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Intel Pentium M 780 as Heart of Your Desktop PC, with ASUS CT-479 Adapter (page 2)

Category: CPU

by Ilya Gavrichenkov

[ 06/20/2005 | 01:46 PM ]

Pentium M Architecture

So, what is the Pentium M processor? Intel isn’t very talkative about the architecture of this product. The official documentation describes its formal characteristics and names just a few of its distinguishing features: a mobile PC oriented architecture, a dedicated stack manager, micro-ops fusion technology and Enhanced Intel SpeedStep technology. This description doesn’t clarify anything about the internal structure of the Pentium M. So, Intel doesn’t seem to want to disclose detailed information on the Pentium M and must have some reasons for that. Why?

The thing is the Pentium M represents a slightly improved P6 architecture that had previously been employed in CPU models from the Pentium Pro to Pentium III. Thus, the Pentium M looks like a step backward against the widely advertised NetBurst architecture. Yet you stop to regard the Pentium M as such after examining its internal structure closely. Taken as a whole, the Pentium M is better than the Pentium 4 in more respects than one! And please do not forget that the P6 architecture is among the best architectures Intel has ever developed – just think how long this architecture has been on the market in its various incarnations. So why can’t the Pentium M – the next incarnation of the Pentium III – become successful like the others?

Let’s first see which improvements were added by Intel to the good old Pentium III to transform it into the much-promising Pentium M.

Pipeline and execution core: The Pentium M is based on the same RISC architecture as Pentium III series CPUs, and there’s little difference in the execution cores of these processors. For example, they each have five execution units in total. But in the execution pipelines of these two CPUs there are already certain differences. While the Pentium III had a 10-stage integer pipeline, the pipeline of the Pentium M is somewhat longer. Of course, we don’t mean the Pentium M has a pipeline comparable in length to the one of Pentium 4 CPUs, but Intel’s engineers actually pursued the same goals when adding extra stages to the Pentium M’s pipeline. As you probably know, making the pipeline longer helps to lift the operational frequency of the processor up but the processor’s heat dissipation and power consumption grow up proportionally to the pipeline’s length which is especially important with mobile CPUs. So, judging by the available empirical data, the pipeline of the Pentium M processor is 12-14 stages long, i.e. a little longer than the Pentium III’s pipeline. Besides pushing the frequency bar higher, the longer pipeline was necessary due to the implementation of micro-ops fusion technology in the Pentium M. We’ll talk about it shortly.

Besides higher heat dissipation and power consumption, a longer pipeline in a modern superscalar processor capable of out-of-order execution leads to other negative effects. First of all, it is the performance hit occurring when the pipeline is refilled after an incorrect branch prediction. In many cases the developers try to reduce this negative effect as they add more stages to the execution pipeline. So, we should next dwell upon the improvements in the branch prediction unit they implemented in the Pentium M.