GURU Needed! - DDR2 memory ?under-clocker? needs help

[Sunbeam65]

If any of you mad modders out there could help out this poor newbie, I will so grateful, otherwise I?m going to have to ?underclock? my CPU. Its an incredibly basic question, but this is my first build, and none of the five books I read actually answer it. I don?t actually want to overclock my first system (at least not right away), but I have a feeling that an experienced overclocker would probably know the answer to this question.

I want to run an Intel E6850 at 3.0 GHz on a 1333 MHz Front Side Bus without changing my Front Side Bus to Memory speed ratio from one to one 1:1. Is this possible?

If I understand correctly the Intel bus is ?quad pumped,? so a 1333 MHz CPU bus means that the actual ?physical? bus speed is 1333 divided by 4 = 333.25 MHz.

I?m wondering what DDR2 Memory I need to buy to run my E6850 at the full 3.0 GHz on the 1333 MHz bus.

Now the plot thickens because, guess what?I don?t understand the DDR2 memory speed ratings.

Can DDR2-6400 rated at 800 MHz run nominally (without overclocking) on a 400 MHz physical bus speed (memory DUAL pumped to 800 MHz) or only on a 200 MHz physical bus speed (memory QUAD pumped to 800 MHz)?

Basically, I don?t know whether to divide by 2 or 4!

I know that old DDR memory can make 2 reads or writes per clock cycle. So (that's easy) old DDR 400 MHz could run on a physical bus speed of 200 MHz. I read somewhere that DDR2 is twice as fast as DDR but I?m not sure about this. So I don?t really know if DDR2 makes 2 or 4 reads per FSB clock cycle. So this really means that I really know DDR2?s true speed capability.

None of my books actually say flat out what the physical bus speed is for DDR2-6400 800 MHz memory. The real problem is that this is SUCH a basic question that all the web sites, even the hand-holding ?how-to? tutorials, assume you already know it.

Every web site I?ve looked at including Corsair?s, if they want to run with an FSB of 1333 MHz they have to slow down their memory by changing the FSB to RAM ratio down from 1:1, which would seem to indicate that even 1066 or 1142 MHz memory can not actually run on a physical bus speed of 333 MHz quad pumped to 1333 MHz?.Oh NO!

Does this mean that I have to wait for 1333 MHz memory to arrive in order to run my E6850 at full speed with 1:1 memory ratio?

I do not particularly want to run my memory at a different speed ratio than my FSB. I'd like to keep it one to one, since I?ve read this is the most stable.

Help!

 

[Jschmuck2]

Welcome to the forums

Are you averse to overclocking your memory?

 

[Sunbeam65]

Hi. Thanks for replying.

No. Not adverse to overclocking RAM (a little) as long as I don't have to use water cooling, and as long as its fairly stable.

 

[MarcVenice]

I think you are just confusing yourself, maybe a few examples will help. For the e6850, and other e6x50 cpu's which have a FSB of 1333, which is in fact qaudpumped, you only need ddr2 667mhz to run your memory and cpu at a 1:1 ratio. Because the ram indeed has to be doubled, ddr = double data rate, so ddr2 667mhz will be able to keep up with an fsb of 1333 just fine.

For the 'older' core 2 duo's, the e6x00 cpu's, with an FSB of 1066 = 4x 266, ddr2 533mhz was actually sufficient to run the memory and the cpu at a 1:1 ratio. Does this all make sense to you now? Ddr2 800mhz is actually overkill for a stock e6850, unless you're going to overclock, then the higher clocked ram will allow you to up the fsb to 1600/4 = 400 and still run at a 1:1 ratio with the ram. So if you put the memory divider at 2, which stands for 1:1 in mobo's nowadays, then the memory will run at ddr2 667 speeds if you keep the FSB of your cpu at it's stock settings of 333/1333mhz.

Usually this is the case if you don't change anything in your mobo, you can always check by running CPU-z and look at the memory tab.

Capice ?

 

[Sunbeam65]

MarcVenice,

Thank you SO much! You understood my question exactly. Your answer makes sense.

My confusion came from the fact that the physical bus speed is multiplied by 4 to get the CPU bus speed, but only multiplied by 2 to get the DDR2 RAM speed. My confusion came from the fact that I was not sure if the "2" in DDR2 meant that the ram was doubling the bus speed AGAIN (or multiplying by 4). Which apparently is not the case, and I was obviously thinking about it a bit too hard.

Your examples clarify everything. An FSB of 1066 = 4 x 266 mhz will work with DDR2 rated at 533 mhz, which is 266 x 2, (not 133 x 4 ! ) at a one to one ram/fsb ratio.

My mistake was trying to treat DDR2 differently than the old DDR for basic calculations.

So it looks like all is fine:

1333 mhz fsb = 4 x 333 mhz. So DDR2 at 667 (333 x 2) will be OK. So obviously even Corsair's DDR2 800 MHz will be fine with some headroom to spare.

Thank you for clearing this up!
Sunbeam

 

[MarcVenice]

No problem, but if I ever need software help, I'm going to bug you about it

 

[Sunbeam65]

Rgr that. I'll try to return the favor sometime. My area is C# Windows UI design. Thanks for help with ram. You may have saved me a lot of waisted money trying to get 1333 DDR3 ram which would have been more twice too fast. (Maybe this is what Corsair wants!)

P.S. I just want to say that this AnandTech forum Rocks! This is the second time I've gotten an answer to a very specific technical question in less than an hour of posting it. Pretty sweet.

Peace.

 

[Jschmuck2]

Yeah. What MarcVenice said :-D

 

[hans007]

if you dont overclock ddr2-667 would be enough.

 

[myocardia]

Originally posted by: Sunbeam65
I know that old DDR memory can make 2 reads or writes per clock cycle. So (that's easy) old DDR 400 MHz could run on a physical bus speed of 200 MHz. I read somewhere that DDR2 is twice as fast as DDR but I?m not sure about this. So I don?t really know if DDR2 makes 2 or 4 reads per FSB clock cycle. So this really means that I really know DDR2?s true speed capability.

The problem with DDR2 is that while the bus speed is twice as fast, the RAM itself runs at the same speed. I think that's what makes it hard to understand, although that wikipedia page should make it fairly easy to understand, for anyone who seems to understand the process as well as you do.

 

[BitByBit]

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