Intel IA-32 - Manual (Page 511)

Vol. 3A 13-1
CHAPTER 13
POWER AND THERMAL MANAGEMENT
This chapter describes facilities of IA-32 architecture used for power management and thermal
monitoring.
13.1 ENHANCED INTEL SPEEDSTEP
®
TECHNOLOGY
Enhanced Intel SpeedStep
®
Technology was introduced in the Pentium M processor; it is avail-
able in Pentium 4, Intel Xeon, Intel
®
Core™ Solo and Intel
®
Core™ Duo processors. The tech-
nology manages processor power consumption using performance state transitions. These states
are defined as discrete operating points associated with different frequencies.
Enhanced Intel SpeedStep Technology differs from previous generations of Intel SpeedStep
Technology in two ways:
•
Centralization of the control mechanism and software interface in the processor by using
model-specific registers.
•
Reduced hardware overhead; this permits more frequent performance state transitions.
Previous generations of the Intel SpeedStep Technology require processors to be a deep sleep
state, holding off bus master transfers for the duration of a performance state transition. Perfor-
mance state transitions under the Enhanced Intel SpeedStep Technology are discrete transitions
to a new target frequency.
Support is indicated by CPUID, using ECX feature bit 07. Enhanced Intel SpeedStep Tech-
nology is enabled by setting IA32_MISC_ENABLE MSR, bit 16. On reset, bit 16 of
IA32_MISC_ENABLE MSR is cleared.
13.1.1 Software Interface For Initiating Performance State
Transitions
State transitions are initiated by writing a 16-bit value to the IA32_PERF_CTL register. If a tran-
sition is already in progress, transition to a new value will subsequently take effect.
Reads of IA32_PERF_CTL determine the last targeted operating point. The current operating
point can be read from IA32_PERF_STATUS. IA32_PERF_STATUS is updated dynamically.
The 16-bit encoding that defines valid operating points is model-specific. Applications and
performance tools are not expected to use either IA32_PERF_CTL or IA32_PERF_STATUS
and should treat both as reserved. Performance monitoring tools can access model-specific
events and report the occurrences of state transitions.