PC hardware--CPU & Motherboard & RAM/BUSS and Memory


If I have a CPU that runs at 2GHz this is 2,000MHz I get this.

And if I can put Memory in my Motherboard that is Rated at
1066 MHz I know because it is DDR type the real speed is 533MHz I get this.

So my Memory of 533MHz is running slower then the CPU that runs at 2,000Mhz.

So when they say you can Overclock the System are you making the Memory Dabble to try to match the CPU speed?

I just wanted to see if I understand thank you for all the time.

Ah, I think you're missing one variable here - the CPU has an FSB (Front Side Bus) that measures how fast it communicates back to the system; let's use a Pentium 4 2.0GHZ as an example:

The CPU has a rated clock-speed of 2.0GHz, or 2000MHz. It has an FSB speed of 100MHz (which is effectively 400MHz, as the Pentium 4 uses a QDR (Quad-Data Rate) type FSB), and has a clock multiplier of 20 (20x100 = 2000). The memory's speed can be 1:1 with the FSB (in this case it would be DDR200 in dual-channel or DDR400 in single channel) or it can be set with a divider (For example if your system uses SDR memory or DDR233) and still work perfectly well.

If the system were overclocked, it would depend on HOW the overclock was applied (it isn't just a simple "raise the operating clock" proposition, at least it shouldn't be) - for example if the CPU supported it, you could lower its clock multiplier and increase the FSB (this would improve performance) - for example setting the multiplier to 10 and the FSB to 200MHz (the Pentium 4 in this scenario is not capable of this but for the sake example let's say it is). To be 1:1 in this case the memory would need to be dual DDR400. However if the system still only had dual DDR200, it would run with a 2:1 divider. Memory performance would not improve as a result, and could present a bottleneck to the CPU (that is, whatever applications are limited by the memory bandwidth would still be limited, even though the CPU is now going to perform better due to the higher FSB).

Alternately if the CPU allowed its multiplier to be raised, say to 40 (also impossible), the memory divider would not change but the CPU's clock-speed would now be much higher (4GHz in this case) - again the same memory bandwidth limits could result in a bottleneck.

In real-world situations such dramatic changes usually aren't possible - more likely you'd be able to raise the FSB to something around 110MHz, and likely bring the memory's clock up to that level too (maintaing 1:1), and you could not change the clock multiplier (very few CPUs come with the ability to change their clock multiplier beyond one or two settings). In cases where the memory cannot be clocked up to the FSB, you would see a divider.

Finally, in the case of very-new Intel CPUs with QPI, and AMD CPUs, there is not a traditional FSB (the CPUs communicate with the system differently) - they will instead have a "reference clock" that serves the same function for setting the CPU's operating clock.


PC hardware--CPU & Motherboard & RAM

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