Since there are protective features that are controlled by the processor, and other features that depend on the platform, it is imperative to get a motherboard that comes with properly implemented thermal features. According to the data sheets you can find on the Web, Intel offers much more than AMD, although both companies offer a basic feature set to prevent a modern processor from dying due to heat.
Intel Thermal Monitor
Intel has been implementing a thermal control circuit that it calls the Thermal Monitor since the Pentium 4. It turns off individual clocks whenever there is a high temperature situation. The consequence is that the CPU will effectively be stopped every once in a while, which makes it appear as if it is running at a lower clock speed. People sometimes refer to this feature as "throttling", although this is not entirely correct.
Obviously, this feature helps to reduce the heat dissipation, but it is entirely controlled by the CPU and is not user or software configurable. Typically you will not notice the presence of the Thermal Monitor as long as your cooling solution works properly. If that is not the case, note that "excessive activation of the
TCC (...) may cause a noticeable performance loss" (Core 2 Duo processor datasheet). If you are familiar with our first Tom's Hardware video (zip download of a DivX video file), you may recall that the Pentium 4 (and consecutive models) delivered substantially lower performance if the Thermal Monitor engaged.
Intel Thermal Monitor 2
Thermal Monitor 2 comes on top of the basic TM feature and makes use of the same corners that enable the Enhanced SpeedStep feature. The latter allows the system to reduce processor clock speed and voltage to save energy; TM2 does very much the same thing, but is triggered by a high temperature situation instead of the processor driver. Also, TM2 doesn't know individual clock speed increments, so it switches straight from maximum clock and voltage to the lowest supported settings.
Intel PROCHOT# Signal
The PROCHOT# (processor hot) signal provides a two-way communication to enable the Thermal Monitor. It can be triggered by the CPU or by the platform, so the system can put the CPU into a state that reduces heat dissipation. This can make sense if other components are about to overheat, and only system-wide power savings will avoid it.
Intel THERMTRIP# Signal
In the event of the CPU temperature exceeding certain threshold values, the processor will automatically shut down the system using the THERMTRIP# signal, to prevent silicon damage.
While an early Athlon 1200 based on the Palomino core died shortly after we removed its heat sink, this doesn't happen any more today. All AMD processors are familiar with the THERMTRIP# signal, which is used to shut down the system. Cool'n'Quiet can also be used by the system to reduce the processor clock speed and voltage, similar to what Intel does with Enhanced SpeedStep or Thermal Monitor 2. However, we found that the AMD Athlon dual core processors don't throttle the clock speed by themselves, but rather shut down the entire system once their temperature exceeds the threshold.