Pipelines and Prescotts

Prescott will be introduced within ten days. A lot of disquieting things are being said about it, presumably by those who are actually playing with the beast a little in advance.

In a nutshell, what these statements boil down to is that the pipeline has allegedly been increased to thirty stages, but it doesn’t seem to do a whole lot of good in getting power and thermal requirements down to what would be historically expected.

Putting Pipelines In Its Place

Attention has been focused on the extension of the CPU pipeline. This is in-and-of-itself not inevitably a bad thing.

Normally, extending the pipeline is a design tradeoff. If you make the CPU do less per clock cycle, you can squeeze in more clock cycles per second. That’s the plus side. The negative side is that you take bigger performance hits when the branch prediction units in a CPU guesses wrong and has to do a do-over on data.

Historically, extending the pipeline is a net plus. That means CPU speed can be increased enough to offset the negative, and then some.

Does that mean extending the pipeline for Prescott has to be a good thing? No, it doesn’t have to mean that at all, either.

It all depends on why you’re extending the pipeline.

If you have a CPU that can run at 4GHz, and extending the pipeline means you can reach 5 or 5.5GHz, that’s one thing, and probably a good thing. On the other hand, if you have a CPU that is supposed to do 4GHz, but can only do 3GHz, and this is the only way you can make it to go to 4GHz, that’s not a good thing, in fact, it’s pretty bad.

If I had to bet now, I would now bet on the latter.

So a longer pipeline isn’t the problem even if there is a problem. It is just a symptom of a deeper problem.

What might that deeper problem be?

The Core Problem: Dubious Dielectics?

Increasing the pipeline of Prescott from twenty to thirty stages at the last minute is a pretty (add expletive of your choice) desperate move by Intel. After all, the latest 130nm Northwoods don’t fall all that short of the 4GHz goal right now. Historically, just a die-shrink ought to get Intel well over 4GHz with no fuss.

It indicates that there is something hellaciously more wrong than 865/875 MOSFETs being a little on the wimpy side. Combine that with the whispers about power consumption, which indicate that the normal big drop in wattage you get from a process shrink just isn’t happening, and there’s something very fundamentally wrong.

What is this likely to be?

Prescott represents a change in how Intel chemically makes CPUs. Prescotts are supposed to be made using strained silicon and low-k dielectrics. These are changes meant to speed transistor and chip speeds, and is rather trickier stuff to use than older higher-k dielectrics.

A lot of other chip manufacturers had problems with 130nm production, and a big reason for that happening was the use of low-k dielectrics at 130nm.

Well, Intel has taken the plunge with low-k at 90nm, along with strained silicon. That’s the biggest difference between Prescott and Northwood: what it’s made from.

And when things aren’t working out, they have to be considered the prime suspects for the reasons why it’s not working out.

Especially when Tejas is being pushed back six months, ostensibly to come with a better strained silicon/low-k formula.

The Building Blocks Are Broken

The last-minute extension of the pipeline and the apparent reason for the delay in Tejas add the last necessary pieces to the puzzle. There may indeed also be some electrical problems that would or would not be there, but the core problem is most probably with the electrochemistry.

And that is a very bad problem, indeed, and not one that can be “fixed” by waiting for socket T.

Immediate Ramifications

Yes, extending the pipeline will hurt performance. If you want a ballpark figure for the moment, figure a Prescott will do about 10% less on average clock-for-clock than a Northwood.

The real test will come when the socket 478 and socket Ts get overclocked. If what I’ve said is true, these chips will probably get into the low 4GHz range. However, it’s going to be a devalued 4GHz which will hardly be any better than the 3.6GHz or so a good Northwood can do today.

I’m assuming that when socket 939 comes, we’ll have a new Hammer stepping which will do a bit better than the current ones.

If I’m wrong, socket 478 Prescott will be rather perkier than a little over 4GHz, and socket Ts should approach and eventually exceed 5GHz.

I really don’t think I’m wrong.

I had said a little while back I was leaning towards buying a socket T system as the least bad upgrade. After this information, not any more.

Intel is going to end up catching a lot of flak for this, including from the mainstream media. After all, they’re going to put out a CPU that will almost certainly be slower than its ancestor.

More importantly, if electrochemistry is causing all this problem, looks like they’re stuck with it for at least a year.

This is pretty bad for Intel. But don’t worry, Intel fans, you’re competing against AMD.

With Enemies Like This, Who Needs Friends?…

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