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Intel ‘Prescott’ 90nm Pentium 4

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Review The year's not even a month old and already the CPU market is shaping up for another exciting 12 months for the consumer. Following AMD's speed bumped Athlon 64 3400+ on 6 January it's now Intel's turn to play its hand and introduce the latest incarnation of the Pentium 4 processor, Prescott, writes Wayne Brooker.

OK, so we've established that there's a new core with a new name, but what exactly makes a Prescott a Prescott? Well, let's run through some of the key features and what they mean to you and I.

Intel Prescott CPU

Prescott is built using the existing NetBurst architecture though it's created using a 90nm process, which in layman's terms means Intel has used improved technology to create a significantly smaller die and as a result is able to put the same elements in a much smaller area than was previously possible. This can bring with it several benefits over the existing 130nm technology in that Intel is able to reduce the core size, which leads to more cores per wafer and thus lower production costs. Alternatively additional features such as extra on-die cache can be added without the core size becoming uneconomically large and expensive to produce. There are other benefits to shrinking the core, including lower power consumption and as a rule faster and more predictable performance too.

Although the 90nm process brings with it all these benefits, Intel has taken further steps to safeguard the future of the Prescott-based Pentium 4s and build in a little headroom. These steps include the use of strained silicon and a low-k dielectric material.

Take the strain

So what's strained silicon then? Inside the core of every CPU are millions of transistors - in Prescott's case, 125 million. One of the limiting factors that dictates how fast these transistors can switch and therefore how high a frequency a CPU can operate at is the speed at which the individual electrons in the current can transport themselves through the lattice of molecules that make up the silicone. Now, although silicon is a very efficient semiconductor as it is, engineers discovered that by stretching it (straining it) during the manufacturing process, the constituent molecules could be spread apart slightly making it easier for electrons to barge their way through.

Sounds simple and it is, once the technology has been perfected. The great thing from Intel's point of view is that the strained silicon process results in around a 10-25 per cent improvement in drive current depending on the type of transistor, while adding only about two per cent to production costs. Ultimately this allows processors to run at much higher frequencies than could be otherwise achieved.

If you're a trivia fan, a thousand of the transistors used in Prescott sat side by side would measure approximately the width of a human hair.

So that's strained silicon, what about this low-k dielectric material business? Well, without getting too scholarly, your CPU is actually created in layers which sit on top of each other, and each of these layers is connected to the one above (or below) using metal connectors known as interconnects. Prescott has seven layers. By the addition of a low-k insulator - carbon-doped oxide (CDO) if you care - between these copper interconnect layers, wire-to-wire capacitance is reduced and internal signal speeds are increased. This is particularly important as the manufacturing process shrinks because the whole circuitry becomes much denser and more tightly packed which in turn increases the risk of signal leakage and cross talk, definitely not a great idea in a mission critical server. By adding an extra layer (previous P4s had 6), Intel has been able to draw a compromise between die density and manufacturing costs.

More SSE

So what else is new? Well, I'm sure you've heard of SSE (Streaming SIMD Extensions) and SSE 2, a collection of specialised instructions designed, in Intel's words to "help accelerate a broad range of applications, including video, speech, and image, photo processing, encryption, financial, engineering and scientific applications". Prescott features 13 new ones, one for floating point to integer conversions, five for complex arithmetic, one for video encoding, four for SIMD-FP using AOS (Array-of-Structures) format and two for thread synchronisation. Not surprisingly Intel has christened this SSE 3.

In true Intel fashion there is already support both available and planned to take advantage of SSE 3. Current titles include MainConcept (MPEG 2/4), xMPEG, Ligos (MPEG 2/4), Real (RV9) and On2 (VP5/VP6) with more planned for this year including Pegasys TMPGEnc 3.0, DVD Author, Adobe Premier (to use MainConcept codec), Pinnacle (MPEG Encoder & use DivX Codec), Sony DVD Source Creator ( bundling Pegasis TMPEG Encoder), Ulead (MediaStudio & Video Studio) and Intervideo, Pinnacle, Showshifter, Snapstream (All using DivX Codec)

Last but not least there are several claimed improvements to Prescott's thermal protection capabilities and enhancements to the existing NetBurst and HT (Hyper Threading) technology along with a full 1MB of on-die Level 2 cache, an increased 16KB of Level 1 data cache and a 12KB micro-ops instruction cache. Also in the design brief comes speculative pre-computation that uses idle time to compute calculations it predicts may be needed next or soon.

HyperThreading is a technology that Intel is very proud of and if it's a term that doesn't mean a lot to you, let me quickly explain the principle. It doesn't take too much technical knowledge to realise that having two CPUs beavering away on your behalf will be more productive than having a single CPU, and in essence that's what HyperThreading does. But rather than you needing to buy two separate CPUs, HyperThreading allows your single CPU to divide itself in half and work on two things simultaneously.

Now you may be thinking that most processor can do that, but in actual fact when you're working on two things at once using a non-HT processor it's actually dividing its time back and forth between those two tasks rather than actually working on both at the same time.

Despite how it might seem, HyperThreading isn't twice as fast as non-HyperThreaded processing, in fact it's not even close, but it does help provide a bit of a boost provided your software has been written to take advantage of it. Think of it kind of like employing a guy who can work with both hands at the same time slowly rather than employing two guys who would be significantly quicker.

With three 300mm fabs online for 2004 Intel is obviously planning on getting large numbers of this chip on shelves in the shortest possible time, an area where rival AMD has struggled to perform recently. Announcing new processors is all well and good but if you can't buy them they may as well not exist.

Performance

Our initial batch of benchmarks certainly seem to suggest that Prescott is slower than its equivalent speed Northwood predecessor but I'd suggest it's not enough to be noticeable in general use. However, it's also perfectly reasonable to conclude that these initial results don't portray the whole story in that none of the benchmarks used were written to take advantage of SSE 3, HyperThreading or indeed a great many Prescott-specific features.

Much as we saw when the Pentium 4 was first introduced it may be a case of having to wait a little while for software to evolve in order to see the full benefits of the newly re-jigged architecture.

Impressive though many of Prescott's new technologies are, most have been implemented in order to give Intel room to introduce faster chips without needing to completely redesign it, probably up to around the 5GHz level if speculation proves accurate.

Verdict

So in summary we have a more feature rich if slightly slower processor with far greater headroom to scale to much higher frequencies. And the price? Well, OcUK has the Prescott 3.2GHz listed at £225.00 + VAT while the Northwood 3.2GHz is selling for only very slightly less at £222.50 + VAT. On that basis your buying decision is simple. If you want maximum speed today and nothing else matters buy Northwood. If, however, you don't want to face upgrading again in the short term and you're prepared to gamble that Prescott's new features will catapult it to ever increasing levels of performance as software learns to make the most of the new capabilities, buy Prescott.

Intel 'Prescott' 90nm Pentium 4
Rating 80%
Price See Intel prunes Pentium 4 prices
More info The Intel web site

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