a question about FSB to memory ratio

[brassbin]

For CPU's with locked(or ceiling'ed one might say) multiplier, raising FSB would achieve the same result. But once the FSB is raised, the corresponding memory speed needs to be raised as well...or does it? I've seen several fairly detailed faq's about overclocking FSB, but not much detail about oc'ing memory. What is it so special about this 1:1 ratio, I often hear people say they can just raise FSB to so ans so to achieve a higher CPU speed, but from some detail posted about the memory speed, the FSB/memory ratio is not 1:1, so does that nullify the higher clocked CPU? Thanks for your help in advance!

 

[sutahz]

"ceiling'ed one might say"
Sure one may say that
"the corresponding memory speed needs to be raised as well"
the corresponding memory speed IS raised, no if, and or buts about it, on a divider or not, memory speed is raised.
"What is it so special about this 1:1 ratio"
In my opinion, the hassle's of other dividers while OC'ing isnt worth it. Using a divider to run the memory faster then the fsb gives 99.89% of people not tangible benefits.
"some detail posted about the memory speed, the FSB/memory ratio is not 1:1"
Those people chose to run their memory faster. It makes them happy, and thats what matters.
"so does that nullify the higher clocked CPU?"
No, they still have the higher cpu clock, so... they still got the extra horse power for free.

 

[brencat]

brassbin, welcome to AT. You cannot run your RAM slower than the FSB on Skt 775. Hence the lowest divider you can use is 1:1. It may be different with AMD but I'm not familiar with that platform. For example PC2-6400 is the most widely used RAM and runs at 400mhz stock. Many people however are running a variety of C2D & C2Q CPUs less than 400 FSB, so the dividers exist for people to keep their RAM speed as close to stock while running their CPU at a lower FSB.

Once you cross 400 FSB however, it becomes increasingly stressful on your RAM the higher you go if you are using anything other than a 1:1 divider, though most decent RAM is capable of speeds in excess of 500 FSB provided timings are loosened to CAS5.

In my case, my CPU is running at 325 FSB and I'm using a 4:5 divider so my RAM runs at 406mhz. I could of course run my RAM at 1:1, but then I'd be underclocking it relative to stock specs. The upside to doing that is that I'd then be able to tighten timings to CAS3, instead of CAS4. But...when I benched it both ways on SuperPi, my system is faster using the 4:5 divider @ CAS4 versus 1:1 @ CAS3. With a 680i mobo which gives users much more control of memory timings relative to P35 or P965, things would likely be different as I'd be able to make true use of a 1T command rate @ 1:1, CAS3. But...I will also caveat that the differences are not noticeable outside of benching, as others have noted. (I do it because it's fun to tweak).

The bottom line is, YOU have to run some tests yourself to find out where the best balance is of performance versus the time you care to invest in tweaking.

 

[brassbin]

hmm..., I'm starting to think I don't really understand how the ratio(or divider, are the two terms interchangeable) is calculated. So if I have a E2xxx CPU with a native FSB of 200Mhz but quad-bumped and DDR2 800 memory, before anything is changed, is the default FSB/memory ratio 800Mhz:800Mhz, hence 1:1?

 

[JAG87]

no, start by completely ignoring quad pumped FSB and double data rate DDR2.

your base clocks are 200 for FSB and 400 for memory. hence your FSB:RAM ratio is 1:2

 

[brassbin]

So what happens when you raise say the example E2xxx FSB from 200Mhz to 300Mhz, is the memory speed up'ed automatically, if so by how much? BTW my spec are, though still in boxes not put together yet, Pentium Dual-Core E2200/DDR2 800 memory/Gigabyte GA-73VM-S2 and the rest. The motherboard supports CPU with 1066/800 FSB and 1333 via OC, and 667/533 memory and 800 via OC, the north bridge is nForce 610i.

 

[brassbin]

ok I did a few more google searches and read a few articles, here's my latest understanding, by manually setting a divider in BIOS, memory speed is raised accordingly whenever FSB is raised. For those boards that allow setting memory speed independently of FSB, which i think my mobo does, one can just set memory speed to be the default spec speed or even above, which might involve loosen memory timing and/or increasing memory voltage, provided it is verified running stable and under commonly agreed upon temperature.

Now my next question then is, given my spec and assuming my mobo does indeed allow setting memory speed independently, if i were to raise the E2200 FSB from 200Mhz to 270Mhz(so to achive a 270Mhzx11=2.97Ghz CPU speed), what is the difference between setting my memory speed at 270Mhz and 400Mhz(which is the default spec speed)?

 

[Xvys]

The speed your ram runs has to be conjunction with your FSB speed. When you set default 200FSB your cpu and ram will run at 200Mhz (this is referred to 1:1 or syncronized). Since it is DDR2 or "double-rate" ram, it's effective speed is 400MHz. Intel claims it "quad-pumps" the cpu, so your system speed is reported as 800Mhz, but in reality your cpu and ram are still running at 200Mhz.

If you increase the FSB to 270Mhz and leave the ram syncronized at 1:1, then your ram will also run at 270Mhz (or DDR2-540Mhz). If you have faster ram, you can also choose to run it A-syncronous and change the ram ratio divider to 5:4, which will run the ram 20% faster than your FSB speed, or 325Mhz (= DDR2-650) or perhaps you could set the divider at 3:2 (400Mhz = DDR2-800).

 

[piasabird]

It might be nice if DDR3 1333 RAM was available for a decent price.

Lets just say "Show me the Benchmark to prove it!"

I like this benchmark but they forgot to include the 800mhz fsb benchmark. Why would they not do that? Need a baseline for comparison.

http://www.tomshardware.com/20...l#to_buy_or_not_to_buy

You could spend between $300 - $600 for faster RAM for 4 Gigs depending on the speed and the latency. Some of the tests were on slower SPEED RAM.

 

[toslat]

A chip on the motherboard - The Northbridge manages communication between the CPU, memory, PCI express and the rest of the motherboard.
In linked mode, the Northbridge has a base clock which is the clock for the FSB while the memory bus clock is obtained from the FSB clock using a multiplier. In unlinked mode both clocks are virtually independent.

The ratio of the memory bus clock to FSB clock in linked mode is referred to as the memory ratio/divider (some bios use FSB:memory). The intel specified/effective FSB speed for a CPU is 4x (cos its quad pumped) of the base clock (e.g 1066MHz =4 X 226MHz) while the DDR2 memory speed is 2x of the memory bus clock.

To overclock a CPU with 'fixed' multiplier one increases the base clock which (at fixed memory ratio) indirectly increases the memory bus speed.

Setting the memory ratio/divider to 1:1 makes the memory bus clock equal to the base (FSB) clock. This gives more headroom to increase the FSB before hitting the speed limit of the memory. If u fix the memory speed, and/or set the memory ratio to auto, the motherboard bios will adjust the memory ratio so that memory bus clock is scaled appropriately to achieve the memory speed that is closest to your specified speed (or auto detected speed). Specifying the ratio explicitly gives u a finer control over ur overclock, while with the 'auto' selection, the bios picks the ratio it feels is appropriate (and safe).

For your Question

at 200MHz base/FSB clock
CPU FSB speed (4x) = 800MHz
CPU clock (11x) = 2.2GHz
memory bus @ 1:1 = 200MHz
DDR2 speed @1:1 =DDR2-400
memory bus @ 3:2 = 300MHz
DDR2 speed @3:2 =DDR2-600
if you set the memory speed to auto, the bios would have selected on your behalf the 3:2 ratio

at 270MHz base/FSB clock
CPU FSB speed (4x) = 1080MHz
CPU clock (11x) = 2.97GHz
memory bus @ 1:1 = 270MHz
DDR2 speed @1:1 =DDR2-540 (not standard)
memory bus @ 3:2 = 405MHz
DDR2 speed @3:2 =DDR2-810
if you set the memory speed to auto, the bios would have selected on your behalf the ratio that seems to provide the closest match to your specified speed. This will likely be a ratio that is just lower than (400/270)=1.48 which i think is 5:4 and will result in DDR2-675. I dont think the board will pick the 3:2 ratio that will result in DDR2-810 (slight overclock) which wld be the preferred choice.

Hope this helps (and does not confuse u)

 

[brassbin]

toslat, not only does your reply answers my question, it also makes other replies clear, thanks so much for all of you guys' help! Had i known what i know now, i would've bought DDR2-667 memory or even a cheaper mobo and use the saving on a better video card or a bigger disk.