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
Problem #1: Can't Use Desktop Chipsets
Although the Pentium M uses the same bus as the Pentium 4, there are two limitations that prevent you from using the Pentium M on regular desktop motherboards or chipsets. The first limitation is that the Pentium M's pin-out is quite different from that of the Pentium 4, despite the similarity in number of pins and socket layout to the Socket-478 Pentium 4s. So, even if the Pentium M could fit in a desktop Pentium 4 board, it still wouldn't work - it's like plugging a USB cable into a FireWire port; even if you could make it fit, you're not going to be transferring anything over that cable.What about a simple converter that modifies the pin-out of the Pentium M to be compatible with a Pentium 4? The reason why a Pentium M to Pentium 4 converter can't be reliably made is explained by the second limitation: the I/O buffers on the Pentium M operate at relatively low voltages (1.05V) while the I/O buffers on a Pentium 4 operate at the CPU's core voltage (1.3V+). In order to work properly with a Pentium M, the North Bridge (MCH) must be able to operate at similarly low voltages, which none of the current desktop chipsets are able to do. There is also additional drive circuitry that is present on the chipset to help deal with operating at such low voltages, which aren't present on desktop Pentium 4 chips.
The problems resulting from these two limitations are three-fold:
- Desktop Pentium M motherboards must use mobile chipsets, which are $10 more expensive on average than their desktop equivalents, leading to more expensive motherboards.
- Current desktop Pentium M motherboards are about 6 - 12 months behind the design schedule of mobile Pentium M motherboards, meaning that they are all using Intel's 855GME chipset, unfortunately, and not the latest mobile 915 chipset. The biggest implication here is that this means that all current generation desktop Pentium M motherboards feature only a single channel DDR333 memory controller.
- The upgrade path for desktop Pentium M motherboards will be quite limited. The problem is that you're pretty much guaranteed only to be able to upgrade to faster 90nm Dothan based Pentium M processors, which aren't going to get many speed bumps between now and when they are replaced by Yonah (which won't work in current motherboards). Remember that the Pentium M is designed with a clock speed limit in mind and that limit is very low, so don't expect too many speed bumps between now and the end of the year (our roadmaps indicate only one new speed this year, that's all).
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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.