What happens when putting very fast RAM in slower Mobo with slower CPU?

Question

This is a dumb question and I'm a bit ashamed of having to ask but it's the first time I stumbled across it.

Let's say I have an i7 5960X with support for DDR4 1333/1600/2133, an ASUS X99E-WS which supports up to DDR4 3200MHz and put DDR4 3600MHz RAM into it. To me the CPU then is the bottleneck. Does the RAM clock down to 2133MHz? Is there a way to have the RAM run faster (BCLK OC?)?

What happens if I'd put it into a Mainboard that supports 1333MHz at max? I know this is non-existent but just for illustration.

Is the entire system running at the speed of its slowest component?

Answer 1

The RAM will clock down to the highest speed the CPU supports. However, you will still get some of the benefit of the faster RAM as the number of clock cycles needed will drop. For example, a RAM that has a CAS latency of 11 at its higher speed might have a CAS latency of 9 at a lower clock speed.

It's not really fair to say the entire system will run at the speed of its slowest components. But the RAM subsystem cannot be assured to reliably run faster than the lower of the maximum speed the memory controller supports and the maximum speed the slowest RAM stick supports. The memory controller is on the CPU for i7 systems.