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A Crusoe for your Notebook

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PCQ Bureau
New Update

Optimizing the performance of a notebook’s battery is a

prime consideration in notebook usage. While it is the hardware in notebooks

that demands power, mobile processors, the operating system, and applications on

notebooks use advanced power management techniques to reduce power consumption.

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Most power management technologies, like the SpeedStep

technology used in Intel’s mobile processors, switch between the maximum

performance mode when on main (or AC) power and to an optimized performance mode

when on battery (DC). Once on battery, however, such technologies don’t switch

the processor’s power consumption on the basis of the intensity of its usage.

For example, processor-intensive applications like games make the processor work

harder, resulting in more power consum- ption than an application like a word

processor. The design of Crusoe processors, by Transmeta, addresses these

issues. Here, we take a closer look at this processor.

Hardware and software instruction set

The code morphing software sits between the x86 applications and Crusoe’s native instruction setMicroprocessors consist of millions of silicon transistors,

which, along with other silicon materials, make up the instruction set of

microprocessors. The more complex the embedded instruction set, like the x86

instruction set used by Intel and AMD processors, the more the number of silicon

devices, and thus more the power required to drive them. To get over this

problem, the instruction set of Crusoe processors has been partly implemented in

software, called Code Morphing software, and partly in hardware, called the VLIW

(Very Long Instruction Word) engine. So, there’s less hardware in the

processor.

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Moreover, the VLIW instruction set is a simpler one than the

x86. These instructions are similar to those of RISC (Reduced Instruction Set

Computing), also a simple instruction set. Each such instruction is called an

atom. A Crusoe processor can process four such instructions–together called a

molecule–at a time (or technically, at one clock cycle). Thus, the parallel

execution of a molecule and a simple instruction set make the VLIW engine a fast

and simple hardware processor.

x86 or Intel compatibility

The software which runs on Intel and AMD machines–like

Windows, MS Office, Netscape Navigator, ICQ–communicates with processors like

the PIII or AMD Athlon using instructions from the x86 instruction set. But as

we said above, Crusoe processors understand VLIW instructions. So does that mean

that your Windows applications, and even Windows itself won’t run on Crusoe

processors? This is where the Code Morphing software comes in. The software is

called so because apart from implementing a part of the VLIW instructions, it

knows how to convert an x86 instruction to corresponding VLIW instructions. The

software forms a layer between the VLIW engine and x86 applications. It’s

embedded in the ROM (Read Only Memory) circuitry and is transferred to the main

memory or RAM on system startup.

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Obviously, the translation of each x86 instruction to VLIW

instructions will retard the processor’s performance, and since the

translation is software-based, it has a more drastic impact. This is overcome by

using a translation cache, where translations between x86 and VLIW instructions

are cached or stored. This is especially helpful if an application is used

frequently, since x86 instructions used by the application will be the same each

time, and the translation can be directly fetched from the cache.

LongRun power management

Ideally, power management techniques should be based on how

intensively the processor is being used, and not only on power supply source (AC

or DC). The power consumed by a processor is directly proportional to the clock

frequency on which it operates, and to the square of the voltage it utilizes.

Conventional power management techniques may turn off the power to the mobile

processor when the notebook is in suspend or sleep mode. However, Crusoe

processors can adjust their operating frequency and voltage as per the demands

of the applications running on it on the fly, using LongRun technology. It’s

claimed that thanks to this technology, Crusoe processors will consume power as

low as 1 watt while running, and about 8 milli-watts when idle. For

processor-intensive applications, the power requirement may go up to 2 watts.

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At present, there are two models of Crusoe processors–TM3200

with speeds in the range of 333-400 MHz, and TM5400 with 500-700 MHz. Both have

built-in memory and PCI controllers for memory modules and add-on cards

respectively. As of now, only TM5400 supports the LongRun technology, whereas TM

3200 with Mobile Linux as the operating system, features standard power

management techniques. Mobile Linux is based on the Linux operating system, but

is optimized for power management and low memory utilization. While TM3200 is

meant to run standard Internet applications like Web browsers and e-mail, TM5400

is intended to give performance like that on standard desktop PCs.

Before you make up your mind about Crusoe processors being

able to give you the ultimate mobile experience, do note that the performance

and weight of a notebook doesn’t depend only on processors, but also on other

components like graphics card, sound card, hard disk, etc. Moreover, Toshiba, a

notebook manufacturing company and an investor in the Crusoe technology, has

claimed that the Transmeta chip doesn’t quite live up to its hype. So, before

you think of buying a notebook with a Crusoe processor–Transmeta has begun

their shipment–wait for a review of these processors by PCQ Labs when we

receive them.

Shekhar Govindarajan

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