Inside Intel's Dothan

Summary:The latest Pentium M processor from Intel improves on the Banias core by shrinking the fabrication process to 90nm, doubling the cache and boosting the clock speed. All this while keeping the die size roughly the same and reducing power consumption by 10 per cent.

Intel's latest processor brings together several important lines of innovation within the company. Dothan -- more properly, a trio of Pentium M Processors with the model numbers 735, 745 and 755 -- is a high-performance IA-32 architecture chip that uses relatively little power and is designed for a mobile environment.


Intel's new Dothan (Pentium M) is the company's first 90nm mobile processor, following the desktop 'Prescott' Pentium 4 chip.
 

Inside, Dothan is the first mobile processor to be produced, like its desktop Prescott counterpart, using Intel's new 90nm process and the first to be made on 300mm wafers -- dinner-plate sized slivers of silicon that allow Intel to make more than twice as many chips per wafer than with the older 200mm size. As the other costs of production are comparable, this results in considerable economic benefits to Intel.


Dothan chips are produced on 300mm rather than 200mm wafers, delivering significant economies of scale.
 

It's also the first mobile chip from Intel to use strained silicon. Here, the atoms in the crystal lattice are held further apart than normal, resulting in much lower resistance for the electrons that move through the lattice. These move up to 70 percent faster as a result, increasing transistor speeds by up to 35 percent. This gives the Dothan design a lot more potential for higher-speed processors later in its life. Those part numbers don't directly refer to the clock frequency, but at the moment the higher the number the faster the clock: the Pentium M Processor 735 is 1.7GHz, the 745 is 1.8GHz and the 755 is the chart-topping 2GHz part. There are no other differences between the models apart from the prices -- the 735 costs $294, the 745 costs $423 and the 755 weighs in at $637, in lots of a thousand.



Dothan (90nm Pentium M) vital statistics

Processor
Enhanced SpeedStep Technology
Frequency

Cache

Fontside bus

Thermal Design Power

Price (in 1ku)

Packaging


Pentium M Processor 755
yes
2GHz - 600MHz
2MB
400MHz
21W
$637
Micro FCPGA & Micro FCBGA
Pentium M Processor 745
yes
1.8GHz - 600MHz
2MB
400MHz
21W
$423
Micro FCPGA & Micro FCBGA
Pentium M Processor 735
yes
1.7GHz - 600MHz
2MB
400MHz
21W
$294
Micro FCPGA & Micro FCBGA

Die shrink
The 90nm process is another factor that gives this chip the potential for a lengthy period in production. The shrink from 130nm ladles out more of the basic driving forces behind Moore's Law -- smaller transistors mean you can pack more on a smaller chip, drive them at faster speeds and reduce their individual power consumption. In Dothan's case, this means 140 million transistors -- just about twice the 77 million used in the previous 130nm Pentium M -- with a current top speed of 2GHz, as opposed to the 1.7GHz limit on the older part. Just about all of these extra components are corralled into a 2MB Level 2 cache, which is double that of the original Pentium M. Despite this, the new component has a maximum Thermal Design Power (TDP) of 21 watts -- more than ten percent down on the 24.5 watts of its predecessor -- and with a die size just one square millimetre larger.


The process shrink from 130nm to 90nm has allowed Intel to boost Dothan's transistor count to 140 million (from 77m), while keeping the die almost the same size as before (1mm2 larger). Most of the new transistors are in the 2MB of Level 2 cache (up from 1MB).
 

Architecture tweaks
There have been a couple of architectural changes as well -- at least to which Intel will admit. The company frequently introduces new undocumented features to processors that it enables and announces later in the parts' life, so there's a good chance that aspects of the LeGrande security system, hyperthreading and other exciting developments are already present. However, the two official changes over the 130nm Pentium M are an Enhanced Register Access Manager and an Enhanced Data Prefetcher. One of the legacy aspects of the IA-32 instruction set is that many of the 32-bit processor registers can also be used as four eight-bit or two 16-bit registers: the Enhanced Register Access Manager uses less power than before when it handles these mode changes between instructions. The Enhanced Data Prefetcher is more efficient at moving data into the Level 2 cache than before: if the processor can operate on cached data rather than repeatedly making accesses to off-chip memory, power requirements diminish.

More power savings are available through the Enhanced Intel SpeedStep technology, which has multiple voltage and frequency points that it can combine in a variety of ways as the need for extended battery life overrides the desire for speed. There are four power-saving states: Auto Halt, Stop Grant, Deep Sleep and Deeper Sleep (presumably leaving Coma, Near-Death and Zombie for future revisions). The part also comes in three voltage variants, standard, low-voltage and ultra-low-voltage, and can run as slow as 600MHz should occasion demand.

Dothan notebooks
Currently, the 90nm Pentium M has a 400MHz frontside bus and works with DDR 333/266 memory in conjunction with the 855PM and 855GME chipsets. Hardware manufacturers building Centrino-branded systems can now combine these components with Intel's latest PRO/Wireless 2200BG 802.11b/g module.


As well as the Dothan Pentium M chip, next-generation Centrino systems will include the 855PM (DDR333) chipset and the PRO/Wireless 2200BG 802.11b/g network module.
 

A 533MHz Dothan variant is scheduled for later this year, to appear with the Alviso chipset and the Calexico 2 802.11a/b/g dual-band wireless. This complete platform is currently codenamed Sonoma, but will be rebranded at launch to something more user-friendly.

 

Topics: Mobility, Hardware, Reviews

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