How does Motherboard and CPU affect RAM choice?

Question

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 (The Qualified Vendor List is a good starting point, but it's a bit limited and sometimes a bit outdated).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 :)

Answer 1

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.

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.

Answer 2

Generally speaking within a given DDR standard there should be very few if any incomptabilities with memory, if your motherboard supports DDR3 up to 1600Mhz then it should do exactly that; after all thats the point of standards right?

As far as your case goes, Phenom II's generally prefer 1333Mhz RAM with tigher timings over higher speeds and looser timings, somewhere between 7-7-7-18 and 8-8-8-22 normally. But generally speaking so long as the RAM you buy is one of the standards supported by your motherboard (DDRx-xxxxMhz / PCx-xxxx) the difference will be mostly academic.

Personally I use a very similar board to that (the AM3+ varient) with at Phenom II x6 and I've been using Kingston HyperX-Genesis as my memory and have found the general case above to be fairly true (benchmarking scores are higher at 1333 8-8-8-22 than at 1600 9-9-9-24).

Update:

There is a very good explaination of this on Tom's Hardware, in the general case it finds that lower timings and higher clock speed are both important to memory performance; but make sure you read the entire article and don't just stop at the synthetic benchmarks.

Answer 3

1)
To answer your first question about how you can be sure that your motherboard supports a special RAM module, you have 3 choices:

• Trust the vendorlist
• check HW forums for threads about your motherboard and compatible RAM lists
• trial & error

2)
Concerning the second question, the answer is very trivial,too.
You won't notice the difference between different RAM speeds and timings. You can check this link, it's german but you will understand it. That's why I would recommend you buying some 1333Mhz CL 9 Kingston RAM, as Kingston is very reliable and has a big compatibility range, too.

Speaking about timings and Mhz in general, lower timings and higher frequencies are always better and faster, however covering your comment on some other answer about the real timings, you will always need a higher supply voltage to run your RAM at those speeds, which again results in more heat and higher powerconsumption.
If you want to have a deeper understanding of the internals, I would recommend you to read this wikipedia article, it is very datailed.

Answer 4

Motherboard has the largest effect on the choice of memory, CPU is much less important.

The simplest way to be absolutely sure that you buy the right RAM, is to use the tools available on the Crucial website.

Their programs will scan your hardware, report on the number of RAM slots used (and by what), and give you a number of alternative configurations. In addition, the quoted prices are always very competitive.

I have used their System Scanner tool several times for more than one computer, and it has never been wrong.

Question: How do I know what kind of memory to purchase?
Answer:

Our System Scanner tool automatically scans your system to tell you about your current memory configuration and recommend compatible upgrades for your specific system.

Or, use our award-winning Memory Advisor tool, where we've compiled all the information you need into one easy-to-use, searchable database.

[EDIT}

Some other RAM analyzers are :

Kingston Memory Search
Corsair Memory Configurator

Of course, you can use a memory configurator to find exactly what you need, and then make your purchase elsewhere.

Answer 5

I think you can buy the cheapest possible ram because the AMD HT can only support a bandwith of 3,2 GB/s. A DDR3 1333 module can go up to 10,6 GB/s.

1. http://en.wikipedia.org/wiki/HyperTransport
2. http://en.wikipedia.org/wiki/DDR3_SDRAM

Answer 6

If you not going to overclock then stick with Chinese produces ram like Kingston.

Why kingston? Because Kingston is produced in China, side by side with many makes of motherbaords, Asus, Asrock, Gigabyte, and most of all Foxcon with their overworked staff.

Kingston is just an example brand.. but like AData, Vdata and Samsung .. the ones that come in laptops direct from the factory.. because they were tried and tested with them.

Kingston with just the labelled speed 1333 or 1600 will be the most compatible timing for most motherboards. If the motherboard.

If you want to over clock then you go and look at Geil, OCZ memory, that will most likley not work on a budget PC, or i will cause it to crash and burn because of incorrect setup. This is where you have to input the correct timings, adjust the voltages and monitor the siutuation. Usually this over-clocking goes hand in hand with over clocked GFX cards too.

With all the units i have assembled in my life . the most problems i have had are the expensive ones that go into gaming boards..