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Vol. 3 14-17
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 14-1). 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
consumption. Power management software can write to the
IA32_CLOCK_MODULATION MSR to enable clock modulation and to select a modula
-
tion 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),
Figure 14-10. IA32_CLOCK_MODULATION MSR
Table 14-1. 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%
63 0
Reserved
13
On-Demand Clock Modulation Duty Cycle
On-Demand Clock Modulation Enable
45
Reserved