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Which RAM Do I Purchase

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System Build - Memory selection
If you are considering building your own computer, you may find this short article on memory selection, of assistance when deciding which memory to purchase.

The main purpose of the article is to highlight the importance of considering not only the ram specified for the motherboard, but also considering the memory specified for the processor.

Once you have selected your motherboard you will see listed on the specifications, details of the ram supported by the board.

Here is an example from Asus for the P7H55/USB3

A quick glance shows the board as capable of supporting the following ram specifications

4 x DIMM, Max. 16 GB.
DDR3 2000/1866/2200(O.C.)/1600/1333 Non-ECC,Un-buffered Memory
Dual Channel memory architecture
Supports Intel® Extreme Memory Profile (XMP)
*Hyper DIMM support is subject to the physical characteristics of individual CPUs. Some hyper DIMMs only support one DIMM per channel.
Please refer to Memory QVL for details.

On the motherboard site, you will also find, usually on the headings for details, specifications, downloads and other items, one named - memory support - or similar wording, as shown in red above

This, usually down-loadable as a pdf document will list all the memory that has been tested with that board.

Obviously motherboard manufacturers cannot possibly test all makes and types of memory and simply because the make of memory you are considering is not listed on the QVL (Qualified Vendors List) does not mean that it will not function perfectly.

However it is generally advisable, unless you have some degree of experience to purchase one of the tested configurations.

Now to look in greater detail at this particular motherboard we see that it can support memory in DDR3 configuration.

DDR2 and DDR3 have 240 pins. However they are NOT interchangeable, as the position of the notch on the ram stick and on the motherboard is different. There are other reasons for their incompatibility, they run at different voltages and different timings

If we then look at the speed of the ram, that is in simple terms the speed at which data is written to it we see, in this example that the board is capable of supporting memory from 2200 MHz down across the speeds to 1333 Mhz. You will also note that speeds other than the 1600 and 1333 are shown with a (OC) entry.

That refers to overclocking and indicates that the voltage and timings of the ram must be manually set for those speeds to work at their full capabilities. That however, is only the start of the overclock configuration, as it is extremely unlikely that the ram will function perfectly without altering the voltage on the northbridge chipset and the CPU voltage.

If you are not experienced in these matters, my advise is to stay well away from overclocking. Generally speaking, the performance gain is relatively small and the risk of damage is relatively high.

Many people building their first system, ask "If overclocking is so risky, why is it mentioned and explained in my motherboard manual?"

The answer is, that it is the manufacturers response to consumer demand, so that motherboards, processors, video cards etc., can meet the increased demands from, for instance the requirements of gaming, without the consumer feeling that he must continually update his hardware with new components.

To return to the selection of ram for this board, we see that it can support up to 16Gb and has four ram slots, supporting dual channel. To use dual channel architecture, there must be at least ONE stick of ram in each channel.
That is one stick in each of the blue slots - the attached image clearly shows the two blue and the two black slots.

These two sticks of ram MUST be exactly matched. Do not try and use different makes of ram, albeit they appear to be the same configuration.

To fully populate both channels, two further sticks are added to the black slots.
If you examine further the details on the tested memory for the board, you will see which ram can be used in which configuration

Side(s): SS - Single-sided DS - Double-sided
DIMM support:

• A*: Supports one (1) module inserted into slot A1 or B1 as Single-channel memory configuration.

• B*: Supports two (2) modules inserted into the blue slots (A1 and B1) as one pair of Dual-channel memory configuration.

• C*: Supports four (4) modules inserted into both the blue and the black slots as two pairs of Dual-channel memory configuration

AND most importantly - these details

ASUS exclusively provides hyper DIMM support function.

• Based on the Intel® specification, you can only install one X.M.P. DIMM per channel.

• Hyper DIMM support is subject to the physical characteristics of individual CPUs.

• According to Intel spec definition, DDR3-1600 is supported for one DIMM per channel only.
ASUS exclusively provides two DDR3-1600 DIMM support for each memory channel.

• According to Intel CPU spec, CPUs with a core frequency of 2.66G support the maximum DIMM frequency of up to DDR3-1333.
To use DIMMs of a higher frequency
with a 2.66G CPU, enable the DRAM O.C profile in the BIOS

and it is in those details, that the information is provided, that is frequently overlooked by people building their own computer.

Some years ago the configuration of ram was mainly dependant on the capabilities of the northbridge chip, on the motherboard. Attached is a simple schematic diagram showing this relationship.
However whilst the northbridge chip is still involved in the control of ram the function has been taken over to some degree by the CPU (Central processing Unit) and it is, both in the case of Intel and AMD processors now the processor that is the MAIN deciding factor for the selection of your ram.

You must of course, still remain within the confines of what the motherboard will accept, on the actual ram limit, 16 Gb in this example, whilst at the same time, carefully examining the limitations of the CPU you choose.

Here is a link to provide you with a full explanation

not only on the aspect of the CPU control of ram, but all you would ever need to know about ram is on that site.

The example motherboard supports
Intel® Socket 1156 Core™ i7 Processor/Core™ i5 Processor/Core™ i3 Processor/ Pentium® Processors

If we now look at one of the Intel i7 processors supported on the example motherboard
Intel® Core™ i7-880 Processor
(8M Cache, 3.06 GHz)

we find this
Memory Specifications
Max Memory Size (dependent on memory type)16 GB
Memory Types DDR3-1066/1333
Number of Memory Channels 2

with the full details here


It is here on these specifications that caution is needed. These details show that the processor is programmed to accept 1066 or 1333MHz ram. This means, that ram operating at those speeds will work, without changing any settings in the BIOS.

Therefore it can now be seen that the ram specifications first examined on the motherboard site, do NOT, unless they are explored in detail show the whole picture. If 1600 Mhz ram was fitted to this motherboard, with this processor, it would either run at 1333 Mhz. or settings would have to be changed.

This, is the start of what is known as overclocking and the essence of this on the motherboard specifications
Supports Intel® Extreme Memory Profile (XMP) which is explained on this link

Even using the semi-automated XMP setting in the BIOS is not without risk
This is explained here on the Intel forums

It may require additional cooling and only if you have some degree of knowledge, should you consider it.

Excellent further advice can be readily obtained from the motherboard site, Intel or AMD, or in most cases the ram manufacturer. Corsair is just one example of dependable, good quality ram, with an excellent support service

On the average self build general computer, not the specialised gaming machine or workstation, there is little benefit in purchasing ram that requires changing settings for voltages, timings, etc. Indeed, any attempts to alter these settings, usually results in increased temperatures and the heat egnerated can do easily damage chips on the ram sticks, and in an extreme situation can drive a CPU beyond its Thermal Design Power. - whch in simple terms is:

Power consumption produces heat that needs to be removed from the system. Naturally, each system has a total cooling capacity per its specific design, and each major component has a cooling limit or power consumption allowance. The system designers usually choose the maximum frequency the system can run at without overheating when running in a "normal" usage model; this is also called the system's Thermal Design Power (TDP). Thus, when an unexpected workload is used the thermal control logic aims to allow the system to provide maximum performance under the thermal constraints

In my experience it is far better, when first building your own computer , to use ram that is in default settings approved for use on the motherboard AND the processor.

Should you be considering building your own system, advice is readily available here at Geeks to Go, you only have to ask.

18 May 2012

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Very informative, excellent article.
I would like to see this as a sticky.
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