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Java Real Machines
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Until this chapter we always assumed that Java program class files provided bytecode instructions to Java Virtual Machine programs that run on conventional processors. However, there is no reason a Java Real Machine that processes bytecodes directly in hardware cannot be built and in fact several have been. Sun Microsystems initially proved the concept in the late 1990s with its PicoJava chip. Since then several independent Java hardware implementations have hit the market.

The JVM specification requires a stack based processing scheme rather than the register approach common in conventional hardware (see the JVM discussions in the web course supplements section.) The language designers wanted Java to run on a wide range of processors, including simple embedded types that had few registers, and so decided the stack approach was the most portable.

Java processors cover a wide range of designs and purposes. A standalone Java chip design provides all the capabilities needed to act as a general-purpose computer. The processor executes the bytecodes directly. Most of the current processors only execute a subset of the full Java instruction set. For example, a chip might leave out the floating-point instructions since for many applications, such as a micro-controller, floating point instructions are not needed.

Another approach is to add a Java co-processor to a conventional processor. The Java co-processor in some designs translates the bytecodes into the instruction set of the conventional processor and accelerates the running of Java programs. In more of a “companion processor” approach (Ref.), the Java hardware takes over the execution of Java bytecodes completely whenever a Java program runs while the operating system and other types of non-Java code run separately in the conventional processor. 

A Java processor may refer to a core, which is a circuit added to FPGA or ASIC/SoC (System-on-a-chip) to provide either the primary processing capability or to act as a co-processor. Such cores are not sold as silicon hardware by their designers but as intellectual property, in the form of a RTL (Register Transfer Language) description of the circuit, to those who make the chips.

Benefits of Java Hardware Processing

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, there may not be sufficient resources for a JVM with Just-in-Time or other sophisticated acceleration capabilities.

However, for small platforms with limited resources there literally may not 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 works well for permanent, non-networked situations such as a controller in a washing machine. For platforms, such as a cell phone, that can download new Java programs, 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 since a low power embedded conventional processor may offer limited performance. If a pure Java processor cannot handle all of the tasks required, an option is to add a Java accelerator core that will assist a conventional processor with running Java programs. Multimedia operations in video, audio, and 2D/3D graphics can especially benefit from the performance enhancements of Java acceleration hardware.

In this chapter we review examples of the following Java hardware:

  • Java Processor Chips - examples of chips that execute bytecodes directly or translate them into instructions for a conventional processor.
  • Java Cores - these processor circuits are sold as IP (Intellectual Property) for implementation on chips, FPGAs, and systems-on-a-chip.
  • Java Boards - board level systems with Java processors. In some cases they are complete standalone computers, while in other cases they plug into a bus to assist a conventional processor with Java programs.

References & Web Resources


Latest update: Dec. 14, 2004
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