumption, the voltage is altered in step with the bus ratio.

The following are key features of Enhanced Intel SpeedStep Technology:

• Voltage/Frequency selection is software controlled by writing to processor MSRs (Model

Specific Registers), thus eliminating chipset dependency.

— If the target frequency is higher than the current frequency, V

is incremented in steps

(+12.5 mV) by placing a new value on the VID signals and the processor shifts to the new

frequency. Note that the top frequency for the processor can not be exceeded.

— If the target frequency is lower than the current frequency, the processor shifts to the new

frequency and V

is then decremented in steps (-12.5 mV) by changing the target VID

through the VID signals.

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