Measuring CPU speeds ( was RE: Why isn't Apple releasing patches for the Month of Apple )

arcarlini at arcarlini at
Wed Feb 14 01:08:24 CST 2007

Jim Leonard wrote:
> aliensrcooluk at wrote:
>> Also, what does the MHz, or GHz, measure
>> exactly?
> The number of machine cycles per second.

It's the fastest clock you can claim to have anywhere
in your machine while keeping a straight face :-)

Acually, to be fair, manufacturers weren't quite that 
bad (usually) but identifying the fastest clock is
actually quite hard, even if you are trying to be honest!
As CPU cycle times dropped, it was quite often the case
that the external clock might be running at say 20MHz
so that the rest of the slow external world could keep up
but that was doubled inside the chip to perhaps 40MHz.
In other cases the external clock might be quite high
but would be split internally to provide the phases
required to clock the various CPU stages.

Sometimes the manufacturer would publish a cycle time,
but as pipelined architectures came to the fore those
cycle times became less useful, so then they started
to publish instructions per tick or clocks per instruction.


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