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Even though newer processors come with scaling (SpeedStep, Cool'n'Quiet), some computers have motherboards with outdated bios which do not support the feature. Thus, they run at full clock speed. Even if they do have it enabled, the processors still have minimum clock multiplier such that they can't run at much lower voltage. For CPU with scaling disabled, both 100% CPU usage and idle CPU should have the CPU running full speed. Scaling-enabled CPU, if the minimum speed is 40% of CPU capacity, the multiplier should be the same whether it's idle or it's using short of 40%. My question: does the power consumption differ between idle and non-idle in both cases?

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Yes.

Every operation a processor performs costs an amount of energy.

Different instructions use different amounts of energy depending on such things as how many clock cycles they take, what parts of the processor they use, etc.

Most processors have a HALT or equivalent instruction which sends the processor into a sort of sleep mode where it waits for an interrupt before continuing processing.

That is why, when you're doing intensive calculations on a computer, whether it has speedstep or not, the processor gets hotter than when it's just idling.

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So does this mean idle CPU is not actually wasting anything, and using distributed computing client is not really environmentally friendly? –  syockit Mar 8 '11 at 8:07
    
I had a brief look at en.wikipedia.org/wiki/CPU_power_dissipation and noticed the "idle power contributes significantly to overall system power usage", which is my suspicion. But the following sentence "CPU uses power management features" seems to agree with yours, though I wonder if HLT instruction is considered one. –  syockit Mar 10 '11 at 0:14
    
A CPU in halted state will still consume power at the base-rate of the processor (the processor is still functioning, but not doing much). A SpeedStep processor will reduce the clock speed thus reducing the base-rate of power consumption making additional savaings over the traditional methods. –  Majenko Mar 10 '11 at 9:17

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