A Different Kind of Synchronization

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XbitLabs has an article based on the AMD Opteron techdocs (page 17).

The article points out that when the memory setting is at 133 or 166MHz, the actual memory speed will usually be slower than that.

This article will explain the apparent reason why, and point out what ramifications this might have with Athlon64s.

How Opteron Is Different

If you look at the chart, and do a little math, you’ll see that all the memory speeds listed are an exact divisor of the CPU speed.

For instance, when the have memory running at 166MHz with a 1.8GHz Opteron, the memory divisor is /11, so the memory speed is 163.63MHz.

When you have the same memory speed with a 2.0GHz Opteron, the memory divisor is /12, which gives you 166.66MHz.

Different CPU speeds, different divisors, different results.

This means the motherboard doesn’t directly determine the memory speed through setting or FSB; the CPU does.

It also means that the memory controller requires a new kind of synchronous speed with the CPU. I say “a new kind” not only because of the CPU rather than FSB link, it’s also new because the specific ratio depends on the processor speed.

It is unclear at the moment whether the synchronous ratio could be changed by a motherboard to allow for higher memory speed (instead of a 2GHz Opteron having a memory divisor of 12 to reach 166MHz, have it divide by 11 instead to get 182MHz).

What is clear is that ascynchronous memory operations of the type we’re used to in the PIV won’t be available.

What About Athlon64?

Athlon64 is supposed to run with 200MHz DDR. So long as Athlon64 have actual CPUs speed divisible by 200 (i.e. 1.8GHz, 2.0GHz, 2.2GHz, etc.), there will be an exact divisor that will give you 200MHz. That means 200MHz memory will always run at 200MHz instead of possibly a somewhat slower speed.

The question for overclockers then becomes, “How do you handle overclocking with this setup?”

If the CPU determines its speed by the FSB, what will happen is that both the CPU and the memory controller will overclock.

For example, if you have a 2.0GHz Athlon64, and raise the FSB from 200 to 220, the CPU will run at 10 X 220 = 2200MHz, and the memory will run at 2200/10 = 220MHz.

A different mechanism, but the same results as a 1:1 ratio. Please note, though, this is a 1:1 ratio with the CPU speed, not the FSB. The Opterons all run at a 200MHz FSB, but can handle 100, 133 and 166MHz memory speeds.

It remains to be seen if the Hammer memory controller is as robust handling overclocked memory speeds as a motherboard north bridge.

If the divisors can be modified by the mobo, you may be able to play around with them a bit, but I wouldn’t bet on it.

The Big Overclocking Question

Since all Opterons run at a 200MHz FSB, given the 1:1 memory ratio, this is not going to allow a lot of room for FSB overclocking unless the memory ratios can be tinkered with.

What will probably prove more important for a big overclock will be the ability to change the multiplier, and right now, we don’t have a clue as to whether it can be done or not, or how easy/hard it will be.

If motherboards don’t allow you to do this painlessly and if a paint/pencil job is still possible, you’re going to have to get a heat spreader off the CPU first.

So overclocking an Athlon64 more than a bit might be easy; it might be brutal. Too early to tell yet.

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