Introduction

Historically, mobile CPUs were designed as derivatives of their desktop counterparts. You'd usually cut down on the cache, lower the clock speed and voltage, and maybe tweak the package a bit, and you'd have your mobile CPU. For years, this process of trimming the fat off of desktop (and sometimes server) CPUs to make mobile versions was the industry norm - but then Timna came along.

Timna was supposed to be Intel's highly integrated CPU to be used in sub-$600 PCs, which were unheard of at the time. Timna featured an on-die memory controller (RDRAM however), integrated North Bridge and integrated graphics core. The Timna design was very power-optimized and very cost-optimized. In fact, a lot of the advancements developed by the Timna team were later put into use in other Intel CPUs simply because they were better and cheaper ways of doing things (e.g. some CPU packaging enhancements used in the Pentium 4 were originally developed for Timna). What set Timna apart from Intel's other processors was that it was designed in Israel by a team completely separate from those who handled the desktop Pentium 4 designs. Intel wanted a fresh approach for Timna, and that's exactly what they did get. Unfortunately, after the chip was completed, the market looked bleak for a sub-$600 computer and the chip was scrapped, and the team was reassigned to a new project a month later.

The new project was yet another "out-of-the-box" project called Banias. The idea behind Banias was to design a mobile processor from the ground up; instead of taking a higher end CPU and doing your best to cut down its power usage, you started with a low power consumption target and then built the best CPU that you could from there. With a chip on their shoulder (no pun intended) and a bone to pick with Intel management, the former Timna team did the best that they could on this new chip - and the results were impressive.

Banias, later called the Pentium M, proved to not only be an extremely powerful mobile CPU, but was also one of Intel's most on-time projects - missing the team's target deadline by less than 5 days. For a multi-year project, being off by 5 days is nothing short of impressive - and so was the CPU's architecture. While many will call the Pentium M a Pentium 3 and 4 hybrid, it is far from it. Intel knew that the Pentium 4 wasn't a low-power architecture. The Pentium 4's trace cache, double-pumped ALUs, extremely long pipeline and resulting high frequency operation were horrendous for low power mobile systems. So, as a basis for a mobile chip, the Pentium 4 was out of the question. Instead, Intel borrowed the execution core of the Pentium III; far from the most powerful execution core, but a good starting point for the Pentium M. Remember that the Pentium III's execution core was partly at fault for AMD's early successes with the Athlon, so performance-wise, Intel would have their work cut out for them.

Taking the Pentium III's execution units, Intel went to town on the Pentium M architecture. They implemented an extremely low power, but very large L2 cache - initially at 1MB and later growing to 2MB in the 90nm Pentium M. The large L2 cache plays a very important role in the Pentium M architecture, as it highlights a very bold design decision - to keep the Pentium M pipeline filled at all costs. In order to reach higher frequencies, Intel had to lengthen the pipeline of the Pentium M from that of the Pentium III. The problem with a lengthened pipeline is that any bubbles in the pipe (wasted cycles) are wasted power, and the more of them you have, the more power you're wasting. So Intel outfitted the Pentium M with a very large, very low latency L2 cache to keep that pipeline full. Think of it like placing a really big supermarket right next to your home instead of having a smaller one next to your home or a large one 10 miles away - there are obvious tradeoffs, but if your goal is to remain efficient, the choice is clear.

A large and low latency L2 cache isn't enough, however. Intel also equipped the Pentium M with a fairly sophisticated (at the time) branch prediction unit. With each mispredicted branch, you end up with a large number of wasted clock cycles and that translates into wasted power - so beef up the branch predictor and make sure that you hardly ever mispredict anything in the name of power.

The next thing to tackle was chip layout. Normally, CPUs are designed to exploit the fastest possible circuits within the microprocessor, but in the eyes of the power conscious, any circuit that could run faster than what it needed was wasting power. So, the Pentium M became the first Intel CPU designed with a clock speed wall in mind. Intel would have to rely on their manufacturing to ramp up clock speed from one generation to the next. This is why it took the move from 130nm down to 90nm for the Pentium M to hit 2.0GHz even though it launched at 1.6GHz.

There were other advancements made to the core to improve performance, things like micro-ops fusion and a dedicated stack manager are also at play. We've talked in detail about all of the features that went into the first Pentium M and its later 90nm revision (Dothan), but the end result is a CPU that is highly competitive with the Athlon 64 and the Pentium 4 in notebooks.

Take the first Pentium Ms for example; at 1.6GHz, the first Pentium Ms were faster than 2.66GHz Pentium 4s in notebooks in business and content creation applications. More recently, the first 2.0GHz Pentium Ms based on the Dothan core managed to outperform the Pentium 4 3.2GHz and the Athlon 64 3000+. Pretty impressive for a notebook platform, but what happens when you make the move to the desktop world?

On the desktop, the Pentium 4 runs at higher clock speeds, as does the Athlon 64. Both the Pentium 4 and Athlon 64 have dual channel DDR platforms on the desktop, unlike the majority of notebooks out there. Does the Pentium M have what it takes to be as competitive on the desktop as it is in the mobile sector? Now that the first desktop Pentium M motherboards are shipping, that's why this review is here - to find out.

Problem #1: Can't Use Desktop Chipsets
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  • bob661 - Tuesday, February 08, 2005 - link

    The only problem with this chip is that the marketing is oriented towards the mobile market and therefore not a direct competitor to the A64. It would be nice if it was. It might bring some cats out of the bag on the AMD side. Competition in the marketplace is good for us all. Reply
  • jvrobert - Tuesday, February 08, 2005 - link

    Really, AMDroids, get a grip. You're all excited because the AMD chips beat a mobile processor pretty handily, and because you are making some silly assumption that the Pentium-M in its current form is Intel's "last chance".

    First, Intel doesn't need a last chance. They make enough money to make AMD look like a Mexico City taco stand. So enough of those delusions of grandeur.

    But on a technical front, if Intel ramps the clockspeed up to the 2.8 range (easy), and releases a desktop class chipset for the Pentium M it would match or exceed any current chip. And these are _basic_ steps. What if they made more improvements?
    Reply
  • jvrobert - Tuesday, February 08, 2005 - link

    Reply
  • bob661 - Tuesday, February 08, 2005 - link

    #45
    You are a rock. The point of the article was to compare the P-M to desktop CPU's because most of us here wanted to know it will perform. And you know what? It performed very nicely.
    Reply
  • classy - Tuesday, February 08, 2005 - link

    I just can't help but to laugh at some folks. Its a nice chip but clearly not in the A64 ballpark. Its that simple. As far as the 2.8 oc, that was only accomplished in one reveiw. All the reviews show the same thing you have oc so it can it compete. What's interesting though is most of these Intel fanboys don't want to see a comparison of an oc'ed A64 vs a Dothan. Smoke city :) Reply
  • FrostAWOL - Tuesday, February 08, 2005 - link

    IF the Pentium-M and P4 are electrically incompatible then someone please explain this:

    HP Blade system Pentium-M with Serverworks GC-SL chipset
    http://h18000.www1.hp.com/products/servers/prolian...

    FrostAWOL
    Reply
  • jae63 - Tuesday, February 08, 2005 - link

    Great review & of interest to those of us with HTPCs. Too bad the price point is so steep.

    One minor correction on page 11:
    "The Pentium M does a bit better in the document creation tests, as they are mostly using applications that will fit within the CPU's cache. However, the introduction of a voice recognition program into the test stresses the Pentium M's floating point performance, which does hamper its abilities here."

    Actually NaturallySpeaking uses almost no floating point but is very memory intensive. The performance hit that you are seeing is because it uses a lot of memory bandwidth and its dataset doesn't fit in the L2 cache.

    Here's some support for my statement, by the main architect of NaturallySpeaking, Joel Gould:
    http://tinyurl.com/6s4mh
    Reply
  • segagenesis - Tuesday, February 08, 2005 - link

    #43 - I think you have the right idea here. This processor is not meant to be performance busting but rather a low energy alternative to current heat factories present inside every P4 machine. I would love to have this in a HTPC machine myself but the cost is still too damn high. Hopefully higher production will bring the cost down. Reply
  • Aileur - Tuesday, February 08, 2005 - link

    I guess the pentium M isnt ready (yet) for a full featured gaming machine, but with that kind of power, passively cooled, it would make for one hell of an htpc. Reply
  • PrinceGaz - Tuesday, February 08, 2005 - link

    #45- It was not an unfair review, on the contrary it seemed very well done. The reason the P-M was compared with fast P4 and A64's is because they cost about the same.

    Maybe someone else buys your computers for you, but most of us here have to spend our own money on them so cost is the best way to decide what to compare it with.
    Reply

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