Verdict: Ultimately, its pretty hard to feel all rosy-cheeked and excited about the Pentium 4e
Intel hasn't made any major revisions to the Pentium 4 architecture since switching from the Willamette to Northwood core in January 2002. Since then, Intel has only made a few minor changes, such as increasing the FSB from 533 to 800MHz and introducing Hyper-Threading.
This year will finally see the relegation of the Northwood core to the history books as Intel launches its successor, the Prescott core. You can tell the difference between the various Pentium 4 cores by looking at the small letter at the end of the name; 'b' denotes a 533MHz FSB Northwood, 'c' an 800MHz FSB Northwood and 'e' an 800MHz FSB Prescott. Whatever happened to the 'd' is a secret that Intel is unwilling to reveal.
Prescott is a very different design to Northwood and the most dramatic change is the lengthening of the pipeline, from 20 stages to 31 stages. Longer pipelines allow for higher clock frequencies, but can also cause problems. Such problems occur when the data being processed in the pipeline is no longer needed and has to be dumped from the chip. The further down the pipe the data is before being dumped, the greater the performance penalty. Consequently, Northwood runs faster than Prescott at the same frequency, as Prescott is doing less work per clock cycle.
To try and counterattack the negative effects of the longer pipeline, Prescott has several other tricks up its sleeve. For starters, the Level 1 data cache has been doubled from 8 to 16KB, although the Level 1 trace cache for storing decoded micro-ops remains the same. More dramatically, there's now 1MB of L2 cache and better prefetching to utilise it more effectively.
Prescott is also considerably denser than Northwood because it's built on a 90nm manufacturing process, the first desktop CPU in the world to use such a fine process. Northwood is built on a 0.13micron process (130nm). A smaller process should make it easier to ramp up the clock frequency and help to make the chips cooler because it's possible to run the circuits at a lower voltage. For example, our sample chip runs at 1.375V, while a 2.8GHz Northwood runs at 1.525V. Interestingly, our test CPU was ten degrees hotter than the Northwood at the same frequency under full load - which gives credence to the rumour that Prescott was delayed (it was due last year) because Intel was having problems controlling heat output.
Finally, there's enhanced Hyper-Threading, although application support is still lacking.
Intel will initially launch the Prescott Pentium 4e in four different speed grades: 2.8, 3, 3.2 and 3.4GHz, but was only able to supply a 2.8GHz chip to us in time for this review. To provide a fair, apples-to-apples comparison, we tested the Prescott chip against a 2.8GHz Northwood.
We had our suspicions that the enlarged Level 1 and 2 cache would not be enough to counter the longer pipeline and were proved correct. At the same frequency, the Prescott performed somewhat slower than the Northwood in all of the Custom PC Media Benchmarks. A close examination of the results shows a performance gap of between 2 - 8 per cent. This meant the overall score dropped from 1.33 on the Northwood to 1.27 on the Prescott. We then ran our Wolfenstein: Enemy Territory gaming benchmark, and recorded a similar drop in performance, as the frame rate was 2 per cent slower.
Prescott does have one potential ace up its sleeve though: SSE3. SSE3 is a set of 13 additional instructions that, just like the new instructions introduced in MMX, SSE and SSE2, should help to improve performance in complex multimedia tasks, such as video encoding. However, because it takes so long for developers to write software, it's unlikely we'll see any major applications that take advantage of SSE3 before the year end, although some apps may be patched earlier.
CONCLUSION
Ultimately, its pretty hard - if not impossible - to feel all rosy-cheeked and excited about the Pentium 4e. The architectural changes from the Northwood to Prescott cores are a mixed blessing. Prescott should theoretically scale much higher than Northwood, although it's likely Intel will have moved to the LGA 775 packaging by then. But, if SSE2 is anything to go by, SSE3 should boost performance in a noticeable way. Intel will sell Northwood and Prescott chips at exactly the same price into the trade, but, according to sources close to Intel, supply will be extremely limited for several months at least, which could bump up pricing by as much as 20 per cent. And because it performs slower at the same clock frequency, it's really not worth buying a Prescott chip at this stage.
