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Could I predict what frequency of RAM should be knowing frequency of CPU ?

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Today not really. In modern motherboard designs the RAM and the CPU are not that tightly coupled as far as the clocks are concerned.

In the past in many cases the CPU base-clock and the RAM base-clock had to be the same or have a fixed ratio.
E.g Socket 370 Pentium designs had a 100MHz base clock and the corresponding 370 Celerons used a 66 MHZ clock (both directed from a 33MHz system clock that also drove the PCI bus and the RAM). The matching RAM could either be 66 or 100. As you see from that they all used a 2x or 3x clock-multiplier.

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  • Actually, a lot of MB's still share a 100 MHz base clock between the RAM and CPU (and usually also the PCH/FCH/whatever-they-call-the-chipset), especially ones targeted specifically at DIY usage where over or underclocking is very prevalent. Jun 15, 2018 at 18:25
  • @AustinHemmelgarn True, as Intel chipset design is in essence a complex set of synchronized busses all based on a 8.33 MHz clock, but those relations are far less obvious on newer systems, with all sorts of fancy multipliers being involved. Often even 2 or more in sequence (eg. ddr4 ram has an internal 4 cycles per clock on top of the actual multiplier on the ram bus). Anyway: the marketing speak GHz numbers are often only tenuously connected to the real clocks and multipliers. I can work them out if I have to, but I rarely see any need to bother.
    – Tonny
    Jun 15, 2018 at 22:44

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