31 Stages: What's this, Baskin Robbins?

Flip back a couple of years and remember the introduction of the Pentium 4 at 1.4 and 1.5GHz. Intel went from a 10-stage pipeline of the Pentium III to a 20-stage pipeline, an increase of 100%. Initially the Pentium 4 at 1.5GHz had a hard time even outperforming the Pentium III at 1GHz, and in some cases was significantly slower.

Fast forward to today and you wouldn't think twice about picking a Pentium 4 2.4C over a Pentium III 1GHz, but back then the decision was not so clear. Does this sound a lot like our CPU design example from before?

The 0.13-micron Northwood Pentium 4 core looked to have a frequency ceiling of around 3.6 - 3.8GHz without going beyond comfortable yield levels. A 90nm shrink, which is what we thought Prescott was originally going to be, would reduce power consumption and allow for even higher clock speeds - but apparently not high enough for Intel's desires.

Intel took the task of a 90nm shrink and complicated it tremendously by performing significant microarchitectural changes to Prescott - extending the basic integer pipeline to 31 stages. The full pipeline (for an integer instruction, fp instructions go through even more stages) will be even longer than 31 stages as that number does not include all of the initial decoding stages of the pipeline. Intel informed us that we should not assume that the initial decoding stages of Prescott (before the first of 31 stages) are identical to Northwood, the changes to the pipeline have been extensive.

The purpose of significantly lengthening the pipeline: to increase clock speed. A year ago at IDF Intel announced that Prescott would be scalable to the 4 - 5GHz range; apparently this massive lengthening of the pipeline was necessary to meet those targets.

Lengthening the pipeline does bring about significant challenges for Intel, because if all they did was lengthen the pipeline then Prescott would be significantly slower than Northwood on a clock for clock basis. Remember that it wasn't until Intel ramped the clock speed of the Pentium 4 up beyond 2.4GHz that it was finally a viable competitor to the shorter pipelined Athlon XP. This time around, Intel doesn't have the luxury of introducing a CPU that is outperformed by its predecessor - the Pentium 4 name would be tarnished once more if a 3.4GHz Prescott couldn't even outperform a 2.4GHz Northwood.

The next several pages will go through some of the architectural enhancements that Intel had to make in order to bring Prescott's performance up to par with Northwood at its introductory clock speed of 3.2GHz. Without these enhancements that we're about to talk about, Prescott would have spelled the end of the Pentium 4 for good.

One quick note about Intel's decision to extend the Pentium 4 pipeline - it isn't an easy thing to do. We're not saying it's the best decision, but obviously Intel's engineers felt so. Unlike GPUs that are generally designed using Hardware Description Languages (HDLs) using pre-designed logic gates and cells, CPUs like the Pentium 4 and Athlon 64 are largely designed by hand. This sort of hand-tuned design is why a Pentium 4, with far fewer pipeline stages, can run at multiple-GHz while a Radeon 9800 Pro is limited to a few hundred-MHz. It would be impossible to put the amount of design effort making a CPU takes into a GPU and still meet 6 month cycles.

What is the point of all of this? Despite the conspiracy theorist view on the topic, a 31-stage Prescott pipeline was a calculated move by Intel and not a last-minute resort. Whatever their underlying motives for the move, Prescott's design would have had to have been decided on at least 1 - 2 years ago in order to launch today (realistically around 3 years if you're talking about not rushing the design/testing/manufacturing process). The idea of "adding a few more stages" to the Pentium 4 pipeline at the last minute is not possible, simply because it isn't the number of stages that will allow you to reach a higher clock speed - but the fine hand tuning that must go into making sure that your slowest stage is as fast as possible. It's a long and drawn out process and both AMD and Intel are quite good at it, but it still takes a significant amount of time. Designing a CPU is much, much different than designing a GPU. This isn't to say that Intel made the right decision back then, it's just to say that Prescott wasn't a panicked move - it was a calculated one.

We'll let the benchmarks and future scalability decide whether it was a good move, but for now let's look at the mammoth task Intel brought upon themselves: making an already long pipeline even longer, and keeping it full.

Pipelining: 101 Prescott's New Crystal Ball: Branch Predictor Improvements
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  • Jeff7181 - Sunday, February 1, 2004 - link

    I'm going to go out on a limb here and say 2004 is the year of the Athlon-64 and Intel will take a back seat this year unless their new socket will help increase clock speeds. When AMD makes the transition to 90nm I think you'll see a jump in clock speed from them too... and I'm willing to bet their current 130nm processors will scale to 2.6 or 2.8 Ghz if they want to put the effort into it before switching to 90nm.

    Intel better hope people adopt SSE3 in favor of AMD-64 otherwise they're going to lose the majority of the benchmark tests.

    On second thought... the real question is how high will Prescott scale... will we really see 4.0 Ghz by the end of the year? Will performance scale as well as it does with the Athlon-64?

    Right now, looking at the Prescott, the best I can say for it is "huh, 31 stages in the pipeline and they didn't lose too much performance, neat."
  • Barkuti - Sunday, February 1, 2004 - link

    Check out the article at xbitlabs:


    Less technical but with a wider set of tests.
  • Stlr22 - Sunday, February 1, 2004 - link

  • Stlr22 - Sunday, February 1, 2004 - link


    Listen,I just want you to know that everything will be alright. Really, life isn't all that bad buddy. It's not good to keep so much hate inside. It's very unhealthy. We are all family here at the Anandtech forums and we care about you. If you ever need to sit down and talk, I'm ll ears pal. So that your brother doesn't feel left out, here's a hug for him aswell.......

  • KF - Sunday, February 1, 2004 - link

    Yeah, the Inquirer was right about 30 stages. Maybe I should start reading it! However I did read the one where the news linked to an article purporting that an Inquirer reporter had bumped into a person who had overheard an Intel executive say Prescott was 64 bit. Maybe Derek and Anand didn't have the space to squeeze that tiny detail into the review.

    I saw a paper on the Intel site a while ago, seemingly intended for some professional jounal, the premise of which was that it is ALWAYS preferable to make the pipeline longer, no matter how long, while using techniques to reduce the penalties. Like, 100 stages would be a good thing. Right then I knew what one team at Intel was up to. The fact that they didn't explain any new penalty reduction techniques only made it all the more sure what Intel had in the works (otherwise why write the paper?), and that they had the techniques worked out, but still under wraps.
  • ianwhthse - Sunday, February 1, 2004 - link

    Err.. *Cramitpal

    Sorry about that. My mind is wandering.
  • ianwhthse - Sunday, February 1, 2004 - link

    Did we actually just get 26 good posts in before crumpet showed up?
  • FiberOptik - Sunday, February 1, 2004 - link

    I like the part about the new shift/rotate unit on the CPU. Does this mean that prescott will be noticeably faster for the RC5 project? Athlon's usually mop the floor with whatever the Northwood can pump out.
  • eBauer - Sunday, February 1, 2004 - link

    "Botmatch has bots (AI) playing, shooting, running, etc. (deathmatch) while Flyby does not. The number that you should be most interested in is the Botmatch scores."

    No, I am talking about the botmatch scores from previous articles. Well aware of the difference between flyby and botmatch. http://www.anandtech.com/cpu/showdoc.html?i=1946&a... In that article, all CPU's had about 10 more fps than the CPU's in the prescott article.

  • AnonymouseUser - Sunday, February 1, 2004 - link

    "I am curious as to why the UT2k3 botmatch scores dropped on all CPU's... Different map?"

    Botmatch has bots (AI) playing, shooting, running, etc. (deathmatch) while Flyby does not. The number that you should be most interested in is the Botmatch scores.

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