In course of normal operation of the Pentium 4 (and Athlon XP) temperature can jump within the range of 30-50?C/s. Digital monitoring ICs can't process diode's measurements faster than 8 times/sec (conversion time is 125 ms and higher). It means that the measured temperature is behind the real temperature of the die. The bigger the jumps, the greater the error: when a cooling system fails the temperature jump can be 60?C/sec and more and the temperature lag will be at least 7?C. That is why classical thermal monitoring schemes do not suit the Pentium 4.
All Intel processors starting from Pentium Pro have a temperature sensor and an analog comparison circuit meant for detecting catastrophic overheating. This sensor, like a thermal diode, is a diode-connected transistor but now based on a reverse-biased p-n junction and a dependence of the junction reverse current on temperature. Diode's current is measured with a comparator with a reference source current which is adjusted so that the comparator would react on a definite temperature value. A response time of such circuit is just several hundreds nanoseconds, that is why it can be found out quickly if a temperature exceeds the limits. As a result, if a temperature of an Intel CPU is more than 125-135?C, this comparison circuit stops sending clocks to all processor nodes, the THERMTRIP# signal reports on catastrophic overheating and the processor VRM turns off.
Look here for more information:
http://www.digit-life.com/articles/pentium4athlonxpthermalmanagement/
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Good luck, hope this helps!
