cool, great link and a recommended read for all...but, for those who are lazy, here is the section of the article that BHC's great post is referring to:
Pentium 4 Power Management – A Performance Limiter
Last year, as details of the Pentium 4 were emerging, Intel's elaborate thermal and power regulation requirements for P4 systems raised many eyebrows. Platforms required new power supplies capable of pumping out more current, and enormous copper heat sinks for CPU cooling. Humorous jabs ensued… "Turns your PC into a toaster oven," some said.
But just prior to the P4 launch, Intel sought to confront the issue by publicizing power specifications for the soon to be released CPU. To many people's amazement, the 1.5GHz Pentium 4 was said to consume only 54.7 Watts - a maximum power dissipation rate well below the fastest Athlon.
Numerous independent product reviews have quoted this value (and still do) particularly in contrast to AMD's latest 73 Watt, 1.33 GHz Athlon. The only problem is that the quoted Pentium 4 power dissipation figure is wrong - or at least the number is misleading.
These two specifications are defined under entirely different conditions. AMD reports the true absolute maximum power dissipation rates without constraint. Intel on the other hand, publishes a compromised figure that is open to lots of interpretation. Page 70 of Intel’s P4 datasheet shows the dissimilarity.
Since most applications are ‘unlikely’ to cause the processor to consume its absolute maximum power, Intel quotes a figure that looks more like a ‘not-to-be-exceeded’ figure. In theory this power level could easily be exceeded under stress tests, benchmarks and particularly challenging application loads. If this were to happen, a thermal diode (required on all P4 platforms) would trigger a power management mechanism that instantly cuts CPU performance, and allows it to cool down.
The P4 spec reads, “The Thermal Monitor feature … is intended to protect the processor from overheating when running high power code that exceeds the recommendations in this table,” (54.7 Watts for the 1.5GHz model).
Looking forward, if you buy a 1.7GHz P4, it will run at that speed when it is idle, or under light loads, when CPU utilization is nominal, or in applications that don’t really need a 1.7GHz CPU. But when you drive it to extremes, or wish to extract all available performance from the processor, you may find yourself spontaneously and unavoidably power managed to a lower effective clock speed. Intel’s motto… “1.7GHz. Its there. Unless you need it.”
Update: Intel’s Thermal Design Guide has revealed that the absolute maximum power dissipation of the 1.5GHz P4 is actually 72.9 watts. This is 33% higher than the published system design specification, and essentially identical to the 1.33 GHz Athlon. In order to prevent the CPU from exceeding 54.7 watt, thermal throttling is used. If performance critical applications drive CPU power above its artificially low 54.7 watt limit, the CPU is halted with a 50% duty cycle (alternating 2 microseconds on; 2 microseconds off) until it cools down. This effectively turns your 1.5GHz processor into a 750MHz processor – just at the moment you demand peak performance. On the other hand, you will probably still be able to check your email at 1.5GHz. This scheme is described on page 23 of Intel’s P4 Thermal Design Guide. Commentary is already floating around the web that perhaps Intel feels guilty about selling 750MHz CPUs in 1.5GHz clothing, and thus has decided to cut the price by 50% as well.