Understanding CPU limitations with memory
When looking at maximum memory speed supported for a specific motherboard or system, one thing that many people forget to take into account is the CPU. With modern CPUs the memory controller is built directly into the CPU itself, which means different types of CPUs may support different speeds of memory. So while a motherboard may support up to 2133 MT/s (Megatransfers per second) DDR3, most CPUs will not support that memory speed by default. Certain types of CPUs also support more advanced ECC memory like you would find in a server or workstation.
To illustrate some differences in CPUs, we will take a look at an older generation i7-2637M. If you scroll down to the memory specifications section for the CPU on the Intel® website, you’ll notice it supports only up to 8GB of total memory, and DDR3 speeds of 1066/1333 MT/s. If you were to install 1600 MT/s Crucial® memory with this CPU, you should expect the memory to downclock and run at 1333 MT/s because that’s the fastest speed the CPU will support.
As an example a CPU like the i7-6700K can handle up to 64GB of RAM, and has added DDR4 support up to 2133 MT/s, with DDR3L up to 1600 MT/s. Just because this CPU can support DDR4 speeds up to 2133 MT/s, does not mean it will support those speeds on DDR3, so pay close attention to what DDR type is specified.
Where do I find this information?
To find this information, look up your CPU model number. If you are unsure what model number your CPU is, this information will be on the invoice for its purchase, the box it came in, displayed in your system BIOS, or you can open up System Information in Windows®, which will show the CPU information as seen below.
A web search for your CPU model number and the term "specifications" will typically provide a document on your CPU manufacturer's website which includes the memory total and speeds supported by a given CPU model. In some cases, third party sites may also feature this information, but may not be as reliable for up-to-date details compared to the original hardware manufacturer.
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