A Review of the Pentium M on the Desktop - Netburst and Dothan
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The Netburst core used in every Pentium4 is not something you want in the mobile space. In fact, it's pretty much the exact opposite. While you can trade performance per clock and get away with it by vastly increasing the speed of the chip, that same trade-off does not work when you have a very limited thermal envelope.
So the Israeli design team took the P6 core, the one that went more or less unchanged from the time of the Pentium Pro up until the end of the Pentium III as their starting point. From there, they took what they knew worked well on the P4 and used that to update the older design to something of a Frankenstein's monster. The end result was a fusion of the Pentium4's ability to rapidly fill the front end of the chip with instructions, with the P6's execution core tweaked for additional clock speed and power saving.
The chip also has less inherent complexity; there's no features like HyperThreading to chew up transistors and power. Instead, that transistor budget is put towards cache, which doesn't take up much power (especially the way it's done in the Pentium-M, where it shuts down sectors that aren't being used) and is easy to create when making the chips themselves. With a full 1MB of L2 on board, the mobile oriented Banias ties a few chips meant for server applications.
What was supposed to quickly follow it out the door was Dothan, a 90nm remake with even more cache and lower voltages to boot. Unfortunately as most of you are aware, Intel's 90nm process had delays -- then when it did appear, it was much hotter than expected as well as leaking power. This is undesirable in a mobile chip.
After a lot of waiting around, a revised Dothan made its way onto the scene. Sporting 2MB of cache, a higher potential for speed, and even more power saving trickery, Dothan looks like a winner. But of course, that's in the laptop realm, where the competition is less than fierce, and it's adapted perfectly to the conditions of life on the move. What happens when you toss one in a desktop?
What happens is that suddenly the other guys aren't playing with one hand tied behind their back. On the desktop, using a weighty heatsink isn't taboo, it's necessary. 92mm fans? Sure. As a result, a 30W power budget is 1/3 of what the big boys have to play with.
The other problem is all the toys surrounding the chip. Or in the case of the Pentium-M, a lack thereof. In a laptop, "feature content" is a limited factor. There's just not much room to stuff in anything other than the essentials. As a result, the i855 chipset that goes hand in hand with Dothan is carrying an airgun compared to the Pentium4s and Athlon64s 12 gauge.
Memory support is rather weak, there is no PCI Express, high speed interconnects between chips, or massive support for overclocking. This is a whole other level of competition. At the moment, the Pentium M is going to have to survive solely on its performance and low heat output, because it's not going to be able to compete on features alone -- or price for that matter. Luckily, both of those are fairly important factors to a growing number of buyers.
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