Introduction
With Intel officially forging forward in the desktop consumer space with Pentium 4, with the Prescott-2M update the most recent, there's also been a lively undercurrent of information, benchmarks and desktop boards for or regarding Pentium-M. Intel's mobile processor of choice, currently based on the Dothan processor core, is widely regarded as Intel's best performing processor, not just in terms of work it can do per clock compared to P4, but also in terms of its heat and power profile. Not only does it appear to be fast, but it's also very cool and frugal from the mains.So it's about time I took a look at the processor in its current form, as a first-look, before I get neck-deep in the latest versions to hit the streets, on their newest platform, over the course of this year. Today's article features one of the mid-range Pentium-M processors and I'll try and explain the basics of how it works and what its outgoing desktop platform is like, so you can judge if it has promise as it transitions to a new platform at ever higher bus speeds and external frequencies.
What is Pentium-M?
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If you read about Pentium-M elsewhere, you'll more than likely see it described as something like the Pentium III on steroids, or the PIII with the P4's external bus, or something similar. Descriptions like that aren't too far from the mark.
Here's the current Pentium-M core, Dothan. in a feature table, up against Intel's newest Pentium 4 core, the Prescott-2M. I've chosen the review processor, a 1.7GHz Pentium-M 735, and the Pentium 4 660 at 3.6GHz.
| Pentium-M 735 | 660 | |
| Frequency | 1.7GHz | 3.6GHz |
| Core | Dothan | Prescott-2M |
| Die Size | 84mm² | 135mm² |
| Bus Speed | 100MHz | 200MHz |
| Major pipeline | 10 to 20 stages | 32 stage |
| Transistors | 140M | 170M |
| Process | 90nm | 90nm |
| Caches | 64KiB L1, 2MiB L2 | 16KiB L1 data, 2MiB L2 |
| ISAs and ISs | x86, SSE->SSE2 | x86, x86-64, SSE->SSE3 |
| Power Techs | TM1, EIST, ACPI C0->C3 | TM1->TM2, EIST, ACPI C0->C3 |
| Socket | µPGA479 | LGA775 |
So, some similarities in certain technology areas, but which in reality are miles away from each other in how they go about things. The first thing to talk about is power consumption and heat output. Dothan at 1.7GHz has a TDP (thermal design power) of only 21W. Think about that for a second and let it sink in that it has 140 million transistors, 2MiB of L2 cache memory and is built using a process that's supposedly power unfriendly. Compare that to Prescott-2M at 3.6GHz with a TDP of 115W (and even the slowest 6-series P4 has a TDP of 84W) and it's clear that Pentium-M's core is designed with low power and low heat output as a first consideration.
You can honestly believe me when I tell you that a 735 P-M will run in free air, without any heatsink whatsoever, in a cold enough room. I kid you not. Passive is a piece of cake on the same CPU with average chassis cooling and active cooling can be done with fans that barely move a gnat's fart of air over the heatsink.
Performance at the same clock as a Prescott-2M, or any other Pentium 4 processor, is much higher, too. Notice the length of Dothan's main integer pipeline. While Intel won't divulge the true figure (a good guess would be 15), or give a map of how it's laid out even in general terms, it is known that Pentium-M is based on the core of the PIII, so comparisons to that CPU's 10-stage integer pipeline can be made. With the last 130nm PIIIs stopping around 1.6GHz, a drop to 90nm and a lengthening of the pipeline for Dothan (and Banias, the 1MiB L2 predecessor) has let Intel scale it to 2.2GHz in recent times.
A shorter pipeline means for more work done per clock cycle, which in turn means higher performance than a CPU at the same clock with a longer main pipeline. Pretty straightforward stuff. Pair that with 2MiB of L2 cache memory, technologies like SpeedStep (EIST at that), the four-samples-per-clock bus that the Pentium 4 pioneered (400MHz effective from a 100MHz base clock) and SSE2 support, and you have a processor that's useful outside of cool, quiet and frugal mobile applications.
What's really piqued the interest of the casual PC user has been the performance of Dothan when overclocked, testing showing the core taking on processors like the Athlon FX-55 when pushed a bit. And that's even when held back by the current desktop boards that are available for the CPU.
Then µPGA479 socket mainboards (actually 478 pins, just like the last pin-based Pentium 4 package, but with different pin layout) available just now are based on Intel's mobile 855GME core logic and they're prohibitively priced and too feature-light for many to consider. Let's talk about that core logic a bit more.
855GME; Holding the Pentium-M back on the desktop?
Besides the fact that Intel don't want Pentium-M in the desktop space to start with, they can't really stop the savvy enthusiast building their own PCs around it. The CPUs are freely available as boxed versions and the mainboards are trickling in, but 855 doesn't really cut the mustard.Implementing a single memory channel on the memory controller, providing only AGP4X as a graphics interconnect and interfacing with a range of I/O controllers that are starting to show their age, comparatively, there's not much to shout about. µATX kind of sucks for the real power user, too.
So you can play with EIST on 855GME and performance isn't bad, as you'll see, but it's the next generation Pentium-M core logic and platform from Intel that'll really set the processor free. Dubbed Sonoma, it provides a dual-channel memory controller (supporting DDR and DDR2), ICH6 for I/O and PCI Express for graphics and peripheral I/O. It should also mean a relative deluge of supporting ATX and µATX mainboards from early supporters of desktop Pentium-M such as DFI and AOpen. Hopefully the cost of the boards won't be too prohibitive.
Look out for that 915-based platform in the near future. Fancy a quick look at a Pentium-M inside one of the only desktop µATX mainboards, before I show you the performance? Thought so.
