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Intel's Prescott chip gets rebuilt for speed
Last modified: January 21, 2004, 3:35 PM PST
By Michael Kanellos and Kai Schmerer
Staff Writer, CNET News.com


Intel has redesigned a key element in an upcoming version of the Pentium 4 processor to allow the chip to hit higher speeds, but analysts say the change may only incrementally improve performance at first.

The Prescott processor, an enhanced version of the Pentium 4 set for release in early February, will feature a pipeline--the equivalent of a assembly line inside a chip--that will be substantially longer than the 20-stage pipeline on current Pentium 4s, according to sources.

Some sources say that the Prescott pipeline will be around 30 stages.

Extending the pipeline in this manner will likely help Intel achieve its goal of selling desktop chips that run at 4GHz in 2004. (The chip will likely debut at around 3.4GHz, according to sources.)

An Intel representative would not confirm the length of the Prescott pipeline but said it was indeed longer than the one found on the Pentium 4.

"It will have a larger pipeline," the representative said. "The larger the pipeline, you can do less work per clock and speed up the processor."

However, the change will also likely create controversy, in that critics are likely to say that Intel is increasing the speed of its chips merely for show. Consumers and many businesses tend to concentrate on chip speed, measured in gigahertz, when choosing a PC to buy. The higher the speed, the higher the price tends to be.

When it went from the Pentium III to the Pentium 4 in 2000, Intel increased the pipeline from 10 stages to 20.

While the longer pipeline has allowed performance to escalate, the additional speed initially provided marginal benefits at best. The 1.4GHz Pentium 4 that came out at that time didn't provide an advantage over the 1GHz Pentium III with many applications, according to testers, and often underperformed the then-state-of-the-art 1.2GHz Athlon, from Advanced Micro Devices, which featured a 15-stage pipeline.

"All of the people who complained about the 20-stage pipeline will complain again," said Peter Glaskowsky, editor in chief of The Microprocessor Forum, who could not discuss any details of Prescott's pipeline. "Intel seems to be in love with deep pipelines for boosting clock speed."

Glaskowsky speculated that the longer pipeline could exist to provide other performance benefits. The complete underlying architecture of Prescott at this point is not fully understood outside of Intel, he said.

In the pipeline
The pipeline is one of the significant elements in the design of a microprocessor. Chips with short pipelines can accomplish more tasks at once, but they run slower, churning at fewer cycles per second.

Increasing the pipeline allows a chip to break down different tasks into smaller ones, but since there are more individual tasks to accomplish, more work doesn't necessarily get done.

A longer pipeline can also cause more delays as a result of errors. If an operation goes awry on stage 18, the processor has to retrace its steps back to an earlier stage so the calculation can be performed with the correct data. With shorter pipelines, there are often fewer stages to redo.

The contrast can be seen in a comparison between the AMD's Athlon 64 3200 and Intel's 3.2GHz Pentium 4. Both chips roughly provide about the same performance. However, the Athlon XP runs at 2GHz but has a much shorter pipeline.

The emphasis on speed dimmed a bit at Intel in 2003: The Santa Clara, Calif.-based chipmaker only sped up the Pentium 4 family twice. It also emphasized overall performance, rather than raw megahertz, when marketing its Pentium M notebook chips.

Still, the subtle shift seems to have been made more as a result of practical considerations than of philosophical conflicts. The Pentium M runs at a far slower speed than the Pentium 4 (and comes with a shorter pipeline) to conserve energy.

"Despite what they say, they have to stay on the gigahertz train," Mark Margevicius, an analyst at Gartner, said.