Overclocking An Athlon64?

AMDZone reports some comments made by some AMD folks during their tech tour in “Tidbits From AMD’s Tech Tour.”

As we pointed out about a month ago, the Hammer processors work a bit differently than earlier processors. Memory speed isn’t determined by an FSB; it’s determined by CPU speed.

This raised the question as to where the circuitry that determined the divisor to be used was inside or outside the CPU. If it were inside, that would be bad news for overclocking; if outside, that would be good news.

Well, the piece says the following, “The memory controller interface will use a BIOS controlled clock divider to set the bus to the memory speed.”

Provided the mobo makers in Taiwan do their usual magic, this should allow overclockers considerably flexibility in matching memory to a system.

For example:

Let’s say you have a 1.8GHz Athlon64 that is capable of running at 2.4GHz.

The way you might be able to overclock it is to set the CPU frequency (more on this later) to 267MHz. You would then have your choice of memory divisors:

A memory divisor of /9 would mean a memory speed of 267MHz (2400/9).
A memory divisor of /10 would mean a memory speed of 240mHz (2400/10)/
A memory divisor of /11 would mean a memory speed of 218MHz (2400/11).
A memory divisor of /12 would mean a memory speed of 200MHz (2400/12)/
A memory divisor of /13 would mean a memory speed of 185MHz (2400/13)/

As you can see, this is a lot more flexible than the Canterwood/Springdale options, so if your memory isn’t quite up to snuff at a certain speed, you just set it back a relatively small notch.

Of course, we don’t know how much the memory controller on the Athlon64s/Opterons are, but even if they don’t take to overclocking too well, BIOS control of memory divisors will let you adjust for that.

But I Don’t Want Divisors, I Want To Run at 1:1!

You can’t, for a very simple reason. You can’t run your memory at the same speed as your FSB because in Athlon64/Opterons systems, memory is detached from FSB.

Instead, you have separate Hypertransport links. For Athlon64s, you’ll have basically one link for memory, and one for everything else.

FSB will be “everything else.”

There was a question as to whether or not Athlon64/Opterons determined their clock speed. Fortunately, these systems will have clock-generators of the usual type, and if you look at the technical sheet for one of them, you’ll see that it provides for FSBs of 267MHz and above.

So at least the mobo shouldn’t stop you from a reasonable level of overclocking.

Overclocking these chips is going to mean some old dogs would need to learn new tricks, but in this case, the new tricks are a net gain.

Ed