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TSK's Opteron/Brisbane Theory

post #1 of 10
Thread Starter 
Okay, before you read any further, all that I am mentioning is purely conjecture, and I have no solid proof that my idea s right not. Its just a theory and you are welcome to disagree.

Intro

This topic is in response to all the people that are like "Omg lyke gets teh Opterawn! it pawns teh Athlons!!111 cuz its lyke made of teh uber silicon from teh center of da wafer!!111oneone1!"Yes, I will admit that the Opterons generally overclock to higher speeds than the Athlons, but I disagree on the fact that its because they are made up of better silicon. AMD won't spend the time looking for the pest part of the silicon. To raise the stabilty, AMD probably lowered the speed of the Integrated Memory Controller, or they could have raised the voltage to the IMC.

The Integrated Memory Controller (IMC)

The Integrated Memory Controller is a memory controller that is part of the CPU die, and not part of the motherboard. AMD did this because it greatly reduces the latency between the CPU and RAM, but its both a blessing and a curse:
Quote:
Originally Posted by Melraidin
Big thanks to YiffyGriffy for suggesting this possible cause for a limited OC.

This isn't so much an answer as a guideline. Myself, I have an A64 3400+ DTR (2200 MHz stock). I've talked with a number of other people who have the same chip, as well as others who have different variants of the Athlon 64.

No matter the cooling, it seems no one has been able to push their Athlon 64's past the point that their memory controller is running at 250 MHz. The Athlon 64's memory controller runs off a divider from the CPU's actual core speed that matches the CPU's multiplier. This works wonderfully for reducing memory latency when running stock: if your CPu is at 11 x 200 MHz (2200 MHz), your memory controller will be running at 2200 MHz / 11 (200 MHz). This is at stock speeds. Now if you overclock your CPU to 11 x 240 MHz (2640 MHz), your memory controller will be running at 240 MHz. Again, most likely to be stable. But as soon as you try to push much further, perhaps to 11 x 255 MHz (2805 MHz), you're likely to encounter instability as the memory controller will be running well above it's rated speed (actual speed of 255 MHz).

This also holds true for other multipliers. For example, if you were to take an Athlon 64 3400+ DTR and lower the multiplier to 10x you would be likely to encounter instability at around 255 MHz HTT (2550 MHz CPU) as the memory controller again would be running at 255 MHz, well above stock.

This would seem to indicate that if you're in the market for an Athlon 64 you should be looking for higher multipliers to enable higher OCs, as this would allow you to push the CPU speed to its limits without hitting the memory controller's limit near 250 MHz.
According to Melraidin, the resason you hit an overclocking "wall" where you can't go any higher no matter what voltages you set it at is because you are running the IMC too fast. The problem is that you cant control the voltage to the IMC, so its going to be unstable at higher speeds, and you can't really do anything about it.

My theory

Since we have now gone over what role the IMC had in overclocking, I can now present my idea.
Quote:
Originally Posted by txtmstrjoe
I'm not sure about the point that the Opteron IMC is slower than an Athlon's. In fact, I'd go on to say it's untrue.

The proof? That information can be found in the stepping codes. Given, say, an Opteron with stepping CCBWE 0534SPMW, and an X2 4400+ with stepping CCBWE 0605XPMW: What do both of these steppings share? CCBWE.

In AMD's stepping code scheme, the first five letters of the code all indicate specific information. The third and fourth letters together ("BW") indicate the version of the memory controller. Obviously, in the given example above (I've got both of these CPUs, so these are actual steppings), they have the same exact memory controller version, so the theory that an Opteron's IMC is inherently slower than an Athlon64's is false. (This is also part of my main support for the argument that Denmark Opterons and Toledo X2s are identical, as are Venus Opterons and San Diego A64s.)
If the above is true the reason that Opterons overclock to higher speeds is because not because of better silicon, its because AMD raised the voltage to the IMC for extra stability so you can reach the magical 3-3.1Ghz, since the IMC are the same, so they had to up the volts for the stablity.As for the Brisbane cores for AM2...somebody got 3.3Ghz stable on almost stock volts. AMD probably put the Brisbane IMC's on a 3/4 or a 1/2 divider, or raised the volts allowing such great OC's

So Opterons and Brisbanes overclock better beause of the lower clocked IMC's or higher voltage I hope you understood my idea
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post #2 of 10
Interesting idea but it's not very conclusive

Perhaps find some documentation to go along with it?
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post #3 of 10
nice theory...makes sense to me. Also, I'm sure he's looking for info, but he did say it was just an idea...
post #4 of 10
Thread Starter 
if you can find proof that an Athlon and an Opteron running at the same speed and cache, and the Athlon performed slightly better, that could prove that the IMC is slower on the opty
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post #5 of 10
Or maybe they push more volts to the IMC .
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post #6 of 10
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maybe...but i swear i've read that they put the opterons on a divider, and i also read something about K8L and IMC dividers (a loong time ago)
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post #7 of 10
Well, too bad you still can't do anything about it
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post #8 of 10
makes sense, my opteron seems to be a bit slower than my athlon at stock -- I havent done any real in-depth testing with the brissy yet though. (thats my next idea )

Id be willing to bet its increased voltage to the IMC, though.
    
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post #9 of 10
Thread Starter 
i can edit in that possibility in too...haha
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post #10 of 10
Interesting.... so with the K10s, since the IMC is on its own power plane and is supposedly ran at speeds higher than core speed, we should see some great OCs.....
    
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