Confused noob

[wgoldfarb]

I am a newbie, and have been doing a LOT of reading on this (great!) forum for a few months. I had been trying to learn as much as possible before trying to build my first system. The level of knowledge and informaiton in this forum is impressive!

After lots of reading I thought I understood the relationship between CPU speed, clock speed, FSB speed and memory speed for both the AMD and Intel platforms, as well as the basics of overclocking. However, some of the articles and tests I have read on Conroe have me a little confused.

Using a Conroe 6400 as an example, if I understand this correctly, the numbers are:

CPU Speed: 2.13 GHz
FSB: 1066 MHz
Multiplier: 8
"real" clock frequency is then: 2.13 Ghz / 8 = 266 MHz
Could also be computed as: 1066 MHz FSB / 4 = 266 Mhz

So, assuming I did not intend to overclock the system, I would need DDR 2 memory running at "real" clock frequencies of 266 MHz. This means 266 x 2 = DDR2 533. Anything slower would not work, and anything faster would be a waste (again, assuming no overclock).

However, most posts and reviews I have read about Conroe mention memory speeds faster than this, often as high as DDR800 and I have seen some test examples going even up to DDR1066. I don't think I have seen any cases where DDR533 is used even when people are talking about lower cost systems. Is overclocking the only reason everyone seems to be using much faster memory, or is there some other advantage of using memory that can run faster than the bus? Even assuming the reason for the faster memory is overclocking, DDR1066 means a clock speed of 533Mhz, a 100% increase over stock clock speed, which does not seem realistic. So, why is everyone spending money on memory that runs so much faster than the bus? Is it simply overclocking (and the apparently large headroom of Conroe), or am I doing something wrong?

Thanks for any help, and thanks again for all the great info!

 

[phile]

The answer = memory dividers. The divider is the ratio between the real FSB clock and the real MEM clock. For instance, you can run PC2-6400 at 800MHz with stock FSB (266) as long as the mobo supports the 2:3 FSB:MEM ratio - 266/400. (The 400 comes from the fact that DDR2-800 has a real clock of 400MHz, with an effective clock of 800). So, getting DDR2-1066 mem would allow you to run your mem without a divider - 1:1 - with a 100% o/c on the FSB (533/533). In other words, unless you're some crazy water-cooling enthusiast, anything faster than PC2-6400 with a conroe is, indeed, overkill.

-phil

 

[Pabster]

Yeah. But not all boards support FSB:RAM dividers.

The OP is correct that PC4200 might be all he needs.

 

[m21s]

Welcome! :beer:

 

[Cheex]

I have some 533 memory in my system (see sig) but its limits my o/c because of my mobo...
Would I have to get some faster memory so that I can o/c?

 

[wgoldfarb]

Phil, Pabster, thanks for the replies. It did not occur to me that this could result from a feature of the MoBo (I guess there is still much to learn!). I started to read more about the motherboards for Core Duo, and quickly came accross a related question. Here at Anandtech I found this:

http://www.anandtech.com/mb/showdoc.aspx?i=2797&p=2

They describe the P965 Express Chipset and show that the bandwith between the DDR2 and the Memory Controller on the chipset is 12.8 GB/s. Since this is dual channel each channel uses 6.4 GB/s = PC 6400 = DDR 800. So, am I correct in assuming that even though the system clock runs at "only" 266 MHz, the RAM can actually communicate with the CPU at a frequency of 400 MHz in any MoBo using this chipset? Is this an example of the divider that Phil was referring to?

If this is correct there would indeed be an advantage of using DDR2 800 in a P965 Express Mobo, even when not overclocking. Yes?

Finally, is that 400MHz somehow a direct result of the real clock speed of 266 MHz? In other words, all else being equal, would increasing the clock speed from the stock 266 MHz also result in the memory bandwith increasing proportionately? Or is the 400 MHz Frequency used by the memory to communicate to the CPU somehow "unlinked" from the real clock frequency? (so that even if I increased the clock speed upwards of 266, the memory Bandwith would remain at 12.8 GB/s).

I apologize for the many questions, this is a very steep learning curve!

 

[myocardia]

Most likely, Cheex, but the only way to find out whether your Dell RAM will overclock is to try it.

 

[myocardia]

Goldfarb, yes, running your RAM at faster than the fsb is advantageous, because it still isn't running as fast as the processor, but the closer that it runs to the processor's speed, the higher the performance. Well, in most apps, but not all. And yes, anytime you run your memory at a speed different from the fsb (266 Mhz on the C2D's), you're running a memory divider.

 

[phile]

Originally posted by: wgoldfarb
Phil, Pabster, thanks for the replies. It did not occur to me that this could result from a feature of the MoBo (I guess there is still much to learn!). I started to read more about the motherboards for Core Duo, and quickly came accross a related question. Here at Anandtech I found this:

http://www.anandtech.com/mb/showdoc.aspx?i=2797&p=2

They describe the P965 Express Chipset and show that the bandwith between the DDR2 and the Memory Controller on the chipset is 12.8 GB/s. Since this is dual channel each channel uses 6.4 GB/s = PC 6400 = DDR 800. So, am I correct in assuming that even though the system clock runs at "only" 266 MHz, the RAM can actually communicate with the CPU at a frequency of 400 MHz in any MoBo using this chipset? Is this an example of the divider that Phil was referring to?

If this is correct there would indeed be an advantage of using DDR2 800 in a P965 Express Mobo, even when not overclocking. Yes?

Finally, is that 400MHz somehow a direct result of the real clock speed of 266 MHz? In other words, all else being equal, would increasing the clock speed from the stock 266 MHz also result in the memory bandwith increasing proportionately? Or is the 400 MHz Frequency used by the memory to communicate to the CPU somehow "unlinked" from the real clock frequency? (so that even if I increased the clock speed upwards of 266, the memory Bandwith would remain at 12.8 GB/s).

I apologize for the many questions, this is a very steep learning curve!

<head spinning>

It's late, I'm tired, and far from being enough of an expert to provide a guru-level response. But, I'll say this much:

Using the example of a conroe setup at stock FSB - you will get better memory performance with the mem @ DDR2-800 (2:3 divider) than with the mem at DDR2-533 (1:1). (Of course, running DDR2-800 at 1:1 would be even better still, but that would require overclocking the FSB to 400.)

-phil

 

[wgoldfarb]

Phil

Thanks again, I think I understand how this works now. I guess it is worth it to get DDR 800. If I do not overclock I will still take full advantage of the available bandwith for the memory through the divider you explained. If I do overclock I can still use the same memory by lowering the divider as needed.

Thanks!

 

[Cheex]

Originally posted by: myocardia
Most likely, Cheex, but the only way to find out whether your Dell RAM will overclock is to try it.

It does overclock but here is what happens...
The BIOS automatically selects a 3:4 FSB:RAM ratio.
When i increase the FSB from 200 to 215, the processor (D915) goes from 2.8 to 3.0GHz (14x Multiplier).
The memory moves up from 266 (532MHz) to 287 (574MHz) (CPU-Z 1.36 verified this).
I was wondering if it was safe to run the memory that high.
I was told however, that as long as i don't increase the voltage on the memory it will be safe.
I don't know about this. Can you shed some light on it for me?

 

[phile]

Going from 532MHz to 574MHz is a modest o/c of the mem. If it's stable, you have nothing to worry about, especially if you haven't boosted the vdimm.

-phil

 

[Cheex]

On my ASRock 775Dual-VSTA motherboard, the BIOS only shows the VDIMM settings: Auto, Low, Normal, High.
What should I run it on?

Any other opinions on my previous question?

 

[Cheex]

Bump...:light:

 

[phile]

If your mem is at stock or only slightly overclocked, set it to AUTO and run a memtest86.

http://www.memtest.org

Burn the ISO to a CD and boot with it.

-phil

 

[Cheex]

I downloaded version 1.65.
I'm gonna burn it to a RW and do that test.
Before i do though...how long does it take?