Intel ARCHITECTURE IA-32 User Manual download pdf (Page 520)

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POWER AND THERMAL MANAGEMENT

The IA32_CLOCK_MODULATION MSR contains the following flag and field used to enable

software-controlled clock modulation and to select the clock modulation duty cycle:

• On-Demand Clock Modulation Enable, bit 4 — Enables on-demand software controlled

clock modulation when set; disables software-controlled clock modulation when clear.

• On-Demand Clock Modulation Duty Cycle, bits 1 through 3 — Selects the on-demand

clock modulation duty cycle (see Table 13-1). This field is only active when the on-

demand clock modulation enable flag is set.

Note that the on-demand clock modulation mechanism (like the thermal monitor) controls the

processor’s stop-clock circuitry internally to modulate the clock signal. The STPCLK# pin is not

used in this mechanism.

The on-demand clock modulation mechanism can be used to control processor power consump-

tion. Power management software can write to the IA32_CLOCK_MODULATION MSR to

enable clock modulation and to select a modulation duty cycle. If on-demand clock modulation

and TM1 are both enabled and the thermal status of the processor is hot (bit 0 of the

IA32_THERM_STATUS MSR is set), clock modulation at the duty cycle specified by TM1

takes precedence, regardless of the setting of the on-demand clock modulation duty cycle.

For Hyper-Threading Technology enabled processors, the IA32_CLOCK_MODULATION

feature to work properly, the feature must be enabled on all the logical processors within a phys-

ical processor. If the programmed duty cycle is not identical for all the logical processors, the

processor clock will modulate to the highest duty cycle programmed.

For the P6 family processors, on-demand clock modulation was implemented through the

chipset, which controlled clock modulation through the processor’s STPCLK# pin.

Table 13-1. On-Demand Clock Modulation Duty Cycle Field Encoding

Duty Cycle Field Encoding Duty Cycle

000B Reserved

001B 12.5% (Default)

010B 25.0%

011B 37.5%

100B 50.0%

101B 63.5%

110B 75%

111B 87.5%

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Intel ARCHITECTURE IA-32 User Manual Page 520

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