FSB & RAM Speed - what it all means

Hi everyone.

Well, I know that people have inquired about these issues before, but I have not found a clear explanation that gives me a satisfactory understanding.

Essentially, the issue is about CPU front side bus (FSB) and RAM speed. Oh, and Santa Rosa.

{Edit: After writing all of this, I realized that it was a mess, so I added numbers to the issues to aid navigation. I hope.}

1. Now, I have come to know (one source being Wikipedia) that Santa Rosa does not support 800MHz RAM. I guess that this means that getting 800MHz RAM is pointless, since it will be throttled back to 667MHz anyway. I think.

However, it does support 800MHz FSB on the CPU.

2. Before I go on, am I correct in presuming that the speed of data transfer between the CPU and RAM will be the lesser of the FSB speed and RAM speed? If so, is there any point to getting a CPU with 800MHz FSB (all other things (eg clock speed & L2 cache) being constant)?

I anticipate that the answer to that question is "yes", because of that thing called "dual-channeling", which - I think - basically doubles the RAM speed. So if I have two 667MHz sticks of RAM, they will be accessible at 1334MHz. But since the FSB can only run at 800MHz, the RAM can only be accessed at 800MHz. So the end result is an 800MHz connection.

3. Also, if I am so far correct, does dual-chanelling require two identical sticks of RAM? What if they are the same brand, but different sizes (eg a 1GB and a 512MB)? What if they are different brands but the same size? What if they are the same brand, purchased at different times? Is brand totally irrelevent?

4. And the most important question of all: Does any of this really matter? Is the difference in performance between a CPU-RAM connection of 667MHz and a connection of 800MHz really quite negligible in every possible situation of usage? Or are there some situations where the difference is significant?

5. And so, at the end of all of that, can someone tell me: have I made any bad assumptions? Indeed, am I totally misconceiving the entire issue?

6. If you can explain it all to me, or point me to previous explanations, I would be very grateful. Alternatively, let me know if, in fact, no-one really knows about any of this and it's all one big question mark.

 

I'm just going to give you REP because I can answer a lot of your questions but not all and many others can do much better! And your 3rd post! I will follow and add as needed but those are very good questions, I am going to my computer book!

 

It may not be pointless. Will have to wait and see the final specs of all the chipsets, or if any BIOSes allow adjustment.

800mhz is the system bus (cpu to northbridge). 667mhz is memory to northbridge.

Dual channel allows for access to multiple ram banks at once, it is not "doubling" the ram speed.

What is important is the number of ranks each memory module has, but you can actually mix and match to your hearts content as long as its stable.

Depends how its handled. With Merom processors Intel had both 533 and 667mhz ram buses. 533mhz memory usually ran cas 4, trcd 4, trp 4, and a tras value near to 12. 667mhz memory ran cas 5, trcd 5, trp 5, and tras near 12. trcd and trp affect performance more than cas or tras, but the end effect is that the performance difference between 533mhz and 667mhz was negligible.

Its all in the previous replies

 

StEvil, you are Evil! Good job! What you said, only extra!

 

I want the Santa Rosa run FSB and ram at 1:1

there are 800Mhz Laptop Rams any where on the market now

 

STEvil - Wow. Just wow. Thankyou for giving the informed, non-oversimplified facts. I don't really understand it, but thanks anyway.

Oh man - so many things to know, so little time to learn it!

If that is a question - yes. Zepto sells them, and I imagine that other vendors would too.

Anyway, if anyone can add to or clarify STEvil's stuff, please do. Especially on whether dual-channel makes a difference and whether I should care about FSB speed.

Alternatively, you don't need to answer my questions in the way that I have set them out. Any way that you can explain any part of it (and you don't have to tackle it all) would be appreciated.

 

Not to get off topic, but we do finally have 667Mhz notebook memory finally on the market running at a Cas Latency of 4. I'll probably upgrade if they can drop that again to 3.

 

Witch - Dual channel does make a performance difference, but the amount of performance difference is going to depend on how much memory bandwidth the program you are using requires, or latency if it is only using small bits of data at a very high rate and again this is going to be affected by cas/trcd/trp/tras (and assorted subtimings). Then you have to hope the chipset is good at what you want it to do, otherwise you end up with problems similar to what everyone who are buying Woodcrest/Cloverton multi-cpu systems are running into (Intel 5000v, 5000x chipsets)...

Yes, I know there is notebook ram with different timings on the market now, was just keeping it somewhat simple

 

The frequency of the ram stays the same, but the bandwidth is doubled. Without dual channel, every access to memory will retrieve 64 bits of data. With dual channel, 128 bits are retrieved instead. This happens in the same amount of time as getting 64 bits without dual channel.

To use dual channel, both memory modules must be of the same capacity. Brand does not matter. Speed rating does not matter, but you will end up using the lower of the two speeds if they differ.

 

Could you give Brand names and part #s please? Ive been looking for lower latency ram for weeks now!