Intel - fsb / ram ratio interaction?

[WintersFury]

Ok, so I'm planning my next build and am trying to nail down the specifics. And I wound up trying to figure out what I'll be able to get fsb wise at certain memory ratios to figure out which cpu to get.

Now the board I'm looking at doesn't currently have a memory setting lower than 667MHz. So this is figuring out the max fsb speed of using the 667 setting with 800MHz memory.



With a 1066 fsb processor it should be 320fsb max (400/5*4) Since I know it's a 4/5 ratio, I'm pretty sure on this one. As 333/5*4 gives right around 266 which is the fsb speed.

Now with an 800 fsb processor I'm thinking it's 1.665/1 fsb ratio (I'm sure it's something more fancy, but 1.665/1 seems to work as well). As 200*1.665 gives 333. So with using 800MHz memory I get 240 fsb (400/1.665) which seems a bit on the low side.

However using the 1066 fsb which I'm pretty sure on I can replicate this by taking 333/266 which gives 1.25 and 400/1.25 gives 320.

So after looking at my math and all, am I right or did I mess something up in the process?

Thanks for checking this out all.

 

[BillParrish]

So after looking at my math and all, am I right or did I mess something up in the process?

hmm

ok, intel boards run best at a 1:1 memory ratio which introduces no wait states or buffering while the cpu and memory catch up with one another, think of it as everyone in the boat rowing at the same speed. (It is true in some instances using a memory mulitplier to boost memory speed when using very high speed memory in a system where the cpu cannot acheive a corresponding high FSB can help performance but it is a special case and we are not talking about it here.

I will simplify, I hope, the whole issue for you.

Any decent enthusiast intel board out there can do 400FSB and with a 1:1 memory mulitplier (on intel memory runs 2x FSB - (DDR3 aside)) so at 400FSB your PC6400 800MHZ memory is running at its exact 800MHz rating.

any core two duo can have the cpu mulitplier set lower than the default on a good enthusiast board.

C2D specs.
(CPU highest default multi, stock speed, cache, highest factory Vid, default FSB)
E4300 9X 1.800MHz 2MB 1.325V 200MHz
E4400 10x 2.000MHz 2MB 1.325V 200MHz
E6300 7X 1.866MHz 2MB 1.325V 266MHz
E6320 7x 1.866 MHz 4MB 1.325V 266MHz
E6400 8X 2.133MHz 2MB 1.325V 266MHz
E6420 8x 2.133MHz 4MB 1.325V 266MHz
E6600 9X 2.400MHz 4MB 1.3525V 266MHz
E6700 10X 2.667MHz 4MB 1.3525V 266MHz
X6800 11x 2.933MHz 4MB 1.3525V 266MHz
Q6600 9x 2.400MHz 4MB 1.372V 266MHz
QX6700 10X 2.667MHz 4MB 1.372V 266MHz

The only issue is the E63XX with a maximum mulitplier of 7 because 7x400 = 2.8GHz and the chips can do more. ANY of the other chips typically can do 8 X 400 = 3.2GHz or more so you use the cpu mulitplier to maximize performance by picking an FSB that gets the most potential out of your ram at the 1:1 everyone rowing at the same speed. Then you play with raising the FSB to get more out of the cpu and see how far your ram will overclock. Loosen ram timings a bit if you have too as the increase in cpu performance more than makes up for the performance hit of going say, 5 5 5 15 instead of 4 4 4 12.


Moral of story, buy a decent board, buy some decent PC6400 (800MHz) memory or faster, the super fast stuff is tossing money in the trash, even non-super fancy stuff will get you to 3.2GHz without overclocking the memory as shown above. Then buy anything other than an 7X mulitplier cpu, whatever fits your needs and budget and you will have tons of room to play.

 

[ziddey]

It would help a lot if you posted what motherboard you're looking into..

As for the dividers, a lot of boards allow you to freely choose a divider, since it manipulates the strap and automatically falls into something else if that's where the divider is. Problem with that is that it becomes harder to scale further in fsb if you goto a lower strap. For example, if you choose to employ the 3:5 divider (you referred to it as 1.666:1), you would be in 200strap still, and it may not be that easy getting higher in fsb's. However, that said, you could very well be ram limited as well.

From the sounds of what you're saying, you're looking to overclock your cpu but not your ram?

Most decent ddr2-800 ram will be able to hit 1000, if you're worried about headroom.

So you're saying that the board doesn't have a 1:1 if I remember reading correctly. Would this by any chance be a g33 based board?

 

[WintersFury]

So you're saying that the board doesn't have a 1:1 if I remember reading correctly. Would this by any chance be a g33 based board?

Yeah, looking at getting one of them new Gigabyte mATX boards when they come out next month for the SFF computer. And currently there is nothing lower than the 667 memory setting.

So I'm trying to figure out if I should grab a e6300 like I have in the tower I'm currently using or one of the e4xxx chips for the higher multi. If my math is correct (Which it seems to be) I'd get a slightly better OC with the 4xxx chips due to the higher multi on them.

I have yet to OC my memory at all, but from what I've read review wise, it doesn't go much above 900MHz, which is why I'm figuring out what I can do keeping the memory at stock speeds first.

Though the fact that I won't be able to go much over 2.5GHz due to the memory and lack of memory settings is kinda making me think of going with an AM2 where I should be able to get 2.9-3GHz as that should be a slight bit faster than the C2D and will save me about $75 to spend on a graphics card.

And Bill, I did some testing with my current setup and memory divider speeds and found that faster memory gave more performance no matter what the situation with my setup. Only reason I'm at 1:1 with it currently is to get the ~20 more fsb out of it than I could with the 4:5.


And thanks for responding guys.