Understanding CPU limitations with memory

Crucial Employee


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 day 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, let us take a look at an older generation i7-2637M. If you scroll down to the memory specifications section for the CPU on Intel’s website, you’ll notice it only supports up to 8GB of total memory, and DDR3 speeds of 1066/1333 MT/s. So if I install some 1600 MT/s Crucial memory with this CPU, I should expect the memory to downclock and run at 1333 MT/s since that’s the fastest speed my CPU will support.




Now let us look at a more modern CPU like the i7-6660K. This CPU 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 as you can see, so make sure to pay close attention to what DDR type is specified.




Where do I find this information?


You first need to figure out the CPU model number you have. 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.




For an Intel CPU: simply type in the CPU model number into a web search, typically the first search result that pops up will be the data sheet on the Intel Website. Otherwise, you can search the model number on the Intel page directly. Scroll down to the Memory Specifications on the Intel data page and you will find the relevant information. For an AMD CPU: AMD provides a guidelines article that breaks down the different CPU types and supported memory speeds. As an alternative, third party CPU websites like CPUboss or CPU-world will provide this information for specific AMD CPUs.

Can I get the advertised XMP speed out of my Ballistix memory even if my CPU doesn’t support that speed?

If your CPU is not able to natively run Ballistix parts at their XMP profiles, then you can possibly achieve these speeds by overclocking your CPU bus. By overclocking the CPU to run at a faster speed, you naturally increase memory speed that the memory controller in the CPU can support. If you need any assistance with overclocking your CPU, you will need to refer to online resources, and be sure to refer to motherboard documentation for navigation of the advance BIOS settings. Overclocking is performed at the user’s own risk, so be sure to research the subject until you are confident enough to attempt it, or decide it is just not for you.