Matching FSB to RAM: I've done all the reading I can - Need Help.

Greetings everyone,

I've come here after reading all that I can get my hands on about matching CPU to RAM to insure that you get the best data transfer rate possible - in short looking for good compatible combo to prevent bottlenecking.

Here's what I'm planning to purchase soon:

(All from NewEgg)
Intel Pentium D 915 Presler 2.8GHz
ASUS P5N-E SLI
G.SKILL 2GB (2 x 1GB) DDR2 800 (PC2 6400) Dual Channel Kit (5-5-5-15 timing, voltage 1.8V - 2.0V)

Now all that I've read (that I could find) suggests this would be a great combo for allowing lots of room for overclocking, but I'm concerned I'm not doing the math correctly and the CPU/RAM will not work well at stock settings let alone when OC'ing begins.

Though the math and how they relate to each other (CPU to RAM and RAM to CPU in terms of compatibility or best match) still escapes me.

I know PD 915 is quad pumped, that gives me a flat out 200mhz FSB.

I know DDR2 is really just DDR x 2 chips therefore DDR2 800 is really DDR 400 x 2. And DDR is dual pumped so that really is 400/2 = 200mhz for it's frequency. I'm assuming that relates to DDR2 in a similar way - 800/4 = 200mhz.

So far it seems that it would be a great match, but I still feel I'm missing something, and I don't trust that I fully grasp the concept of the CPU/RAM bus relationship for compatibility.

Could someone provide some light on this further? Or fill in the "missing" parts of my understanding as I've listed so far.

I hope this helps explain my dilemma, in a (semi-)short way.

I believe the math is the same for DDR & DDR2. For instance, if I run my E6400 stock, that's an FSB of 266, which equates to DDR 533. So in order to run my DDR2 800 at it's rated speed on a 1:1 divider, I have to set the FSB to 400.

So if you run the Presler at stock clock (FSB 200) the memory will be running at DDR 400. If my math is correct you would have to run the Presler to 5.6 GHz to get the DDR2 800 to rated speed on a 1:1 divider.

I'm sure if I'm wrong someone will correst my math!

My first and only question is why, if you are going to buy new stuff, your aren't picking up a C2D processor and instead are going with the Presler core?

An e4300 or e6300 is about $170 or $185, repectively and both will drop to under $150 by April, with the e4300 coming in around $130 or less. The difference is price, versus the $100 cpu that you've picked out will be just a few dollars a month, if spread out over the 2 to 3 year useful life...not too mention the resale value will be greater.

Performance, however, will be night and day better with the Conroe.

As always, your call, but perhaps you might want to give this one a little more thought.

bloodshedder said:

If my math is correct you would have to run the Presler to 5.6 GHz to get the DDR2 800 to rated speed on a 1:1 divider.

Click to expand...

Could you show your math, with short notes, so I can see how you came up with that? It would appear I'm way off on my own math...way off.


Reefa_Madness: I absolutely agree with you on going the Core2 route. I didn't see prices like that on NewEgg, but then again I can't find milk in the fridge.

I will say this, I wanted a MoBo that was decent in OC'ing, and offerred alot of options for future expansion, but on a budget. I choose the Presler as a "starter" CPU for the moment. I'm simply looking for a semi-budget upgrade with focus on highly upgradable mobo, and decent gpu (I'm going for the EVGA GeForce 7900GS KO).

Though I admit your comment on "resale value" did strick a bell. I didn't consider resale options for later. That changes some of my considerations for the parts I've choosen (specifically the CPU). But up until this, my choices were strickly on meeting my budget, and reaching the desired goals to meet my long-term and immediate needs.

Oh, and my computer at home is 4 framerates away from release it's magic blue smoke...4 years, and I've kept it hopping along as far as I could. It's mostly an emergency upgrade.

bloodshedder said:

I believe the math is the same for DDR & DDR2. For instance, if I run my E6400 stock, that's an FSB of 266, which equates to DDR 533. So in order to run my DDR2 800 at it's rated speed on a 1:1 divider, I have to set the FSB to 400.

So if you run the Presler at stock clock (FSB 200) the memory will be running at DDR 400. If my math is correct you would have to run the Presler to 5.6 GHz to get the DDR2 800 to rated speed on a 1:1 divider.

I'm sure if I'm wrong someone will correst my math!

Click to expand...

An added thought to your math: I thought DDR2 was just DDR chips x 2 (note I said chips, not sticks). So DDR2 achieves it's high ratings due to chicanery. Example DDR2 800 is nothing more than DDR 400 chips meshed together to work as one stick, giving you DDR2 800.

So I guess the confusion I'm having is: DDR 400 works 1:1 at 200mhz on the FSB - That I can agree with. So if DDR2 800 is just DDR 400 in disguise, doesn't that mean it's still 200mhz on a 1:1 setting? Otherwise, if the math works the same for DDR and DDR2, then you are correct in your math, bloodshedder.

Anyone else care to weigh in on this? I believe the knowledge of this is beginning to sink in here...but I have doubts still. I just can't figure out why I'm having difficulty on this one aspect. LOL

DDR = Double Data Rate
This means it can read and write on the same clock cycle
Marketing guys decided to "double" the rated speed to imply this new technology was twice as fast as the old stuff (SDR = single data rat)
DDR-400 runs at 200 mhz. DDR2-400 also runs at 200mhz. DDR2-800 runs at 400mhz. Twice the frequency of the DDR-400.

Though DDR2 800 is rated twice as fast as DDR 400, it can only be realized if you have the FSB pumped up to 400 MHz at a 1:1 divider.

So a Presler 915 has a native clock of 200 MHz and a multiplier of 14. 200 MHz*14 = 2.8 GHz. At a FSB of 200 and a 1:1 divider DDR2 800 will be running at 400 MHz. Now if you use a 5:4 divider it will be running at 500 MHz ((200 MHz * (5/4))*2). So to get that DDR2 800 up to rated speed you would have to run at:

1:1 Divider ((400 MHz * (1/1))*2) = 800 MHz. 400 MHz*14 multi = 5.6 GHz.

5:4 Divider ((320 MHz * (5/4))*2) = 800 MHz. 320 MHz*14 multi = 4.48 GHz.

3:2 Divider ((266 MHz * (3/2))*2) = 798 MHz. 266 MHz*14 multi = 3.724 GHz.

I believe all this is accurate, though I might have transposed the values for the dividers. Is it 5:4 or 4:5? Heh, I always forget!

But like Reefa said, you can easily get that DDR2 800 to rated speeds with a C2D. I'm no expert at OC'ing. Not by a L-O-N-G shot, but I've easily got my E6400 running at 400*8, which matches up very nicely with my G.Skill DDR2 800.

I felt the light go off in my brain.

Thank you all so much for your help. I'll let this info and advice marinate before I chuck out the $$$.

nCoded said:

Could you show your math, with short notes, so I can see how you came up with that? It would appear I'm way off on my own math...way off.


Reefa_Madness: I absolutely agree with you on going the Core2 route. I didn't see prices like that on NewEgg, but then again I can't find milk in the fridge.

I will say this, I wanted a MoBo that was decent in OC'ing, and offerred alot of options for future expansion, but on a budget. I choose the Presler as a "starter" CPU for the moment. I'm simply looking for a semi-budget upgrade with focus on highly upgradable mobo, and decent gpu (I'm going for the EVGA GeForce 7900GS KO).

Though I admit your comment on "resale value" did strick a bell. I didn't consider resale options for later. That changes some of my considerations for the parts I've choosen (specifically the CPU). But up until this, my choices were strickly on meeting my budget, and reaching the desired goals to meet my long-term and immediate needs.

Oh, and my computer at home is 4 framerates away from release it's magic blue smoke...4 years, and I've kept it hopping along as far as I could. It's mostly an emergency upgrade.

Click to expand...

First off, I see that someone else has already explained the math, while I was busy trying to find you links. I'll just leave what I've posted below, even though it is likely a duplication in many respects.

Math:
With a default multi of 14 (x 200 MHz clock speed = 2.8GHz), if the ram is DDR2-800 running at 1:1, then it would be 400 MHz clock speed x 14 multi or 5.6 GHz cpu speed (which isn't going to happen without extreme cooling), so you would have to run a divider to run the ram at 400 MHz. For example, with a 1:2 divider you could run the ram at 400 MHz (DDR2-800), but keep the cpu running at stock speeds of 200 x 14 or 2.8 GHz (400 MHz ram speed, divided by memory ratio of 1:2 (50%) equals the 200 MHz cpu speed). The quad pump is not the memory speed, but the FSB and I believe that is where the math is getting you confused.

In the example above, if you were able to run your cpu at 400 MHz and 1:1 memory ratio then the FSB would be 1600 (400 x 4), instead of the default 800 (200 x 4).

The whole purpose of the dividers is to allow you to run the ram at high speeds and improve memory bandwidth, but yet slow your processor down to achievable speeds.

Now, on to the processors...

The big drop in prices will come in April, but the current prices that I quoted above can be found here:

e4300 for $169 (retail box w/3 yr warranty):
http://www.newegg.com/Product/Product.asp?Item=N82E16819115013

e6300 for $183 (retail box w/3 yr warranty):
http://www.newegg.com/Product/Product.asp?Item=N82E16819115005

The beauty of the 4300 is that with a multi of 9 you don't need the same high fsb capacity board that you would need with the 6300. The Asus board that you have picked out would be plenty for the 4300 (but maybe not the e6300) or an e6400.

Nice comparison of these two chips (4300 and 6300):
http://www.techspot.com/review/40-core2-e4300-vs-e6300-overclocking/

I'll try to find you a link to the projected price drops for April, just in case you want to try to wait them out.

By the way, an alternative cpu (slightly cheaper), if you are looking for a bandaid fix, and one that generally has alot of overclocking headroom, would be the 631, a 3.0 GHz (15 multi) single core (Cedar Mill) which regularly hits in excess of 4.0 GHz (and beyond with add'l voltage). It would be worth keeping afterwards just to play with.

http://www.newegg.com/Product/Product.asp?Item=N82E16819116004

Yeah, like I said, I'm certainly no expert, and if I'm wrong in my math I'm sure someone will correct it. You might want to wait for someone to verify what I posted.

The C2Ds have a native clock of 266 MHz. My E6400 has a multiplier of 8. With the P5B Deluxe it barely breaks a sweat running at 400 FSB which gives me 3.2 GHz and DDR 800.

Also Intel will be revamping the E6300 & E6400 (April I think?), coming out with the E6350 & E6450. They will have a full 4 MB of L2. Not sure if they will also be dropped in price as the other Intels will?

Doh, I did leave out the 1:2 divider which would let you run the CPU at stock and still hit DDR 800. But anyway, Reefa is correct about taking a route other than the Presler. I'm loving my E6400!

Here is a link to those reported price cuts.

http://www.hkepc.com/bbs/itnews.php?tid=735045

nCoded to the forums

Basically the same math (÷2) that goes for DDR 400(200MHz) goes for DDR2 800(400MHz) only difference is that DDR2 is able to go at higher speeds, thats all.