Vol. 3B 14-25

POWER AND THERMAL MANAGEMENT

14.7.5.1   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. Software reads output of the digital 
thermal sensor using the IA32_THERM_STATUS MSR.

14.7.5.2   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.
Temperature measurements returned by digital thermal sensors are always at or below TCC activation tempera-
ture. Critical temperature conditions are detected using the “Critical Temperature Status” bit. When this bit is set, 
the processor is operating at a critical temperature and immediate shutdown of the system should occur. Once the 
“Critical Temperature Status” bit is set, reliable operation is not guaranteed. 
See Figure 14-27 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.

•

Thermal Status Log (bit 1, R/WC0) — This is a sticky bit that indicates the history of the thermal sensor 
high temperature output signal (PROCHOT#). Bit 1 = 1 if PROCHOT# has been asserted since a previous 
RESET or the last time software cleared the bit. Software may clear this bit by writing a zero.

•

PROCHOT# or FORCEPR# Event (bit 2, RO) — Indicates whether PROCHOT# or FORCEPR# is being 
asserted by another agent on the platform. 

•

PROCHOT# or FORCEPR# Log (bit 3, R/WC0) — Sticky bit that indicates whether PROCHOT# or 
FORCEPR# has been asserted by another agent on the platform since the last clearing of this bit or a reset. If 
bit 3 = 1, PROCHOT# or FORCEPR# has been externally asserted. Software may clear this bit by writing a zero. 
External PROCHOT# assertions are only acknowledged if the Bidirectional Prochot feature is enabled.

•

Critical Temperature Status (bit 4, RO) — Indicates whether the critical temperature detector output signal 
is currently active. If bit 4 = 1, the critical temperature detector output signal is currently active.

Figure 14-27.  IA32_THERM_STATUS Register 

63

0

Reserved

15

Reading Valid

1

2

3

4

5

8

10

16

22

23

27

Resolution in Deg. Celsius
Digital Readout

Thermal Threshold #2 Log
Thermal Threshold #2 Status 
Thermal Threshold #1 Log
Thermal Threshold #1 Status
Critical Temperature Log

6

7

9

31

32

Critical Temperature Status
PROCHOT# or FORCEPR# Log
PROCHOT# or FORCEPR# Event
Thermal Status Log
Thermal Status

11

Power Limit Notification Log
Power Limit Notification Status