CPU vs mobo Memory Controller

[Apk07]

I'm helping a friend build a computer and hes looking to buy the Core i3-560 http://www.newegg.com/Product/Product.aspx?Item=N82E16819115068

It says it's integrated memory controller supports up to 1333 DDR3, but his Mobo says it can take up to 1600. If he gets this CPU/Mobo and 1600 RAM, will it underclock the RAM to 1333? If he got 1600mhz RAM, would the mobo's memory controller negate the CPU's and allow the 1600 instead? What are the ups and downs?

 

[jaquith]

The i3's are better served with 1333 MHz RAM; I know the i3-560 runs 1600 MHz RAM. The ups 2~3% faster CAS to CAS {same IC}, the downs more difficult on the i3's to obtain 1600 MHz. Good quality, low & tight CAS {7-7-7-24}, 1333 MHz RAM can outperform 1600 MHz RAM {9-9-9-24}.

The CPU IMC does not fully determine supported RAM Frequency; e.g the i7 9XX has a 1066 MHz IMC but can run up to ~ 2200 MHz RAM.

Unless the 'plan' is to OC the i3-560 then my best suggestion is 1333 MHz. Also, use either the Certified {QVL} listed RAM per MOBO Mfg or use the RAM Mfg's Tested Configuration tools to determine the best RAM for your MOBO.

 

[4745454b]

There is no motherboard memory controller for those chips, its in the CPU. The CPU might officially support DDR3-1333, but it can go faster with overclocking.

 

[Apk07]

So if I bought 1600 anyways, would it work -at all-? Or would it only work if the CPU was overclocked

 

[4745454b]

It should work. It should downclock to the fastest speeds that the IMC supports. If the chip you put in there only officially support DDR3-1333 then that's what it should clock itself as.

 

[jaquith]

There is no motherboard memory controller for those chips, its in the CPU. The CPU might officially support DDR3-1333, but it can go faster with overclocking.

Maybe I worded it poorly by "CPU IMC does not fully determine supported RAM Frequency" -- I mean the RAM Frequency is not limited by the spec limits of the CPU IMC. CPU IMC - means the CPU Integrated Memory Controller; I know VERY clearly where the memory controller is located.

The i3's perform better and more reliably with 1333 MHz RAM as do 'some' of the lower i5's.

 

[4745454b]

I was trying to tell the OP there is only one memory controller. Current systems its found in the CPU, there is none in the board that can "override" it.

Actually, the CPU IMC DOES fully determine supported RAM frequency. If the memory controller doesn't support it, its not going to work.

 

[jaquith]

As I stated above, 'ultimately' yes there's a limit, but that limit varies on several factors. Most importantly the CPU itself, for example the SB has a very hard wall above 1333 MHz RAM frequencies, whereas the i7 9xx supports and benefits from RAM with frequency close to almost double its 'rated' 1066 MHz IMC. Further, BCLK increases provide increased RAM Frequencies to IMC 'limits' - the newer SB CPU do NOT support BCLK increases so ultra fast RAM > 1333 MHz will add very little benefits. Other CPUs like AMD Phenom II also have 1066MHz/1333MHz IMC; the 6-core can support and benefit from 2000 MHz+ RAM whereas the 4-core has limits between 1333~1600~1800 MHz depending on the CPU model.

So "CPU IMC DOES fully determine supported RAM frequency" is not an accurate statement; other than to some 'ultimate' number above the rated IMC.

 

[4745454b]

Maybe its because I just woke up, but I didn't follow that.

Most importantly the CPU itself, for example the SB has a very hard wall above 1333 MHz RAM frequencies, whereas the i7 9xx supports and benefits from RAM with frequency close to almost double its 'rated' 1066 MHz IMC.

I'm assuming SB stands for Sandy Bridge here and not south bridge. I fail to see the point you are trying to make. You are mentioning different CPUs, and their IMC. If there is anything else that might influence the supported memory, why are you mentioned the memory controller?

Further, BCLK increases provide increased RAM Frequencies to IMC 'limits' - the newer SB CPU do NOT support BCLK increases so ultra fast RAM > 1333 MHz will add very little benefits.

Ok, and this is a limitation of??? The CPU and its memory controller? I'd actually argue that the memory controller can go faster, but because all the buses are tied together its everything else that can't keep up. But its still the CPU that decided what it can use. Furthermore keep in mind that SB can change the memory ratio to OC the ram without changing the BCLK. Won't provide the same benefits as increasing the BCLK but its possible.

The memory controller does determine what your ram support is. Always has, always will. (until they invent something really weird/new.)

 

[jaquith]

Actually, the CPU IMC DOES fully determine supported RAM frequency. If the memory controller doesn't support it, its not going to work.

You asked why I'm [IMO wasting my time] arguing? I guess I have (2) two choices: 1. Ignore what you're saying, 2. Defend myself. Apparently, I choose the later.

The BCLK increase, decreases the disparity between the CPU IMC and high RAM Frequencies from the bus.

Further, I was pointing out not everything is the same between CPUs {vanilla/one size fits all} and therefore the answers are not the same per CPU nor are they the same for different GHz speeds of CPU. Take a look sometime in, say an ASUS, manual there you'll note how the IMC is handled in the BIOS varies on CPU speed; the best example is 2.66GHz CPUs.

SB {Sandy Bridge} is an oddity, good article to read/see the results {good starting point} -> http://www.bit-tech.net/hardware/memory/2011/01/11/the-best-memory-for-sandy-bridge/4 However, in other current CPUs this is not the case of the 'wall' as I described.

Clearly, if we all went on your logic and statements 1600, 1800, 2000 MHz all would FAIL in the i7 9XX slower 1066 MHz IMC, or 1600 MHz on the OP's 1066/1333 MHz IMC, etc. In the OP's case of a 3.33GHz it can handle the extra frequency but the i3's as I've stated see little gain.

 

[4745454b]

Whoa, slow down here. I'm not trying to argue with you nor did I mention attacking you or cause you to defend yourself. (your words, I don't see where I said anything of the kind.) I even in the post previous to this said possibly because I was still waking up failed to see the point you are trying to make. I think you have one, but I can't see it.

My main "objection" was that you said there are other things that determine memory support other then the memory controller (MC). I fail to see how thats possible and I'm asking for help in understanding your thinking. Just because I don't understand it doesn't mean its not possible/true.

Take a look sometime in, say an ASUS, manual there you'll note how the IMC is handled in the BIOS varies on CPU speed; the best example is 2.66GHz CPUs.

Could you point out two? Asus has a lot of manuals and I'm not sure what I'm looking for.

Clearly, if we all went on your logic and statements 1600, 1800, 2000 MHz all would FAIL in the i7 9XX slower 1066 MHz IMC, or 1600 MHz on the OP's 1066/1333 MHz IMC, etc.

Where did I write such a thing? As a matter of fact I could swear I wrote this earlier in the thread.

It should work. It should downclock to the fastest speeds that the IMC supports. If the chip you put in there only officially support DDR3-1333 then that's what it should clock itself as.

I never said faster ram would FAIL in a machine that only supported 1066 or 1333, but it will be default follow the SPD and downclock to whatever the MC supports. You can go faster with OCing, but its not a guarantee.

 

[jaquith]

Let me turn it over to you, you prove to me everything what you're saying.

edit: I had this image already

 

[4745454b]

My point is easy to prove, your little graph even mentions its. Whats the first line/sentence after the chart? "when using DIMMs with a frequency higher then the Intel CPU spec," Where is the MC again?

Its the MC that determines what RAM can be run. Do you remember the first S939 CPUs? DDR400 was possible, but only if you used half the banks. Once you used up all the ram slots, the MC found in those early CPUs downclocked all the ram to DDR333. This was fixed with the next CPUs as AMD beefed up the MC. Same is true for this gen. Ram manufacturers might be able to make DDR3-3500, but if the MC on the CPU can't run that fast, it isn't going to happen. There isn't anything else that determines what speed ram you can run, other then the MC as far as I know. I turn the floor back over to you if you can show me otherwise.

 

[jaquith]

Actually, the above chart proves everything that I said so far in the post. As I recall, this is from a P55 MOBO manual. Further, I have all along clearly stated RAM Frequency is CPU {IMC} dependent; different CPUs IMC will provide different frequencies that are obtainable. My experience here is to know what will or won't work, and decimate that information correctly.

If you want to run your, say, i7 9XX at stock with 1066 MHz RAM, and run your GPU, etc at stock -- go for it. Meanwhile, my i7 9XX @ 4.6GHz with 2000 MHz RAM, and OC GTX 4XX's will be running circles around it just fine.

 

[4745454b]

With that last post I *think* I finally understand what point you are trying to make.

I mean the RAM Frequency is not limited by the spec limits of the CPU IMC./quote]

This is what I was trying to wrap my head around as the ram frequency is very much "controlled" if you will by the IMC. If the MC only goes to 1333 officially but can usually go as high as 1600, you'll never get your ram up to 2000MHz. I'm sure you'd agree with that. Your last post referenced back to the chart finally brought your point home. (again, I think.)

different CPUs IMC will provide different frequencies that are obtainable. My experience here is to know what will or won't work, and decimate that information correctly.

This is also correct. Different CPUs might use different IMCs, and will have different obtainable frequencies. I would not argue against that at all. I'm still a bit clueless why you objected to my saying that it is the MC that limits what ram you can run but perhaps I'll never know.

 

[jaquith]

My argument was because you implied that it's all the same, and implied that if you breach the IMC spec limit that it would fail.

Trust me it's a PITA, for me, to have to sift through what CPU supports what RAM frequency and/or if there's a benefit to faster frequency RAM.

Initially, I in 'the opening day of the P67/H67 'SB' and in 1 or 2 posts assumed the SB would have an advantage to 1866 MHz RAM -- I assumed incorrectly. The second I found that the SB could not benefit and utilize RAM with higher yet obtainable frequencies -- my advice immediately changed to either 1333 ultra-low CAS or 1600 MHz ultra-low CAS. My H-O-P-E is the SB 6 & 8 core for the LGA 2011 will be different, but my 'guess' is there will be the same wall.