For reference, the datasheet for the Pentium D 800 series is here:
Two temperatures measurements are significant for processors: Tjunction and Tcase. Tjuntion is the temperature measured at the actual CPU circuits within the die. Ultimately, this is bottom line. If Tjunction gets too high, Bad Things happen. Tcase is the temperature measured at the center of the top surface of the CPU package. That is, the surface that the heatsink mounts to. The Intel specs don’t specify maximum Tjunction, only Tcase. The reason for this is that the thermal design guide is provided for the benefit of heatsink/fan manufacturers. They can’t control how temperatures are propagated from Tjunction to Tcase, so their job is simply to keep the CPU package below a certain temperature. The rest is up to Intel.
Here’s the rub: The Pentium D line has an embedded thermal diode for measuring processor temperature. However, the diode is embedded at the edge of the die, so it really measures Tjunction (or very close to it) rather than Tcase. The temperature drops by some amount from Tjunction to Tcase, sometimes referred to as Tjunction-to-Tcase delta. Intel doesn’t publish what the delta is, so I can’t tell you with certainty. It’s somewhere on the magnitude of 10-15C for most of their processors. Conservatively, I’d assume 10C of drop between the thermal diode and the processor case.
So, putting it all together for an 805 model:
Tcase max at 0W (idle processor) - 43.2C
Tcase max at 95W (Thermal Design Power) - 64.1C
Thermal Design Power is the theoretical absolute maximum power the CPU could ever consume. Even at 100% CPU usage, you will probably never quite reach that power level. Assuming a 10C delta to Tjunction, the maximum temperature readings in BIOS/temp measurement software should be:
Idle: 53.2C
100%: 74.1C
As I said, 10C is me being conservative. I can’t find much experimental data on Pentium D chips. You might get away with temps 5C higher, but I wouldn’t count on it.
Also, be aware that your motherboard may also have a temperature sensor mounted below the CPU package. Its reading may be shown in some temp monitors in addition to the built-in CPU temp diode. If you see two temp. readings for the CPU, the higher one is the built-in CPU diode. Always use that one.
A few other miscellaneous observations about temperatures:
Exceeding the maximum temps by a small amount probably will not cause any immediate trauma to the CPU; however, it might cause unstable behavior and shorten the life span of the processor over a longer period of time. Also, you might find it odd that the maximum Tcase at full TDP is 64.1C, but only 43.2C at idle. If the processor can stand 64.1C, then surely 50C won’t hurt it. This is true, but modern processors use a variety of tricks (like lowering the core voltage) to reduce power consumption at lower processor utilization. Some of these tricks are temperature-sensitive. If your processor is sits at 50C idle and 60C at 100%, you aren’t going to damage the processor, but the temperature at low power might cause instability due to the reduced voltage. This can lead to the odd situation where your processor may be more stable at full power than at low power.
