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Looking at the specification of a RAM memory module from Kingston, it says:

800MHz fCK for 1600Mb/sec/pin

What does "fCK" stand for?

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2  
I'm guessing its Front-side bus clock, as a reference to the speed of the system bus between the CPU and ram. This used to be called Front side bus speed. –  Frank Thomas Sep 4 at 19:12

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up vote 18 down vote accepted

It means clock (CK) frequency (f) - check out the table here for how the clock frequency generally translates into the memory bandwidth for DDR flavors. It's more commonly used in digital design texts and more formal engineering notes, though I have usually seen the CK as a subscript instead. I think it's just a case of some of the more formal notation used by the Kingston engineers bleeding into their documentation :)

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Per US Patent 6914851, it is indeed the clock speed:

The first column of the table represents the clock frequency fCK of the clock signal 12, where fCK=1/tCK

The f does not seem to come from "Front-side", at least as far as that patent is concerned.

More specifically, from the patent's language this may be specifically referring to the clock generator, which generates the clock signal (which governs the speed of the memory bus). IE, a fCK of 800MHz means the clock generator produces a signal 800 million times per second, which translates to 1600 million cycles per second per pin (as this is DDR). Increasing this number (by increasing the speed the clock generator operates at) is how you overclock memory (see the table in the patent for an example of this).

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That is the I/O bus clock speed. I'm not sure exactly what the f is short for, but CK is short for clock. This seems to be a Kingston-specific name for this parameter, as Google isn't turning it up for any other manufacturers.

This means that the data bus is operating at 800MHz, which allows each data module to transfer 1600MB/s. There are 8 data modules, allowing for a total transfer speed of 12800MB/s.

Note, the I/O bus clock speed is different from the memory clock speed, which is usually 1/4th the data speed for DDR3 memory.

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Mb not MB, from the text of the question. –  Joe Sep 4 at 19:01
    
@Joe Mb and MB are the same, perhaps you meant MiB ? –  Darth Android Sep 4 at 19:27
5  
Mb commonly denotes megabits, or 2^20 bits. MB denotes megabytes, or 2^20 bytes, or 2^23 bits. –  Joe Sep 4 at 19:28
    
(I suppose you can argue Mb denotes 1,000,000 bits, if you prefer, and Mib would be 2^20 bits; in memory language 1 Mb is commonly used for 2^20 bits, and in any event is irrelevant to this specific issue.) –  Joe Sep 4 at 19:39
    
@Joe Ah, I see the difference now. I was just using the units that the wiki page had. Memory module bandwidth specifications are in megabytes per second for DDR3 memory; The memory module is either wildly out of JEDEC specifications, or the technical writer of the spec sheet was careless. –  Darth Android Sep 5 at 14:37

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