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Intel's Pentium 4 600 series processors

Plus: Prescott goes extreme
— 12:01 AM on February 21, 2005

A LITTLE OVER A YEAR ago, while media attention was affixed firmly on the Superbowl, Intel discreetly let slip a brand-new, vastly rearchitected CPU core that, by all rights, should have been called the Pentium 5. The "Prescott" CPU core, as we now know, became somewhat infamous for its particular combination of tepid performance and gluttonous appetite for power (and corresponding prodigious heat production). This was the processor that was supposed to make it to 4GHz and never did, the CPU that convinced Intel that the future was in dual-core designs and "platformization." It may not have been a resounding success or a complete failure, but it was certainly consequential, despite its quiet introduction.

Today, in the dead of early Sunday morning, Intel is meekly unveiling another new Pentium 4 processor core, and it may be just as consequential. The Pentium 4 600 series is a new tier of performance-oriented Pentium 4 processors that will be sold alongside the existing P4 500 series. Based on the Prescott design, the 600-series core adds key features intended to pep up Prescott's performance and curb its power consumption. Not only that, but these are 64-bit CPUs. With the introduction of a 64-bit version of Windows approaching, Intel has finally turned on Prescott's dormant support for the 64-bit extensions to the x86 instruction set pioneered by AMD.

Recent lottery winners will also be pleased to learn of the emergence of a new Pentium 4 Extreme Edition processor. Based on the same new CPU core as the 600 series, this puppy runs at 3.73GHz on a 1066MHz front-side bus, and it has 64-bit support, as well.

Can this new variation of the Prescott core help Intel recapture its supremacy in desktop processor performance? We've had Intel's new CPUs on the test bench for over a week now, and we have some answers.

The Pentium 4 660

What's new
Intel's new CPU core packs fistful of enhancements over the original Prescott core. I'm gonna bust out the bullet points in order to give you the highlights.

  • 2MB of L2 cache — In terms of performance, this is the number-one change. The 600 series and the new Extreme Edition both pack a robust 2MB of L2 cache now, twice as much as older P4s. The extra on-board cache memory will boost performance in situations where the CPU can avoid accessing slower main memory in order to complete a task. The benefits of extra cache RAM aren't universal, though. Some programs cycle through quite a bit more data than 2MB, and won't benefit from additional cache. Others already fit nicely into a smaller cache, and therefore aren't helped by more of the same. We'll explore this dynamic in our performance tests, of course.

    The addition of another meg of L2 cache raises the new core's transistor count to roughly 169 million, well above the 125 million transistors in the original Prescott core. Thanks to Intel's 90-nanometer manufacturing process, the chip isn't incredibly large by today's standards. Die size is up from 122mm2 to 135mm2. Larger chips generally tend to consume more power and generate more heat, all other things being equal. In this case, though, other things are not entirely equal.

  • Enhanced power management — The 600 series finally brings Intel's Enhanced SpeedStep technology to the desktop. Previously used in Intel's mobile processors, SpeedStep dynamically scales CPU clock speed and voltage in response to load. The new core also includes the enhanced halt state from the Pentium 4 500J-series processors we reviewed not long ago. I'll explain more about how these new power management features interact shortly.

  • 64-bit extensions — Intel has dubbed its 64-bit extensions EM64T, for Extended Memory 64 Technology, but they are really just a functional clone of AMD's AMD64 extensions, first implemented in the Opteron processor a couple of years ago. With these extensions and the right software, including a 64-bit operating system and applications compiled to use 64-bit extensions, the Pentium 4 gains the ability to address more than 4GB of RAM (without any workarounds). AMD64 and EM64T also include some additional registers, or local slots on the chip for storing data, that should provide a bit of a performance boost in 64-bit applications. The move to 64-bit computing won't bring revolutionary new heights of CPU performance overnight, but it will prevent us all from bumping our heads on the 4GB memory address space limitation in the next few years.

  • Execute Disable Bit support — Like the 500J series processors, the new Intel core includes support for the Execute Disable Bit, also called the No Execute (NX) bit by AMD. Operating systems can use this "no execute" capability to help minimize the risks of certain types of security threats, such as buffer overflow exploits.
Notably missing from the features list of the 600 series is support for faster 1066MHz front-side bus speeds. Instead, the P4 600s will roll on an 800MHz bus, as did their predecessors. The 1066MHz bus is reserved for the Pentium 4 Extreme Edition processors.

Speaking of which, the Pentium 4 3.73GHz Extreme Edition is quite a change from the 3.46GHz model. This new Extreme Edition is based on the same Prescott-derived CPU core as the 600 series, while previous Extremes were based on the pre-Prescott "Gallatin" core. That means the new Extreme Edition now has a longer, 31-stage main pipeline and lower clock-for-clock performance. The old EE's L3 cache is gone by the wayside, replaced by the beefy 2MB of L2 cache in this new core. The new EE can also do the 64-bit dance, but it doesn't have the fancy power management or enhanced halt state that the 600 series does. The EE 3.73GHz ought to outperform the 600 series thanks to its 1066MHz bus and higher clock speed, but whether it can outperform the EE 3.46GHz is another question.

The original Prescott die (left) versus the new die with 2MB of L2 cache (right)