Intel IA-32 - Manual (Page 520)

13-10 Vol. 3A
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
register is duplicated for each logical processor. In order for the On-demand 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%