Reviewed by: Carl Nelson [02.02.04]
Prices: 3.2GHz: $278
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Preparing for the Future
Today Intel is releasing the latest core revision to their Pentium 4 lineup: Prescott. This is the second revision from the original "Williamette" core P4, and it offers some pretty significant changes to the CPU.
This is a pretty wide launch for Intel; the new Prescott core will be available in 2.8, 3.0, 3.2, and 3.4 GHz, with a version of the 2.8 available without Hyperthreading support. You should have no trouble finding a Prescott up to 3.2 in stores in the very near future, although it would appear that 3.4's are pretty scarce at the moment. It is a rare occasion that Intel launches a new core revision without sending the fastest version to reviewers. I am sure we'll get a 3.4 soon enough though.
Prescott version P4's will be differentiated from Northwoods via the "E" designation. So a P4 3.2E is Prescott, while P4 3.2C would be a Northwood.
Interestingly, Intel is also releasing what will probably be the last Northwood available, at 3.4 GHz. They also have a new 3.4 EE available, but not too many people in their right mind would be willing to shell out for one of those.
So What's New?
Usually when Intel launches a new core revision, it is in preparation for the future, and doesn't offer much immediate benefit. We're going to find out what Prescott means today, and tomorrow as well.
New Manufacturing Process
Intel is the first to perfect the 90nm fab process, and that is what the Prescott is based upon. Because of this, Intel was able to increase the amount of transistors on a smaller die; the Prescott die measures in at 122mm squared with around 125 million transistors, while the Northwood was a bulky 145mm with only 55 million transistors.
Intel has introduced many new techniques to allow them to significantly increase ramp production, and lower fabrication cost. This will benefit Intel immediately, although the end user will have to wait a while to take advantage, as Intel will be able to quickly increase clock speeds and lower prices if they have to.
New SSE Instructions
You knew as soon as AMD started using SSE2 on their CPU's that Intel would quickly have something to top it. Prescott P4's introduce SSE3, which include 13 new streaming instructions that will increase the performance of various operations as programs start to make use of them. We won't see much immediate benefit from these instructions, so once again you can see this as a preperation for the future.
Larger L2 Cache
This is what will probably give us the most immediate performance benefit compared to the Northwood, if any benefit is to be seen. The L2 cache has been increases to 1MB from 512KB. Unfortunately with an increased cache size, you also have to cope with increased access latencies. But again, the Prescott is all about the future, and eventually applications will greatly benefit from a larger cache more than they would from a lower latency. AMD hasn't given up on a 512KB cache though, since they have gone back to making Athlon64's with that amount of L2 cache. Of course, nobody really knows where AMD is going with odd decisions such as this...
Larger L1 Cache
The L1 cache has also been upgraded - to 16KB that is 8 way set associative on the Prescott from 8KB, 4 way associative on the Northwood. Again latencies will be slightly increased, but eventually the size increase will outweigh the drawbacks of that.
To combat against the increased latencies of the larger L1 and L2 cache, Intel has improved the prefetching abilities in Prescott.
Prescott introduces an improved Hyperthreading technology from Northwood. When Hyperthreading was first introduced, we didn't see a lot of applications making heavy use of it. We haven't gone back to look at a non-HT P4 since then, so today we will have another look at Hyperthreading, using a pair of Northwood CPU's and Prescott CPU's of matching speeds. This way, we should find out exactly what HT does for us, and what kind of difference Prescott's improvements make.
Again we are seeing Intel's dedication to the future with the Prescott. They have significantly lengthened the data pipeline to 31 stages, from 20 with the Northwood. We know that this will allow them to increase clock speeds, which seems to be the way Intel wants to go. AMD is trying to get away from that, and they opt to keep their pipelines short, allowing for higher performance-per-clock speed. In the end, clock speed itself doesn't matter to us, since competing products are priced the same anyway.
The issue with a longer pipeline TODAY is the clock speed of Prescott is no higher than that of Northwood, and so we will find that Northwood will perform better clock-for-clock than Prescott in many applications, much the same way Athlons do (although to a much lesser extent, of course).
As you can see, Prescott is indeed a significant revision. If this doesn't warrant a name change to "Pentium 5", I can't wait to see what will!
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