Inside Pentium M Architecture


In this tutorial we will explain you how Pentium M CPU works in an easy to follow language. Since all new CPUs from Intel will use Pentium M’s architecture, studying this architecture is very important to understand the architecture of Core Solo e Core Duo (Yonah) CPUs and also to understand the foundation layer for the forthcoming Core microarchitecture, to be used by Merom, Conroe and Woodcrest CPUs. In this tutorial you will learn exactly how its architecture works so you will be able to compare it more precisely to other processors from Intel and competitors from AMD.

Pentium M is based on Intel’s 6th generation architecture, a.k.a. P6, the same used by Pentium Pro, Pentium II and Pentium III CPUs and not on Pentium 4’s as you may think, being originally targeted to mobile computers. You may think of Pentium M as an enhanced Pentium III. Pay attention to not confuse Pentium M with Pentium 4 M or with Pentium III M, which are different CPUs. Read our tutorial All Pentium M Models to learn about all Pentium M versions released so far.

Several times Pentium M is called Centrino. Actually, Centrino is when you have a laptop with a Pentium M CPU, an Intel 855 or 915 chipset and Intel/PRO wireless LAN. So, if you have a laptop based on Pentium M but without Intel/PRO wireless LAN, for example, it cannot be called Centrino.

In this tutorial we will basically explain how P6 architecture works and what’s new on Pentium M compared to Pentium III. So, in this tutorial you will also learn how Pentium Pro, Pentium II, Pentium III and Celeron (models based on P6 architecture, i.e., slot 1 and socket 370 ones) processors work.

In order to continue, however, you need to have read our tutorial “How a CPU Works”. In this tutorial we explain the basics about how a CPU works. In the present tutorial we are assuming that you have already read it, so if you didn’t, please take a moment to read it before continuing, otherwise you may find yourself a little bit lost. Actually we can consider the present tutorial as a sequel to our How a CPU Works tutorial. It is also a good idea to read our Inside Pentium 4 Architecture tutorial, just for understanding how Pentium M differs from Pentium 4.

Before going further, let’s see the main differences between Pentium M and Pentium III CPUs:

  • Externally, Pentium M works like Pentium 4, transferring four data per clock cycle. This technique is called QDR (Quad Data Rate) and makes the local bus to have a performance four times its actual clock rate, see table below.

Real Clock Performance Transfer Rate
100 MHz 400 MHz 3.2 GB/s
133 MHz 533 MHz 4.2 GB/s

  • L1 memory cache: two 32 KB L1 memory caches, one for data and another for instructions (Pentium III had two 16 KB L1 memory caches).

  • L2 memory cache: 1 MB on 130 nm models (“Banias” core) or 2 MB on 90 nm models (“Dothan” core). Pentium III had up to 512 KB. Celeron M, which is a low-cost version of Pentium M, has a 512 KB L2 memory cache.

  • Support for SSE2 instructions.

  • Advanced branch prediction: branch prediction circuit was redesigned (and based on Pentium 4’s branch prediction circuit) to improve performance.

  • Micro-ops fusion: The instruction decoder fuses two micro-ops into one micro-op in order to save energy and improve performance. We’ll talk more about this later.

  • Enhanced SpeedStep Technology, which allows the CPU to reduce its clock while idle in order to save battery life.

  • Several other battery-saving features were added to Pentium M’s microarchitecture, since this CPU was originally designed for mobile computers.

Let’s now talk more in depth about Pentium M’s architecture.

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Author: Gabriel Torres

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.

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