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If I understand it correctly, FSB and processor frequency should have a 1:1 ratio, how can I calculate this ratio with DDR RAMs? I need a simple example to make my mind

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FSB is generally much slower than the processor speed. An example FSB may run at 133 MHz, with a (slow) processor running with a multiplier of 9 to give it a speed of 1.2 GHz. Some hardware can accept different clock settings, so if the front side bus is stable to run at 166MHz, the CPU can either be run at 166x9 = 1.5GHz (giving a combined boost to memory access and processor speed) or if the CPU is unstable at this speed, the multiplier can be lowered to 7.5, giving a CPU speed of 1.25GHz but allowing the increased FSB for faster memory access.

Memory should generally operate at least as fast as the FSB. Above this speed there is little advantage to the memory running faster, if the bus cannot provide it with data at the same rate. However, the effective speed of memory modules is not the same at the clock rate it actually runs at.

The effective speed of DDR depends what kind of DDR you have. DDR stands for double data rate, meaning those modules run at twice the FSB frequency, such as 2x200MHz = 400 MHz. DDR2 effectively operates at four times the FSB, and DDR3 at 8 times.

You can therefore decide which memory is required for your bus speed with this calculation

FSB x 2^your DDR version = memory speed

So in the example, FSB =166, DDR version is DDR2:

166 x (2^2 = 4) = 166x4 = 667MHz effective speed.

Another example: FSB = 133, DDR version is DDR3:

133 x (2^3 = 8) = 133x8 = 1066MHz effective speed.

It is also worth noting that (most) DDR modules are backwards compatible within the same family. If, (for example) a 1.5 GHz DDR3 module is too fast for your needs, it will work in any slower DDR3 system, but not in a DDR2 system, nor in any system requiring memory faster than 1.5GHz.

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The front side bus hasn't been one to one with the CPU in a long time. Nowadays the CPU clock runs as a multiple of the FSB. Memory usually runs at a 1:1 ratio but even this is configurable and changeable on some motherboards.

This wikipedia article is a good stating point. There are lots of tech sites with overclocking guides that also go into the relationship between the three clocks.

CPU

The frequency at which a processor (CPU) operates is determined by applying a clock multiplier to the front-side bus (FSB) speed in some cases. For example, a processor running at 3200 MHz might be using a 400 MHz FSB. This means there is an internal clock multiplier setting (also called bus/core ratio) of 8. That is, the CPU is set to run at 8 times the frequency of the front-side bus: 400 MHz × 8 = 3200 MHz. By varying either the FSB or the multiplier, different CPU speeds can be achieved.

Memory

Setting an FSB speed is related directly to the speed grade of memory a system must use. The memory bus connects the northbridge and RAM, just as the front-side bus connects the CPU and northbridge. Often, these two buses must operate at the same frequency. Increasing the front-side bus to 450 MHz in most cases also means running the memory at 450 MHz.

In newer systems, it is possible to see memory ratios of "4:5" and the like. The memory will run 5/4 times as fast as the FSB in this situation, meaning a 400 MHz bus can run with the memory at 500 MHz. This is often referred to as an 'asynchronous' system. It is important to realize that due to differences in CPU and system architecture, overall system performance can vary in unexpected ways with different FSB-to-memory ratios.

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out of interest, I know(jf I recall) that DDR RAM Has its own clock.. but are there any systems in the last 10 years where RAM derives its speed from the FSB? –  barlop Dec 3 '13 at 12:54
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If you want to overclock your FSB for example:

You have DDR 3 Memory which has a optimal (Standard) settings of FSB of 1600 MHz, You find these details on the DDR 3 Memory itself or in the description.

So if you don't wanna overclock your CPU you want it to run at the original settings like 3 GHz then you need to go and take your FSB speed and calculate it.

Values first: Quadcore = 4 cores, Dualcore = 2cores. For example. (We take the quadcore which means 4) Then take your FSB in the Bios, it would be 400. Because 400 * 4 = 1600MHz ok that's done. Now don't forget the CPU ratio, lets say original settings is 9. Ok then find by calculating like this, ratio * the FSB = CPU clock speed In this example you'll need the ratio of 7,5.

So at the end your clock settings would be like this:

FSB * Amount CPU core = FSB clock speed
400 * 4 = 1600 MHz
FSB clock speed * CPU ratio = CPU clock speed
1600 MHz * 7,5 = 3 GHz (3000 MHz)

Thats it and if you want you can set the voltage of your memory which are in the description too of the memorys.

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