Understanding the relationship between RAM speed and FSB

[Stu]

I have been attempting to further understand the relationship between RAM speed and FSB.

I was always taught that, where possible, you must match the speed of the CPU's FSB to the RAM clock. So if for instance I had an AthlonXP 2400 with a 333MHz FSB (166MHz double pumped), I ought to be using DDR-333 (PC-2700) RAM. I always found this concept quite simple.

What I never really understood though, was the system for Intel. Say for instance I had a 2.4GHz P4 with 533MHz FSB (133MHz quad-pumped), going by the AMD system, I should be looking at RAM with a clock of 533MHz, which, as far as I'm aware didn't exist when the 2.4GHz socket 478 P4 was in production. Instead we saw them with DDR-266 RAM, which at least makes sense in matching it's "real" unpumped clock. Where they really started to lose me was when I started seeing motherboards with DDR-333 memory support for the 533 FSB CPU's.

Obviously it got a lot more complicated with DDR2, but I'll cross that bridge when I come to it.

In the meantime, can anybody enlighten me on the above?

 

[Methical]

I to would like a better understanding on relationships between CPU & FSB/BSB & RAM. Maybe a request for an article should be put in place

But I guess the answers are probably on Google somewhere lol

 

[Stu]

I to would like a better understanding on relationships between CPU & FSB/BSB & RAM. Maybe a request for an article should be put in place

But I guess the answers are probably on Google somewhere lol

There is tons of info on Google, but a lot of it is contradictory. It would be nice if someone could clear it up once and for all, in one concise article, that is also in laymen's terms. Take this article for instance - http://icrontic.com/articles/core2_fsb_explained/3, it attempts to explain a difficult issue, but fails in my opinion.

 

[Simmy]

Taking that P4 2.4Ghz 533mhz (133mhz Bus) as an example, this is my understanding of it.

The true clock speed is 133mhz. DDR RAM runs at double the true clock speed, hence the name Double Data Rate. Therefore, the speed of the RAM you need is twice the real bus speed, which will be 2 x 133 = 266mhz (or PC2100). The quad speed is pretty much irrelevant and you're right when you say you just need to observe the "real" unpumped clock speed.

Where they really started to lose me was when I started seeing motherboards with DDR-333 memory support for the 533 FSB CPU's.

That just means the RAM has the potential to run faster than the default cpu fsb. Perhaps it was meant to allow the cpu to be overclocked from 133mhz to a 166mhz fsb? The RAM speed doesn't necessarily HAVE to match the cpu clock speed - just as long as the RAM speed is equal to or greater than it (as in the DDR333 case above).

Did they use RAMBUS memory on P4 2.4s? I have no idea how RAMBUS works Although I have a feeling it was only used on the very early P4s.

 

[Stu]

Taking that P4 2.4Ghz 533mhz (133mhz Bus) as an example, this is my understanding of it.

The true clock speed is 133mhz. DDR RAM runs at double the true clock speed, hence the name Double Data Rate. Therefore, the speed of the RAM you need is twice the real bus speed, which will be 2 x 133 = 266mhz (or PC2100). The quad speed is pretty much irrelevant and you're right when you say you just need to observe the "real" unpumped clock speed.

Did they use RAMBUS memory on P4 2.4s? I have no idea how RAMBUS works Although I have a feeling it was only used on the very early P4s.

Simmy, you have arrived at the same conclusion as I would have. But the difficulty I have is understanding the quad pump - why is it there if it's irrelevant? Also, why and how did these P4s "support" DDR-333 (actual 166MHz) when the 533MHz FSB was really 133MHz? Take for instance I have an old Dell machine lying around. It has the 2.4GHz P4 533MHz, and DDR-333 RAM. Does this mean that the DDR-333 RAM is essentially wasted as the true speed it runs at will be DDR-266?

As for RAMBUS, my understanding is that Intel pulling its support early on meant the technology never really took off.

 

[Simmy]

Does this mean that the DDR-333 RAM is essentially wasted as the true speed it runs at will be DDR-266?

Yeah, essentially it is wasted. But DDR333 also might have allowed for lower latency timings than DDR266 could handle?

So DDR266 might run at 266mhz with a CAS of 2.5
DDR333 could at 333mhz with a CAS of 2.5 by default, but might be able to handle a CAS of 2 at 266mhz