I don't know if there's any instruction that could make the CPU idle. And I also notice that there's a System Idle Process that use 99% of the CPU when the CPU usage is shown to be 0%. What is this System Idle Process? If it is using 99% of the CPU, why is the CPU temperature lower than other process taking 99% of CPU?

Allow me to put it a little further, how is the usage percentage of CPU is calculated?

Maybe this is a dumb question. Hope the geeks here won't feel tedious.... ;)


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    Blongs on superuser? Jul 28, 2010 at 14:08
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    Migrate to superuser. In the meantime, read codinghorror.com/blog/2007/05/… to see that if it takes 99% of your time, then 99% of your time is empty.
    – MvanGeest
    Jul 28, 2010 at 14:08
  • I am not sure...Please give this question a chance... ;) Jul 28, 2010 at 14:09
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    Just because the question doesn't belong on Stack Overflow doesn't mean it's not a valid question. Don't take the migration to Super User the wrong way. And don't read the double-negatives not the right way :P
    – Matt Ball
    Jul 28, 2010 at 14:10
  • What we mean with migrate is that it'll be on superuser.com, another site that is more suited for this question. Please don't do anything yourself, it will be migrated automatically after enough votes are collected.
    – MvanGeest
    Jul 28, 2010 at 14:12

3 Answers 3


The system idle process is a program that is an infinite loop of IDLE commands with the priority lower than the lowest possible priority.

Your CPU can never "stop" so to speak, however there is usually an IDLE command (or something similar) that can be run on a CPU.

The IDLE command is simply a command that is meant to use the smallest number of circuits possible on your CPU, thus keeping its temperature as low as possible.

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    The x86 instruction set "HALT" command tells the processor to stop doing anything until an interrupt is received. When there's no active processing going on, issuing that command may be appropriate. Jul 28, 2010 at 15:40
  • Idling it all the time will let it stay awake (important for notebook CPUs!) As BrianKnoblauch said, the HALT command will leave it off until a interrupt is received, which is the correct way on a notebook as far as I know.
    – sinni800
    Aug 23, 2011 at 12:00

What stargazer said, plus CPU usage is calculated as the percentage of capacity (calculations per second aka flops) that each process is using.

  • And this means that a processor can use like 0.1%, but it's rounded down to 0% //No point in adding this comment as an answer... Jul 6, 2011 at 14:59

It means that all the processes are stuck waiting... they are "blocked" on thing like user input, network card, hard-drive data, or even ram.

Basically it means that the threads are all waiting for the operating system to let them run again. it does this using something called an interrupt. Some interupts are timed, which is why your clock changes every second or minute, but your computer doesn't have to keep track of it 100% of the time.

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    why the downvote?
    – Ape-inago
    Dec 12, 2010 at 7:46

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