clockcpucpu-speed
I was reading an article Clock rate from wikipedia which has to say that:
The clock rate typically refers to the frequency that a CPU is running at.
The clock rate of a CPU is normally determined by the frequency of an
oscillator crystal.
So, i'm wondering if I have 1 GHz clock rate (or processor speed?) what will be the frequency of oscillator crystal? It can't be just same as the clock rate?
Here are some of the things that can affect overall CPU speed:
The quoted CPU speed is the speed at which the internal clock runs. This is faster than the external clock (the FSB clock) which determines how fast the CPU can access the resources on the motherboard. A fast internal clock means the CPU can process faster, but a slower FSB means it takes longer to access memory and such.
This can be partially offset by the L1 cache (very fast memory inside the CPU itself) which can pre-fetch data from memory while the CPU is doing other things, so a larger L1 cache can yield a faster performance in certain circumstances.
The number of cores is important as well. A 1.6GHz dual core processor has approximately twice the processing speed (in optimal circumstances) of a single core 1.6GHz processor. This never realistically gets achieved in real life, but it is still much faster to have 2 cores than one. It's almost like having 2 processors.
The efficiency of the CPU itself has an effect too. Each instruction that the CPU performs takes a certain number of clock cycles to complete. A more efficient CPU will use less clock cycles to perform complex operations (floating point, for example) which makes the overall processing quicker.
The chipset on the motherboard can have an effect as well when it comes to the CPU accessing resources on the motherboard.
Pretty much everything in the computer has a bearing to a greater or lesser extent on the overall speed of the system.
CPU clock speed = cpu multiplier x mobo clock speed.
This one is a bit vague since there is not one clock speed on the motherboard. There are several. E.g. the PCI bus runs at 66MHz or 33MHz. PCIe runs at several speeds. Ram IO at several speeds. Most of the time these are derived or synchronised to one base clock.
When people used to talk about bus speed on the motherboard they usually meant a clock which was used to talk to the CPU's FrontSideBus.
E.g. A Klamath CPU (pentium 2) would run at 233Mhz.
What you call 'motherboard clock' would be 66MHz.
Back then increasing that clock yielded performance gains in all four items. If one of them could not keep up the system would not work.
Since then things have changed. The design is much more modular.
If the CPU clock speed is dependent upon the mobo clock speed, then how is the clock speed of the CPU predetermined upon buying the CPU (i.e. written on the box without knowing what mobo is being used)?
It is determine by testing how fast a CPU can work reliably. And then labeling it as such.
E.g. I produce a 1000 pentium-1's chips and and test them I may find that:
If I need more CPUs marked -120 and I got a surplus from the 133 bin then some get moved.
The key point however is that the CPU has no speed of its own. It is tested and guaranteed to work with a specific multiplier and a specific bus speed. It may work if these are exceeded. It may not. And if you set these values lower it will simply run slower.
Back in ye old days the person buying the CPU had to jumper the motherboard correctly.
E.g. for a pentium 1 120MHz you would jumper the motherboards main clock for 60MHz, set the jumpers for a x2 multiplier, and select the right voltage. On boot the CPU would read the multiplier and act accordingly.
If you made an error (e.g. by setting the jumper to indicate a 1½ multiplier) then it would simply run at a different speed. If it was slower then that was no problem. If it was faster then it might not work at all (but sometimes you got lucky).
Some people tried to maximise performance of their CPU by doing their own testing. Often you could gain some speed by playing with bus speed, multiplier and voltages. Equally often people pushed their CPU too far and it ran unstable.
This is not a problem. As an overclocker you know what you are doing.
However some shops also relabeled chips and sold their CPU as rater for a higher speed. I think this was the main reason why Intel started locking their multiplier maximum.
After installation, does the CPU adjust it's multiplier based upon the mobo clock speed to achieve advertised speeds?
No, most of the time the CPU is configured for one specific multiplier.
The speed it is sold as it expected base clock time that multiplier. (You can often lower the multiplier though. This results in a lower speed but also in lower power usage).
For example, if the CPU clocks speed is advertised at 2.4GHz and the mobo clock speed is 100MHz, will the multiplier be automatically set to 24x?
No. It will start at 24. Either because it is locked at 24x, or because it reads you BIOS and finds that that auto configured itself for a 24 multiplier.
If you try to set a higher multiplier then the CPU will mostly ignore that.
(Exception: unlocked CPU's which as sold as black editions.)
Why does mobo clock speed seem to not be very important / talked about? For example, when I search on Newegg, mobo clock speed never seems to be listed. When I search enthusiast forums and overclocking forums, mobo clock speed is rarely mentioned. To me, it seems like the mobo clock speed would be pretty important.
Speeds on the motherboards are important. They are also more or less the same for all same generation boards.
If I am understanding things correctly, a lower mobo clock speed means that you CPU must work harder to achieve advertised clock speeds.
No, it would not archive the same speed. It would just run slower.
Best Answer
Quartz crystals can vibrate at different frequencies.
Source
So, motherboard manufacturers will pick a crystal that vibrates at a suitable frequency and then pass that through some electronics to multiply it up to the desired clock speed for the CPU.
There's a section in the Wikipedia article that lists the commonly used crystal frequencies