Ram speed and dividers

ok i am currently running my opty 165 at 301x9 which gives me 2709 mhz. my ram divider is the 5/6 divider. So as this should be i should be at 301 *5/6=250mhz Ram speed with the cpu speed being 2709 as reported shortly after post screen. My question is is that cpu-z reports cpu speed at 2724 and a ram speed of 247.7mhz. So what is right cpu-z or my calculator. Here are 2 screens

CPU-Z is correct, and the calculator would be too if it had the right numbers

If you look at your first screen shot you'll see that the FSB is actually 302.7, multiply by 9 and you get 2724.3, I normally see the FSB vary a little from one minute to the next, or from one boot to the next.

On your ram speed, CPU-Z shows FSB to DRAM ratio as CPU/11 and 2724.3/11 = 247.7MHz. I'm not totally sure how that relates to the 5/6 divider?? But the math works out.

ok on my asus board the 5/6 divider is 333mhz if i am not mistaken. Its Listed in my bios as 166,200,333,400 and so on. 400 is 1:1 so the 333 should be 5/6 correct? i just wonder why cpu-z shows a different bus speed then i set in bios, i mean i set 301 not 302.7!

Now that you mention it, 1.7MHz more than what your bios shows is quite a bit. I think on my setup that cpu-z might show .1 or .2 more than my bios shows, but that's it.

Are you running cpu-z 1.38? I have seen some posts at other forums about v1.38 showing slightly higher FSB and Core speeds than v1.37 - nothing changed on the system, just a different version of cpu-z. I haven't found any explanation for it though, maybe the previous versions were just reading the bios instead of actually testing for the speed. I think I'm running v1.37 and will take some screen shots, then upgrade to v1.38 and take some more - should be interesting.

As far as the divider goes, I have been trying to rap my head around that all day and can't quite comprehend what's going on. I'm still learning the mysteries of the bios and the relationships between the different clock speeds and this is just a little beyond me at this point. I can almost put my finger on what's going on, but I'm missing some little thing that will allow me to figure it out. Maybe someone can explain it to the both of us.

Help, anyone???

Yea its version 1.38, get some screens from both 1.38 and 1.37 and let me see the difference. What do you need help figuring out? Its simple math but it turn out that 90% of the time that we are wrong. I meabn the equation would be 301 times 5 divided by six which would give 250mhz(ddr500) but its not, AMD is weird.

I'll try to get some screens later tonight.

On the dividers, I'm assuming there is a logical reason for the lower memory speed, especially since the math works out perfect when using CPU/11. I figure that there is some relationship between the 5/6 divider and the FSB and the multiplier that I'm missing.

I have looked at several other threads on the forum, and after doing the math on all of them there is always a 3-4 MHz difference between what the FSB*divider equation gives and what the memory speed actually ends up being (when a divider is used, 1:1 seems to work out). There's gotta be a reason.

Can't wait to get home and check this out on my rig, I haven't tried anything other 1:1, so I never ran into this.

Ok cool lemme know what you find out, maybe we can figure out exactly whats going on here.

Well, I played around with the different versions of cpu-z and just for fun I tried to push my OC a little further, sure is nice to have that cmos reset button on the mobo I'm running into a wall and haven't taken the time to see what it actually is.

I had unexpected results with the cpu-z upgrade, my FSB and CPU speeds actually got more accurate (closer to what I actually had in bios), the values where higher with 1.37... I tried to lower my multiplier and raise the FSB like yours is to see if my numbers would be off, but had to use "the" button 2 more times

My RAM speed did exactly what yours did except it went to CPU/14 since I normally run CPU/11.

I'm still trying to figure out how to post my screens. I guess I can't just copy/paste or insert them, looks like I have to upload the to the web first. What process did you use?

I usually upload to image shack. so what we are seeing here is that at high bus speed with a lower multi then cpu-z will read farther off, the higher the ht speed the higher cpu-z will read off i bet.

CPU-Z v1.37 with 1:1 and 5/6 divider

I'm betting the same thing, but then again I've lost a few bets in the past. Ok, I'm going to try some screens now.

Here are the 1.37 shots with 1:1 and then 5/6, bios was 2585 235*11. One thing I noticed is that if I made a bios change, F10 and then booted, my FSB and CPU were higher, like 235.6 and 2591. If I shut completely down and then restart, they would go back to 235.2 and 2587??? Didn't happen with 1.38 though which was even stanger.

CPU-Z v1.38 with 1:1 and 5/6 divider

Hey, that came out kind of nice. Here's the 1.38 at 235*11 for 2585 in bios.

Memory speed did the same thing yours did (with both versions). Notice that the FSBRAM went from CPU/11 to CPU/14, need a math wizard to figure out the relationship.

My other clocks went to an almost perfect match to bios settings with v1.38, exact opposite to what happened on your setup. Wondering if I can get a lower multiplier and higher bus speed to boot and test our theory.

hmm very interesting stuff, we need some one that has a better understanding of this stuff to shed some light on the subject.

AS far as the CPU-Z and the CPU/X where X is a number. It is correct that it is the CPU speed divided by that number to give you your Ram speed. If you set you RAM to 166 and then overclock it by 50mhz (FSB, or the equivilant) then the ram will be at 50mhz.... Take your current CPU speed and divide it by that number in the CPU-Z screen shot and you will have your ram speed... now as far as CPUZ(the newest version) showing higher or lower CPU Speed. I have seen that too....

maelstrom
I didn't quite understand your explanation. We have figured out that the CPU/X on the memory tab gives you the memory speed, but what determines X? Seems that the bus speed should just be muliplied by the divider to get the memory speed, but it doesn't work out.

I found something that tries to explain the memory speeds that we are getting. In the A64 101 link, down towards the middle of the first page:

http://www.ocforums.com/showthread.php?t=306418

"What complicates what multiplier and HTT speed you should use are the CPU/memory dividers. No motherboard allows you to manipulate them directly. Instead, they provide “maximum memory clocks,” or supposed HTT/memory ratios. Make no mistake, no such thing exists. The memory is derived off of the CPU speed, but it’s never made clear, and the dividers need to be manipulated indirectly. Also, CPU/mem dividers are integral only; there are no half dividers, so it’s advisable not to use half multipliers...."

"To find the dividers mathematically, use the formula CPU Multiplier/Ratio and take its celing (e.g. anything above 10.0 becomes 11, anything above 11.0 becomes 12, etc). Formula found by Hitechjb1 from the Overclockers Forums."

So, in your case we would take muliplier of 9 divided by 5/6 (.83) or 9 * 6/5 to get 10.8 and then round up to 11, which we see on your cpu-z memory screen as CPU/11 and then gives you your true memory speed of 247.7

In my case we get 13.2 (11/.83 or 11 * 6/5) and round up to 14, which matches my memory screen shot and gives a true memory speed of 184.7

So far I can't find anything wrong with cpu-z 's numbers, so I'm just going to trust them in the future. This is weird stuff, I kinda understand it but it's making my head hurt

HeavyD said:

I found something that tries to explain the memory speeds that we are getting....
...CPU/mem dividers are integral only...
...and take its celing...

Click to expand...

The nOObs have enough trouble with FSB x Divider, don't be confusing them with all this stuff..lol

Have a look at Blue's AMD Athlon 64 Overclocking Calculator, it calculates both ways. http://www.gurushane.com/site/software.aspx

And if you know how to use them, half multis are a good thing... sometimes.

Hey, that calculator is cool. Thx!!

I posted on the Corsair forums and got a response that has given me another way to look at. With AMD64, when choosing the memory clock, the true divider used to determine the memory speed depends on your current CPU muliplier.

In my case, the true divider is 11:14, not 5:6. On my screen shot, CPU/14 is saying to divide 11 by 14 and then muliply by the bus speed (11/14 = 0.786 * 235 = 184.71) My last post shows how we got to the 14, in a round about way.

In lifted's case, the true divider is 9:11 (9/11 = 0.81 * 302.7 = 247.66)


Ok, I think that's enough math lessons for the day (week/month/year)!!! I'll be using Blue's calculator from now on.

thats a handy little tool there. good find, now i can figure this crap put and maybe get a half multi to work to my advantage!

HeavyD said:

Hey, that calculator is cool. Thx!!

In lifted's case, the true divider is 9:11 (9/11 = 0.81 * 302.7 = 247.66)

Click to expand...

That kinda works, but it's a klunge of the true method and the 11:14 can only be back calculated if you know the true memspeed first. It just compensates for skipping the rounding up.

The correct formula is:
Multi x FSB / (Multi x 200/Divider) Rounded up

For example:
11 x 235/(11 x 200/166 rounded up) Note that 11 x 200/166 = 13.253... which rounds to 14)
2585/14 = 184.64

Note that if you use the 5:6 ratio for a 166 divider:
FSB x Ratio = wrong answer
235 x 5/6 = 195.8383

Note that if you use FSB x Divider/200:
FSB x Divider/200 = wrong answer
235 X 166 /200 = 195.05