Introduction
In March of 2003, Intel officially launched its Centrino mobile computing platform, at the center of which was a new x86 processor designed specifically for the mobile computing space: the Pentium M. That the world's largest microprocessor maker had taken the unprecedented step of designing a mobile-oriented processor for the world's most popular instruction set architecture (ISA) was clear sign that the mobile revolution had finally arrived. Indeed, just two months prior to Centrino's launch Steve Jobs had declared 2003 "The Year of the Laptop," in clear recognition of the growing size and influence of the portable computing market.
By 2003 the mobile revolution had been some time in the making, though, and it was helped along by Transmeta's Crusoe. TM put battery life on the map as a key issue for mobile products, and they succeeded in convincing both Intel and AMD that in many situations, portability and flexibility are more important to mobile users than performance. Not long after Crusoe was unveiled, Intel got mobile religion in a big way, and they put together a team in Israel to work on a new microprocessor architecture designed specifically for the mobile space. The original rumors around Banias, as the processor was codenamed, had Intel starting over completely from scratch and designing a new CPU from the ground up; the new chip was said to have little in common with either the PIII or the P4. As it turns out, these rumors were wrong.
Banias, or the Pentium M (as it's now called) is clearly the latest and greatest version of Intel's venerable "P6" microarchitecture, on which Intel's 32-bit desktop chips from the Pentium Pro down through the PIII were all based. But Banias includes a few tricks from the P4, too, as well as some innovations that set it apart from both processors.
The present article takes a look at Banias a.k.a. the Pentium M (or PM), the processor at the heart of Intel's Centrino platform. Unlike with some of my previous articles, I won't even attempt to cover most of what a CPU geek could possibly want to know about the PM. I cover only the points that I think are the most interesting and the most important from a big-picture perspective. Of course, this means that I'll gloss over a few things, and that I'll leave other things out entirely. That being the case, I've made it a point to resume my recently lapsed practice of including a bibliography with links to sources for more information on the PM. If you want a more general overview of the processor's features, especially with regards to power-saving technologies like Enhanced Speedstep, then you should look there.
As I noted above, the PM is the latest iteration of the venerable PPro (a.k.a. "P6") architecture, probably the most commercially successful microprocessor architecture of all time. Intel's standard line about the PM is that they took what they learned from the P4 and mixed it with the PIII, and that's true in a certain sense. But the best way to look at the PM is as an evolutionary advance of the P6 microarchitecture.
In the following few sections, we'll take a look at what's known about the Pentium M's microarchitecture. Unfortunately, Intel has been close-fisted with the details, but there's enough out there to allow us to put together a general picture of the core.
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