Re: Why do CPUs run hotter...?

In article <1150667780.533129.170350@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
irwin@xxxxxx says...

mc wrote:
Why does a Pentium CPU run hotter when "working hard" (executing complex
software) than when the OS is idling?

It is always executing *some* instruction, and in that sense, is never idle.

I understand that CMOS gates dissipate power only when changing state. I
*presume* what's going on is that when the CPU is executing software, it is
executing a greater variety of machine instructions and therefore heating up
a larger proportion of the circuitry in the chip (since there is special
circuitry for each kind of instruction).

Am I on the right track? This is one of those dumb questions where I'd like
to know the exact answer, rather than just guessing.

1) Yes you are on the right track as idling will only involve
regurgitaing a few instructions in the cache and will require less use
of CPU cct's.

Nonsense. If nothing is happening reasonably current processors do
.... *NOTHING*. The clock tree accounts for something on the order
of 40% of the power and leakage as much of the rest. If the
processor is doing nothing, shut off the clocks and lower the
voltage.

2) The OS can throttle back the CPU when it is lightly loaded (mostly
used on portables)

It's called *HALT*, and has been done for at least a decade.

--
Keith
.

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