Embedded Java - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 658

used far more often than others and some instructions carry out more compli-
cated operations than others. So many of the hardware Java processors directly
implement only a subset of the instructions and emulate the others.
24.6 Benefits of hardware processors
Real Java machines offer a number of advantages for embedded applications
where the processor must typically work with limited memory and power
resources. As we mentioned in the J2ME section, for many of the small platforms
there may be insufficient resources for a JVM with Just-in-Time compilation or
other sophisticated acceleration capabilities. Furthermore, there may not even be
room for both a JVM and a program to run. A JVM can easily take up a megabyte
or more of RAM, while the entire memory available on a micro-platform like a
cell phone may consist of 500 kilobytes or less.
One option is to use AOT (ahead-of-time) compilers that interpret the Java
code in advance and transform it into machine code for a target platform. The
code then runs at full native speeds. This obviously eliminates portability, but
it works well for permanent, non-networked situations such as a controller in a
washing machine. For platforms that can download new Java programs, such as
a cell phone or PDA, an interpreter either as a JVM or in hardware is required.
Even when there is sufficient memory for a JVM, a hardware Java processor
could provide greater speed than a low power embedded conventional processor
running a JVM program. If a processor needs to handle both Java and non-Java
programs, an option is to add a Java accelerator core that assists a conventional
processor with Java programs. Multimedia operations in video, audio, and 2D/3D
graphics can benefit especially from the performance enhancements of Java accel-
eration hardware.
24.7 Java processors
We survey here a sample of commercially available systems to illustrate the
range of capabilities and designs of hardware Java processors. (We warn that
some of these products, and even some of the companies, will leave the scene in
the coming years as the marketplace determines which designs are viable.) We
consider Java processors that come as a complete chip and those provided as
cores to be implemented as part of other systems.
24.7.1 Java chips
The following Java processors are available as hardware chips and are usually
sold commercially as part of an electronics module for embedded applications:
aJile aJ-100, aJ-80 - the 32-bit aJ-100 derives from the JEM TM Java processor first
developed at Rockwell Collins [14]. It executes the full bytecode instruction set,