Pentium 4 processors have a feature called Thermal Throttling which protects the CPU from overheating, preventing it from burning. Case the processor reaches a "trigger" temperature (which varies acordingly to the CPU model), the processor automatically lowers its speed, in order to make the generated heat to decrease. In the event of an overheating condition, your PC will be slower, but on the other hand your processor won't burn. If even lowering the processor speed the temperature doesn't drop to a safe level, the computer is turned off, in order to protect your CPU.

Actually there are two types of Thermal Throttling:

• TM1: Available in Pentium 4, Xeon, Celeron (Northwood and Prescott cores only) and Pentium M processors. In this type the Thermal Throttling function does not physically lower the CPU clock, but it inserts idle cycles between the instructions sent to the CPU core (i.e. it inserts wait states inside the processor), which lowers the processor performance, hence its temperature.
• TM2: Used on socket 775 Pentium 4 and Celeron and Pentium M processors, this type really lowers the CPU clock. This is done by lowering the CPU clock multiplier.

Average users won't know if the Thermal Throttling is activated or not in their computers. If this feature is activated in your PC, it will run slower and also this means that you have an overheating problem in your computer, which has to be solved.

There is a very small program called Throttle Watch (http://www.panopsys.com) that can tell you if the Thermal Throttling is being activated or not. Just uncompress and run the program and you will see two windows. The top window shows how much the processor is being used. You can access a similar windows by pressing the Control Alt Del keys at the same time on Windows XP and then selecting the Performance tab.

But it is on the lower window that you will find the advantage of this program. This window shows if the Thermal Throttling feature is being used or not, and how much.

click to enlarge
Figure 1: Throttle Watch.