How To Overclock An Opteron . . .

Someone over at the TBreak forums found out something very interesting.

The person was running an Athlon 64 3200+. While keeping the memory at a 1:1 ratio, he was only able to raise the FSB from 200 to 215MHz.

However, if he changed the memory speed from 200MHz to 166MHz, he was able to increase the FSB to 239MHz (and at least be stable enough to run 3DMark).

This is a matter of major importance to future Hammer overclockers, because unless someone comes up with a way to change multipliers, this is going to likely be the only way to overclock them very much.

While the socket 754 motherboard was not named in the piece, the manual for the Asus SK8N Opteron motherboard indicates that the BIOS gives you various options for memory speed

The Opteron/Athlon 64 determines memory speed a bit differently than other processors. Memory speed is not determined by the FSB, but rather as a divisor of the clock speed.

What these settings do is determine the divisor for memory. In the situation above, setting the memory at 200MHz means a divisor of 10, while setting the memory at 166MHz means a divisor of 12.

You can read AMD’s description of this for Opteron here (page 17). This appears to also be the case for Athlon 64s.

While the mechanism is different, the end results are pretty much the same as the 5:4 and 3:2 ratios found in PIV 865/875 boards.

So for this person, it looks like his RAM (or perhaps the Athlon 64 memory controller) couldn’t handle more than 215MHz, but by shifting to 166MHz, he could run the CPU at 239MHz FSB, while running RAM at 2390/12 = 199MHz.

This approach would perhaps be optional for a 2.2GHz or perhaps even a 2GHz Athlon FX/64. It becomes a necessity should AMD release CPUs at speeds lower than 2GHz.

Let us assume for argument’s sake that a Hammer is capable of 2.4GHz. To reach that speed with an Athlon FX/64/Opteron with a 200MHz FSB, the required FSB would be:

CPU Speed
FSB
2.2GHz
218
2.0GHz
240
1.8GHz
267
1.6GHz
300

Whether it is the RAM, the memory controller, or both, trying to get the maximum speed from a lower-end Hammer will require running memory at 166 or even 133MHz.

Probably the best mix of CPU and memory speed would come at 1.8GHz. Running at 2.4GHz would get you an FSB of 267 (at least the SK8N allows up to 300MHz FSB) and a 166MHz (actually 163.63MHz) overclocked memory speed of 218MHz.

Steppings

While there is no Athlon FX/64 technical documentation that talks about stepping, the Opteron documentation does (page 6). So far, there’s been two steppings, and it looks like the later stepping (which should show up as F-5-8 on Sandra/CPU-Z) is the equivalent of the Athlon 64 stepping F-4-8.

So there’s some reason to expect that at least at some point in time, lower-speed Opterons might prove good overclockers.

Price Is The Real Problem

The real problem is what AMD is going to put out at what price. On the one hand, you have a report that they aren’t going to put out FX/64s at less than 2GHz.

On the other hand, there is a recent review featuring a 1.8GHz Athlon 64. What’s particularly interesting about this is that the PR was revised during the review from 3100+ to 2900+.

What will AMD do? Who knows? Best guess is we’ll see lower-speed Athlon64s when they start cranking up production.

In the Meantime . . .

Those who already have Hammers might want to try what the person at TBreak did, and see how far they can now go with their systems. Those who are going to get one in the near future might want to do the same in their testing.

Ed

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