How does Motherboard and CPU affect RAM choice

amd-phenomcpumemorymotherboard

I am seeking advice on which memory to use with my motherboard and CPU. Below are my components and some relevant information extracted from their official websites.

NOTE: Although this is a very specific case, a general answer is equally good for me — and also alot more useful for everyone else!

CPU: AMD Phenom II X4 810

  • ASUS M4A785TD-V EVO listed as one of many recommended motherboards.

Motherboard: ASUS M4A785TD-V EVO

  • 4 x DIMM, Max. 16 GB, DDR3 1800(O.C.)*/1600(O.C.)/1333/1066/800 ECC,Non-ECC,Un-buffered Memory
    Dual Channel memory architecture (Complete list of specs: here).

NOTE II: I wouldn't want to do any overclocking as I have no experience with that. So I think the 1333 MHz is the most appropriate. Since the higher frequencies are marked with "O.C."

Questions:

  1. How can I be certain that a specific RAM will work well (or not work) together with my CPU and motherboard? (The Qualified Vendor List is a good starting point, but it's a bit limited and sometimes a bit outdated).
  2. Is it true that choice of memory is not independent of the processor? How?

I'm worried I'll be missing something essential when buying my next RAM chips 🙂

Best Answer

I'd like to comment on the RAM frequency (1333/1600/etc) part. Generally, the best stick is the one that has the ideal combination of:

  • lowest timings
  • highest frequency
  • lowest voltage
  • lowest price
  • being compatible with your motherboard.

But the first 3 factors are not set in stone. For example, if for the same price, you can get:

  1. a stick of 1333mhz ram rated at 9-9-9-9 at 1.5V
  2. a stick of 1600mhz ram rated at 9-9-9-9 at 1.5V

Stick #2 is the better stick here. Because if you "slow it down" to 1333mhz, you may be able to run it at better timings such as 8-8-8-8, or at 9-9-9-9 with a lower voltage, 1.4V perhaps, or just run it at 1333mhz and call it a day. They're practically the same chips, just tested to perform at the stated minimum specs. In other words, don't give up a good sale because it's a 1600mhz stick!

Compatibility is not set in stone either! If a 1600/9-9-9-9 stick doesn't run at this speed on a motherboard, it may actually run fine at 1333/9-9-9-9. Just like the 1333 stick of the same brand would. Of course avoid any stick you know beforehand may not be compatible.

And that is why most RAM default to 1333mhz in the BIOS: for best compatibility. It's often up to the user to configure it optimally (higher frequency, lower timings, or lower voltage) as per the rated specs, if he so desires.

Example

You can use CPU-Z to figure out the rated specs at different frequencies. Below are the specs for my ram module, officially rated 1600mhz, CL-9-9-9-9-24, 1.6V. This JEDEC table is embedded of the RAM chip itself.

enter image description here

As you can see, the official specs match the column for 1600mhz (actually 800mhz, remember DDR stands for double data rate). If I were to run the ram at 1333mhz (666), I could safely set the BIOS to run the RAM at 1.5V instead - in fact I should since anymore is wasted heat. At around 1200mhz, I could safely lower the timings to 8-8-8-8-22.

Now you may ask what timings could this particular ram achieve at 1333mhz and 1.6V? Unfortunately, that falls in the realm of the unknown (or the overclocking). In this case, it would be much safer to buy a chip that guarantees 1333mhz, 8-8-8-8-24 at 1.5V or 1.6V.