A Crusoe for your Notebook

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.

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

Microprocessors 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.

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.

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.

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