Prescott pipeline longer than Northwood's – Intel

New P4s may be slower than old

Intel has confirmed that its 'Prescott' processor features a long instruction pipeline than the current desktop Pentium 4. The announcement lends weight to media claims that the chip will run more slowly than its predecessor.

When Intel launched the P4, it upped the chip's pipeline to 20 stages, from ten. The move was widely criticised at the time because the greater the number of stages the more drastic the effect of pipeline stalls.

A stall takes place when the chip suddenly finds it has the wrong data. Sophisticated circuitry allows the chip to predict where a program is likely to take it and to pre-load and pre-process instructions that may be needed in the future. When it works, it keeps the processor fed with instructions, so it works more efficiently. When it goes wrong, the chip has to flush out the pipeline to rid itself of instructions and data it doesn't need.

And the longer the pipeline, the more pre-processed instructions and data have to be dumped before the chip can begin loading and handling the correct instructions.

Since pipeline stalls are inevitable, it might be sensible to limit the length of the pipeline. But longer pipelines are necessary to make ever higher clock frequencies operate efficiently. Ramp the clock speed high enough, and pipeline stalls and their repair take less time. That's what's happened as the P4 went beyond 3GHz - the kind of speed the 20-stage pipeline was designed for.

Other chip makers, less keen on offering as steep a clock frequency growth curve as Intel, can get away with shorter pipelines, but need other ways of boosting a chip's performance, such as the ability to handle multiple instructions at the same time.

An Intel spokesman this week told Cnet that Prescott has a longer pipeline that Northwood, the current, 130nm version of the P4. He wouldn't say how much longer the Prescott pipeline is, but 30 stages seems a reasonable estimate.

And just as the PIII proved faster than the early P4s in some applications, it's likely that Northwood will similarly prove faster than Prescott, which has clearly been designed for speeds of the order of 4GHz - which is what Intel's internal roadmaps suggest it's destined for by the end of this year. ®

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