Gecko's CPU Library

Intel Pentium M (Dothan) processors

Introduction: May 2004


The Pentium M brand refered to only two single-core 32-bit x86 microprocessors (with the Intel P6 microarchitecture) introduced in March 2003 (during the heyday of the Pentium 4 desktop CPUs), and forming a part of the Intel Centrino platform. The Pentium M branded processors had a maximum power consumption (TDP) of 3-25W, and were intended for use in laptop personal computers (thus the "M" moniker standing for mobile). They evolved from the core of the last Pentium III branded CPU by adding the bus interface of Pentium 4 one, an improved instruction decoding/issuing front end, improved branch prediction, SSE2 support, and a much larger cache. The first Pentium M branded CPU, codenamed the Banias, was followed by the second one - the Dothan. After the Pentium M branded processors, Intel released the Core branded dual-core mobile Yonah CPU with a modified microarchitecture. Pentium M branded CPUs can be considered as the end of the Intel P6 microarchitecture.

The Pentium M represented a new and radical departure for Intel, as it was not a low-power version of the desktop-oriented Pentium 4, but instead a heavily modified version of the Pentium III Tualatin design (itself based on the Pentium Pro core design). It was optimised for power efficiency, a vital characteristic for extending notebook computer battery life. Running with very low average power consumption and much lower heat output than desktop processors, the Pentium M ran at a lower clock speed than the laptop version of the Pentium 4 (the Pentium 4-Mobile, or P4-M), but with similar performance - a 1.6GHz Pentium M was typically able to attain the performance of a 2.4GHz Pentium 4-M. The Pentium M (model 740) had been tested to perform up to about 7,400MIPS and 3.9GFLOPS (using SSE2).

The Pentium M coupled the execution core of the Pentium III with a Pentium 4 compatible bus interface, an improved instruction decoding/issuing front end, improved branch prediction, SSE2 support, and a much larger cache. The usually power-hungry secondary cache used an access method which switches on only the portion being accessed.

Other power saving methods included dynamically variable clock frequency and core voltage, allowing the Pentium M to throttle clock speed when the system was idle in order to conserve energy, using the SpeedStep 3 technology (which had more sleep stages than previous versions of SpeedStep). With this technology, a 1.6GHz Pentium M was effectively able to throttle to clock speeds of 600MHz, 800MHz, 1000MHz, 1200MHz, 1400MHz and 1600MHz; these intermediate clock states allowed the CPU to better throttle clock speed to suit conditions. The power requirements of the Pentium M varied from 5 watts when idle to 27 watts at full load. This was useful to notebook manufacturers as it allowed them to include the Pentium M into smaller notebooks.

Although Intel had marketed the Pentium M exclusively as a mobile product, motherboard manufacturers such as AOpen, DFI and MSI had been shipping Pentium M compatible boards designed for enthusiast, HTPC, workstation and server applications. An adapter, the CT-479, had also been developed by ASUS to allow the use of Pentium M processors in selected ASUS motherboards designed for Socket 478 Pentium 4 processors. Shuttle Inc. offered packaged Pentium M desktops, marketed for low energy consumption and minimal cooling system noise. Pentium M processors were also of interest to embedded systems' manufacturers because the low power consumption of the Pentium M allowed the design of fanless and miniaturized embedded PCs.

The Dothan core

Intel launched its improved Pentium M, formerly known as Dothan, named after another ancient town in Israel, on May 10, 2004. Dothan Pentium M processors (product code 80536) were among the first Intel processors to be identified using a "processor number" rather than a clockspeed rating, and the mainstream versions were known as Pentium M 710 (1.4GHz), 715 (1.5GHz), 725 (1.6GHz), 735 (1.7GHz), 740 (1.73GHz), 745 (1.8GHz), 750 (1.86GHz), 755 (2.0GHz), and 765 (2.1GHz). The processor supported neither hyperthreading nor SSE3.

These 700 series Pentium M processors retained the same basic design as the original Pentium M, but were manufactured on a 90 nm process, with twice the secondary cache. Die size, at 84 mm², remains in the same neighborhood as the original Pentium M, even though the 700 series contained approximately 140 million transistors, most of which made up the 2MB cache. TDP was also down to 21 watts (from 24.5 watts in Banias), though power use at lower clockspeeds had increased slightly. However, tests conducted by third party hardware review sites showed that Banias and Dothan equipped notebooks had roughly equivalent battery life. Additionally third party hardware review sites had benchmarked the Dothan at approx 10-20% better performance than the Banias in most situations.

The processor line had models running at clock speeds from 1.0GHz to 2.26GHz as of July 2005. The models with lower frequencies were either low voltage or ultra-low voltage CPUs designed for even better battery life and reduced heat output. The 718 (1.3GHz), 738 (1.4GHz), and 758 (1.5GHz) models were low-voltage (1.116V) with a TDP of 10W, while the 723 (1.0GHz), 733 (1.1GHz), and 753 (1.2GHz) models were ultra-low voltage (0.940V) with a TDP of 5W.

Revisions of the Dothan core were released in the first quarter of 2005 with the Sonoma chipsets and supported a 533MT/s FSB and XD (Intel's name for the NX bit) (and the PAE support required for it was enabled, unlike earlier Pentium Ms that had it disabled). These processors included the 730 (1.6GHz), 740 (1.73GHz), 750 (1.86GHz), 760 (2.0GHz) and 770 (2.13GHz). These models all had a TDP of 27W and a 2MB L2 cache.

In July 2005, Intel released the 780 (2.26GHz) and the low-voltage 778 (1.60GHz).

The CPUID signature for a Dothan was 0x6DX.

Source: Wikipedia, the free encyclopedia.