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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  • terrywongintra - Monday, February 2, 2004 - link

    anybody benchmark prescott over northwood in entry-server environment? i'm installing 3 servers later by using intel 875p (s875wp1-e) entry server board n p4 2.8, need to decide prescott or northwood to use.
  • sipc660 - Monday, February 2, 2004 - link

    i don't understand why some people are bashing such a good inovation that was long overdue from intel.

    a pc that doubles as a heater and at only 100-200W power consumption.

    Let me remind you that a conventional fan heater eats up a kilowatt/hour of power.

    Think positive

    * space reduction
    * enormous power savings (pc + fan heater)
    * extremly sophisticated looking fan haeter
    * extremly safe casing. reduces burn injuries
    to pets and children.
    * finely tunable temperature settings (only need
    to overclock by small increments)
    * coupled with an lcd it features the best
    looking temperature adjustment one has ever
    witnessed on a heater
    * child proof as it features thermal shutdown
    * anyone having a laugh thus far
    * will soon feature on american idol
    the worst singers will receive one p4 E based
    unit each. That should make people
    think twice about auditioning thus making
    sure only true talent shows up.
    * gives dell new marketing potential and a crack
    at a long desired consumer heating electronic
    * amd is nowhere near this advancement in thermal
    thechnology leaving intel way ahead

    hope you enjoyed some of my thoughts

    Other than that good article and some good comments.

    on another note i don't understand why people run and fill intels pockets so intel can hide their engineering mistakes with unseen propaganda, while there is an obvious choice.

    choice is Advanced Micro Devices all until intel gets their act together.

    go amd...
  • Stlr22 - Monday, February 2, 2004 - link

    INTC - "Intel roadmap says Prescott will hit 4.2 GHz by Q1 '05. My guess is that it is already running at 4 GHz but just needs to be fine tuned to reduce the heat."

    Maybe they are trying to keep it under the 200watt mark? ;-)
  • INTC - Monday, February 2, 2004 - link

    I think CRAMITPAL must have sat on a hot Prescott and got it stuck where the sun doesn't shine - that would explain all of the yelling and screaming and friggin this and friggin that going on. "Approved mobo, approved PC case cooling system, approved heatsink & fan - and you better not use Artic Silver or else it will void your warranty..." gee - didn't we just hear that when Athlon XPs came out? It brings to mind when TechTV put their dual Athlon MP rig together and it started smoking and catching on fire when they fired it up the first time on live television during their show.

    Intel roadmap says Prescott will hit 4.2 GHz by Q1 '05. My guess is that it is already running at 4 GHz but just needs to be fine tuned to reduce the heat. I bet the experts (or self proclaimed experts such as CRAM) were betting that Northwood could not hit 3 GHz and look where it is at today. Video card GPUs today are hitting 70 degrees C plus at full load but they do fine with cooling in the same PC cases.
  • CRAMITPAL - Monday, February 2, 2004 - link

    Dealing with the FLAME THROWER's heat issues is only one aspect of Prescott's problems. The chip is a DOG and it requires an "approved Mobo" and an "approved PC case cooling system", a premo PSU cause the friggin thing draws 100+ Watts and this crap all costs money you don't need to spend on an A64 system that is faster, runs cooler, and does both 32/64 bit processing faster. How difficult is THIS to comprehend???

    Ain't no way Intel is gonna be able to Spin this one despite the obvious "press material" they supplied to all the reviewers to PIMP that Prescott was designed to reach 5 Gigs. Pigs will fly lightyears before Prescott runs at 5 Gigs.

    Time to GET REAL folks. Prescott sucks and every hardware review site politely stated so in "political speak".
  • Stlr22 - Monday, February 2, 2004 - link


    It's ok man. It's ok. Everything will be alright.

  • scosta - Monday, February 2, 2004 - link

    #38 - About your "Did anyone catch the error in Pipelining: 101?".

    There is no error. The time it takes to travel the pipelane is just a kind of process delay. What matters is the rate at witch finished/processed results come out of the pipeline. In the case of the 0.5ns/10 stage pipelane you will get one finished result every 0.5ns, twice as many as in the case of the 1ns/5 stage pipeline.

    If the pipelines were building motorcycles, you woud get, respectively, 1 and 2 motorcycles every ns. And that is the point.
  • LordSnailz - Monday, February 2, 2004 - link

    I'm sure the prescotts will get hotter as the speed increases but you can't forget there are companies out there that specializes in this area. There are 3 companies that I know of that are doing research on ways to reduce the heat, for instance, they're planning on placing a piece of silicon with etch lines on top of the CPU and run some type of coolant through it. Much like the radiator concept.

    My point is, Intel doesn't have to worry about the heat too much since there are companies out there fighting that battle. Intel will just concentrate on achieving those higher speeds and the temp control solution will come.
  • scosta - Monday, February 2, 2004 - link

    You can find thermal power information in the also excelent "Aces Hardware" Prescot review here:

    In resume, we have the following Typical Thermal Power :
    P4 3.2 GHz (Northwood) - 82W
    P4E 3.2 GHz (Prescot) - 103W

    Note that, at the same clock speed and with the same or lesser performance, the Prescot dissipates 25% more power than Northwood. This means that with a similar cooling system, the Prescot has to run substancially hoter.

    As AcesHardware says,
    [Q]After running a 3DSMax rendering and restarting the PC, the BIOS reported that the 3.2 GHz Northwood was at about 45-47°C, while Prescott was flirting with 64-66°C. Mind you, this is measured on a motherboard completely exposed to the cool air (18°C) of our lab.[/Q]

    So, what will the ~5GHx Prescot dissipate? 200W ?
    Will we all be forced to run PCs with bulky, expensive, etc, criogenic cooling systems?. I for one wont. This power consumption escalation has to stop. Intel and AMD have to improve the performace of their CPUs by improving the CPU archytecture and manufacturing processes, not by trowing more and more electrical power at the problem.

    And those are my 2 cents.
  • CRAMITPAL - Monday, February 2, 2004 - link

    Prescott will never go above 3.8 Gig. even with the 3rd revision of the 90 nano process. Tejas will make it to just over 4.0 Gig. with a little luck but it won't be anything to write home about either based on current knowledge.

    Intel has fallen and can't get it up!

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