Intel i7: CPU speed vs memory speed

[BLaZuRE]

I've been learning a lot of how technology has changed over the last couple years and how behind I've gotten. I was looking at the Core iX cpus and noticed i7's mostly support only DDR3 1066/1333. Can anyone point me to why these two speeds (i.e. no 1600, and how overclocking works in relation to this limitation)? FSB previously limited the maximum memory speed, what limits it now?

Thanks!

 

[MobiusPizza]

In Sandybridge, the memory controller resides on the CPU now, there is no such thing as FSB anymore. The time keeping is performed by a base clock at 100Mhz called BCLK. The headroom for changing that frequency is very low, about 5% or so before everything become unstable. In a sense, overclocking is all performed with CPU multiplier on Sandybridge platforms.

Within BIOS you can choose arbitarily what memory speed to run in standard multiples of BCLK (1066, 13333, 1666, 1866, 2133, etc), idependent of CPU multiplier. For stability most people would just leave BCLK at 100Mhz. They are completely decoupled now.

Although officially Intel states support for DDR3 1333, but in reality you can specify the maximum speed your RAM runs at. I think it's just a logistic issue and cost of testing and specification preventing Intel from bloasting support for all memory. Either this or the system is 'optimised' for 1333.

You can also see from benchmarks, memory speed advantages of DDR3 2133 versus DDR3-1333, you see real world performance gains of only 0-7% (not synthetic benchmarks). Not a whole lot, which may support the later argument.