Intel 64 and IA-32 Architectures Software Developers Manual Volume 3A, System Programming Guide, Part 1

13-12 Vol. 3A
POWER AND THERMAL MANAGEMENT
13.4.4 Detection of Thermal Monitor and Software Controlled
Clock Modulation Facilities
The ACPI flag (bit 22) of the CPUID feature flags indicates the presence of the
IA32_THERM_STATUS, IA32_THERM_INTERRUPT, IA32_CLOCK_MODULATION
MSRs, and the xAPIC thermal LVT entry.
The TM1 flag (bit 29) of the CPUID feature flags indicates the presence of the auto-
matic thermal monitoring facilities that modulate clock duty cycles.
13.4.5 On Die Digital Thermal Sensors
On die digital thermal sensor can be read using an MSR (no I/O interface). In Intel
Core Duo processors, each core has a unique digital sensor whose temperature is
accessible using an MSR. The digital thermal sensor is the preferred method for
reading the die temperature because (a) it is located closer to the hottest portions of
the die, (b) it enables software to accurately track the die temperature and the
potential activation of thermal throttling.
13.4.5.1 Digital Thermal Sensor Enumeration
The processor supports a digital thermal sensor if CPUID.06H.EAX[0] = 1. If the
processor supports digital thermal sensor, EBX[bits 3:0] determine the number of
thermal thresholds that are available for use.
Software sets thermal thresholds by using the IA32_THERM_INTERRUPT MSR. Soft-
ware reads output of the digital thermal sensor using the IA32_THERM_STATUS
MSR.
13.4.5.2 Reading the Digital Sensor
Unlike traditional analog thermal devices, the output of the digital thermal sensor is
a temperature relative to the maximum supported operating temperature of the
processor. Tj(Max).
Temperature measurements returned by digital thermal sensors are always at or
below Tj(Max). Over temperature conditions are detected using the out-of-spec
status bit. When this bit is set, the processor is operating out of specification and
immediate shutdown of the system should occur. Once the out-of-spec status bit is
set, reliable operation is not guaranteed.
See Figure 13-8 for the layout of IA32_THERM_STATUS MSR. Bit fields include:
• Thermal Status (bit 0, RO) — This bit indicates whether the digital thermal
sensor high-temperature output signal (PROCHOT#) is currently active. Bit 0 = 1
indicates the feature is active. This bit may not be written by software; it reflects
the state of the digital thermal sensor.