I am just curious!
Todays computers are running on high clock frequencies.
If I have CPU that runs on (let say) 1 GHz what parts of hardware
are actually running on this speed? (except crystal
I understand that memory chips are much slower than CPU so every
time that CPU need data that is not in cache it must slow down.
I also read somewhere that CPU is internally dividing clock impulses
and run on slower speed that it is advertised.

So where is this high speed used?
Thanks
Misko


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On Mon, Jan 22, 2007 at 10:13:29PM +0100, Misko wrote:
> I am just curious!
> Todays computers are running on high clock frequencies.
> If I have CPU that runs on (let say) 1 GHz what parts of hardware
> are actually running on this speed? (except crystal
> I understand that memory chips are much slower than CPU so every
> time that CPU need data that is not in cache it must slow down.
> I also read somewhere that CPU is internally dividing clock impulses
> and run on slower speed that it is advertised.
>
> So where is this high speed used?

marketing material? ;-)

A

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On Mon, Jan 22, 2007 at 10:13:29PM +0100, Misko wrote:
> I am just curious!
> Todays computers are running on high clock frequencies.
> If I have CPU that runs on (let say) 1 GHz what parts of hardware
> are actually running on this speed? (except crystal
> I understand that memory chips are much slower than CPU so every
> time that CPU need data that is not in cache it must slow down.
> I also read somewhere that CPU is internally dividing clock impulses
> and run on slower speed that it is advertised.

> So where is this high speed used?

Actually, the crystal generally runs more slowly than the CPU clock.
Generally, a CPU has a phase-locked loop clock generator on the chip for
use by the core logic. A smaller multiple is used for the interface
logic.

In short, the high frequency is used by the arithmetic units, internal
cache, and instruction pipelines, all internal to the CPU chip. The CPU
does not normally "slow down" when a cache miss occurs, it idles until
the data comes from memory, but the clock doesn't vary.

The CPU can be told to go into a low power state in which the clock
frequency is reduced.
--
Rob

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> cache, and instruction pipelines, all internal to the CPU chip. The CPU
> does not normally "slow down" when a cache miss occurs, it idles until
> the data comes from memory, but the clock doesn't vary.

Indeed. Although it doesn't strictly idle right away: it first tries to
keep working on other things, but since memory can easily take about 100
cycles to answer, the CPU pretty quickly runs out of things to do and ends
up idling most of those 100 cycles.


Stefan


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