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Looking at the processor socket, we see only a few differences between the Pentium 4 and the Pentium M . While Socket 478 hosted all Pentium 4 Northwood processors and the Prescott at up to 3.4 GHz, Socket 479 takes care of the Pentium M. Thanks to the almost identical physical specs, this strategy makes it possible to use the same packaging for two different processor families. However, Socket 479 does not have a lever for locking the CPU; it comes with a little screw instead.
Intel is trying hard to increase Socket 775 platform sales, but processors for the Socket 478 platform are still around. Here, you should definitely go for the 130 nm Northwood core with 512 kB L2 cache, since these do not get as hot as the 1 MB 90 nm Prescott models, yet deliver the same level of performance. There is, however, one argument that speaks against using Northwood: the old core still does not recognize Intel's SSE3 instruction set. Here we should mention that the Pentium M does not support it either.
The Pentium 4 is based on Intel's well known NetBurst architecture, which is designed on the principle of rather long pipelines. Northwood has a total of 20 stages while the Prescott core pipelines have 31. This has some advantages in conjunction with Intel's Hyper-Threading technology, as it allows two threads to be processed simultaneously on only a single processor core. Intel's basic approach behind this technology was to go for long pipelines and high clock speeds.
The Pentium III pipeline has only 10 stages, while the Pentium M is supposed to have a few additional ones. Neither processor supports Hyper-Threading, but the Pentium M comes with Intel's SpeedStep technology. Also, the cache line size of the Pentium M has been increased from 32 to 64 Bytes.
SpeedStep is one of the most important features for saving energy and ensuring longer battery life, since it allows the operating system to reduce the processor clock speed when there is not much work to be done. While the first implementation of SpeedStep only supported two modes (energy saving or max performance), Intel modified the feature to support 200 MHz increments in clock speed with the release of the Pentium M. In any case, the minimum clock speed is 600 MHz. The table on the following page shows all available Pentium M models.
As we mentioned above, perhaps the most interesting aspect of the comparison between the Pentium 4 with the Pentium M is related to thermal design power (TDP). The fastest P4 processors are specified for up to 115 or 84 Watts in Socket 775, depending on the classification and validation of each part after production. For Socket 478, the maximum TDP is 103 W for the 3.4 GHz part. In contrast, the Pentium M numbers are much, much lower. Looking at Dothan, the maximum TDP is only 21 watts, while Banias consumes a maximum of 22 watts.
All current Intel processors have a quad-pumped system bus (the Front Side Bus or FSB). This results in marketing-friendly buzzwords such as FSB400, FSB533 and FSB800, while the basic clock speeds are 100, 133 and 200 MHz. Pentium 4 systems have been running at FSB533 and FSB800 for over a year now, while the Pentium M architecture is going to be transitioned from FSB400 to FSB533 with the Alviso platform launch in Q1/2005.