Intel's Pentium M on the Desktop - A Viable Alternative?
by Anand Lal Shimpi on February 7, 2005 4:00 PM EST- Posted in
- CPUs
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.
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bluesdoggy - Tuesday, February 8, 2005 - link
...in the mobile world, the Pentium 4 and Athlon 64 are often castrated or limited either by low clock speeds...Mommy, is that processor a steer?
valnar - Tuesday, February 8, 2005 - link
As usual, an unfair review. Comparing a 2.0Ghz 400FSB laptop CPU against 3.0Ghz desktop heatmonsters? Of course it won't beat them. But look at how well it does, and probably would do (if reviewed correctly) against Pentium 4 2.4-2.8Ghz CPU's. Considering the ultralow power it needs and lack of heat it generates, this WILL be the hot (err... cool) ticket for Shuttle XPC's and the like in the near future. For anyone who doesn't need the fastest processor at the moment, the Banias designers did a fantastic job.EODetroit - Tuesday, February 8, 2005 - link
Great article, its about time that you did this one. And you compared both P-M motherboards on the market, I don't remember the other web sites doing that.You stated that the P-M won't scale, and that's the reason this isn't Intel's desktop future. One thing though... Intel's other desktop CPUs aren't going to scale much this year either. In fact, on a percentage basis, the P-M might actually scale more this year than the various P4-Kiln edition CPUs after all.
Combine that with a mobile-915 chipset for the desktop, and therefore the elimination of the huge memory bottlenecks (and hopefully a little more voltage adjustments) and all of the sudden we may see all those Losses and Ties turn into Ties and Wins.
Whatever happens, don't be the last enthusiast site to review the mobile-915 desktop motherboards when they arrive, like you were with this. We need a trusted source to know what to buy.
mickyb - Tuesday, February 8, 2005 - link
The performance per watt is awesome. Great for SFF. The article is good, but until there is a newer chipset for this CPU, we won't be able to determine a final performance ruling. I am dissappointed in the lack of desktop MB offerings. This will be the challenger to the MAC mini in near future. Someone will be putting laptop components in a box and call it done.I found a couple of things interesting. Taking the memory out of play, it seems the A64 is still better optimized. L1 cache of Northwood is pretty impressive. AMD has an opportunity to improve performance just by improving the L2 cache latency.
I really don't think the Pentium-M limits are around 2.6 GHz by the end of the year. At 22W, this could probably reach higher speeds. I think the upper limit that Intel is publishing is in context of a laptop and the cooling challenges in that platform. If you put a chip in a DT, then it is a different story.
AtaStrumf - Tuesday, February 8, 2005 - link
A great article! Another Anand classic :-)I'd just like you to add an Athlon XP 3200 to the lineup and at least one more Newcastle (which is just the most popular A64 at the moment ;-) May I suggest a 3000+ 2,0 GHz/512/1CH? With just one dot on the graph extrapolating anything becomes a nightmare :-(
As for P-M it's one hell of a CPU considering it's limitations and we just can't stop wondering what it could become if Intel decided to remove them. Sonoma will party answer that question, but unfortunately the ultra low voltage cap will still remain, so we may never really know.
On the other hand I think an A64 will still be a nice enough desktop CPU so we really have no need for P-M on the desktop side of things. With Lancaster-Turion supposedly on S754 we may be in for a very nice successor to 2500+ Mobile, so to hell with P-M >;-)
bob661 - Tuesday, February 8, 2005 - link
C'mon guys. These tests aren't showing that the P-M is crap, just not what we originally thought it was. I am surprised as hell at these results. For a laptop CPU, it still kicks ass. And with two A64 and three AXP machines, I am no Intel fanboy.paulsiu - Tuesday, February 8, 2005 - link
For folks who want to have a mobile chip lower power solution, why not just go to the mobile Athlon 64? The CPU performance should be about the same as their desktop counterpart (at least the socket 754 version) and you can often use the same motherboard as the desktop.The Pentium Mobile idea seems nice, but I can't imagine spending $300 on a board that contains outdated technology.
MIDIman - Tuesday, February 8, 2005 - link
Superb article.Granted, this is a "desktop" review, but I think the P-M is a completely different world from the P4-775 and A64, and I'm not entiely sure how people can compare them. This was built to be a portable solution and has been moved to desktop. Put that into account, and you have an extremely capable system that is silent, passive, and can be extremely small (matx here, but ITX is out there). I'm just trying to figure out why I didn't just read a Sonoma-based review, since it is out and being made (i.e. Dell's new 6000 laptop), or at least a 2.2ghz Dothan.
I think Sonoma will bridge a bit of this performance gap, but consdering that these types of chipsets and CPUs will always be low voltage, I think we'll always see places where its performance is maybe not up to par, but well worth every penny for small and silent with desktop performance. THey'll only get smaller and faster, and IMHO, this is pretty damn close to desktop performance.
muddocktor - Tuesday, February 8, 2005 - link
I agree withpost #36 about the benchmarks seemingly being picked to go for the P-M's weaknesses, but I guess that's how you get article hits. ;) I do fully agree that the present motherboards and chipsets they use hold back the perfromnace quite a bit; it might be a different story when the new mobile 915 chipset mATX boards come out for desktop use though.One glaring weakness in this comprehensive test though is the utter lack of numbers on system power usage and noise. If I were deploying a whole bunch of new systems for a corporation, I would give serious thought to a P-M setup even though the initial outlay would be more than a comparable P4 setup due to the decreased wattage used by the P-M system and the resultant heat from operation being much less, leading to lower environmental costs. Face it, in typical office applications the P-M is more than powerful enough for 90% of the users for the forseeable future and if your company has hundred or thousands of computers, the power saving should more than compensate for the higher pricetag of aquiring the P-M systems.
Anand, when the new mobos based on the mobile 915 chipset come out, you need to revisit Dothan and it's performance.
msva124 - Tuesday, February 8, 2005 - link
What were people expecting out of the pentium m? I have always multiplied the Mhz by 1.5 and used that number as the speed rating. So for instance the 2.0Ghz Dothan would be 3000+. The benchmarks confirm this - with the exception of one or two tests, it met or exceeded the performance of the Athlon 64 3000+.Whenever it was discussed as a desktop alternative I always assumed the implication was that this would be way off in the future, once clock speeds were ramped up.