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Pentium 4: In Depth
Copyright (C) Darek Mihocka.


Introduction

The Pentium 4 processor - Intel's "next generation" of processor to succeed the Pentium III and Celeron processors - was released on November 20 2000. This was an important day in CPU history and for Intel. Some OEM web sites such as Gateway.com claimed it to be "the most powerful processor available for your PC". Unfortunately, that claim was far from true in November 2000.

Despite a huge pavilion at COMDEX Las Vegas in November 2000 to launch the product and system prices that launched in the $3500 to $4000 price range, the Pentium 4 failed to outperform existing Pentium III and AMD Athlon system. Intel engineers themselves in a December 2000 interview at http://www.eet.com/story/OEG20001213S0045 admitted that the original Pentium 4 processor failed short of the published specification of that processor with entire sections (such as the planned L3 cache) missing or being trimmed down.

I'm a very curious guy whenever a new microprocessor is released. How fast does it run? What new features does it have? How much does it cost? What changes do we need to make to our products to take advantage of the new processor? Those are some of the questions I try to answer.

So I did with the Pentium 4 what I do with every new processor - I studied its performance and compared it against existing processors. After a month of close study, I understood why it was easily losing benchmarks to the older AMD Athlon processor and even to the Pentium III.

Although the road was a long one, involving changes to the design and manufacture of the Pentium 4, severe price reductions, the release of new compiler tools, and code rewrites by third party developers, the Pentium 4 has ultimately reached the end of its life.

Starting in March 2003 with the release of "Centrino" branded Pentium M (M is for "mobile") notebook computers, the original Pentium III design was brought back to life in mobile form an even better than before - low clock speed to conserve power, large L2 cache to reduce memory traffic, highly efficient instruction decoder and out-of-order execution core that executes up to three instructions per clock cycle.

On average, at the SAME clock speed, say, 2.0 GHz, the Pentium M core executes up to twice as many instructions per second as a Pentium 4. In fact it even outperforms AMD's technology, as our SoftMac benchmark numbers show.

It's been a good 20+ years since I started studying computer architecture in college and started my career path down the road of working on software CPU emulators and virtual machine. And a lot has changed since then - a thousand fold increase in clock speed and memory size, out-of-order execution, multimedia extensions, etc - and yet the PC of today is able to reliably run MS-DOS code written literally a quarter of a century ago.

This kind of stuff really fascinates me of course, since understanding how a CPU works internally allows one to better simulate it in software. I was delighted to read a couple of recent books while I was on my winter vacation. Both of these books are available from amazon.com and come highly recommended by me if your goal is to understand the whole AMD vs. Pentium III/M vs. Pentium 4 issue:

The first book is The Pentium Chronicles, written by Bob Colwell, one of the men who designed and built the microprocessor core that is after 10 years still at the heart of today's latest Centrino laptops and yes, even the new Apple Mac Mini and Apple iMac Duo. The book is a fascinating behind-the-scenes look at the thought process that went into the redesign of the Pentium processor.

For the more technically minded I recommend Modern Processor Design, a textbook that covers numerous popular microprocessor architectures and includes a very technical chapter written by Bob Colwell on the P6 core. There is also a chapter on the 64-bit PowerPC processor which is the predecessor to the chips used in today's Xbox 360, Playstation 3, and Nintendo Revolution game consoles.

I originally started posting my thoughts and analysis of the Pentium III, Pentium 4, and AMD Athlon processors back in late 2000. Those three architectures are still alive and kicking and much of my analysis from 2000 still applies today to today's chips.

My apologies go out to any Intel engineers I may have offended with my past comments about the Pentium 4. I'm not a chip designer and certainly shouldn't be telling them how to do their jobs. With 6 years of hindsight I can understand now (but did not in 2000) why Intel did certain things the way they did, why the Pentium 4 was designed the way it was and why it ended up behaving the way it did.

Below are links to my three postings from late 2000 to late 2001 on the topic of the Pentium 4 and what I understood of it back then. WARNING! I would highly recommend that you read the above two books first before delving into this very geeky territory!

(Dec 27 2000) ROUND 1 - The first Pentium 4 systems

(March 3 2001) ROUND 2: Pentium 4 takes on Xeon and Athlon

(Oct 22 2001) ROUND 3: Pentium 4, Athlon XP, Pentium III Tualatin - which runs Windows XP best?

(Sept 3 2007) NO EXECUTE! Taking the AMD/Intel issues to a whole new level


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