Re: Why do CPUs run hotter...?


"kidkv" <ZACKSELE@xxxxxxx> wrote in message
news:1150699422.655256.244430@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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.
The key thing that has not been said so far in this thread, is
_parallelism_. Modern chips are not internally single threaded. Even units
with only a single processor core, have large sections designed to perform
work in parallel. The first 'obvious' one, is the maths co-processor
module, but in fact the degree of parallelism, is much larger. Most chips
will be executing 'parts' of multiple instructions, ahead of the current
location, all at the same time, and then deciding which one to use, or
combining multiple results, based upon latter instructions. When a chip
sits in a simple loop, or even better 'halts', these other parallel
components, can stop running completely. Historically 'halt', was
introduced in the early 1980's, but was first used in NT. Latter, Intel
throttled the speed of various parts, and stopped the MCP in particular,
with their 'SpeedStep' system. AMD, use a similar system with their
"Cool'n'Quiet" system, which slows the clock rate, and drops the votage on
parts of the chip that are not currently being used. A slightly modified
version called 'PowerNow', operates on the 'mobile' processors. These
throttling operations, help to reduce power when work is actually being
done, as against the 'halt', which only operates during the system idle
state.

Best Wishes


.

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