PRODUCT HOW-TO: Use ARM DBX hardware extensions to accelerate Java in space-constrained embedded apps

Performance is an issue constantly raised about the Java platform. Java's portability is also a major disadvantage, as bytecode must always undergo some form of conversion to run on the native instruction set of the underlying architecture. The feature-rich demands of next-generation Java applications will quickly outstrip the capabilities of current massmarket Java handsets.

Hardware graphics accelerators, increasing processor clock speeds, and fast data transfer rates are all changing the application types that can run on mobile devices. If Java is to keep pace, Java platform performance must improve and a powerful Java Virtual Machine (JVM) must be used.


Figure 1: To reduce die size and improve performance, Jazelle DBX is implemented in the ARM pipeline as a finite state machine rather than a traditional microcode engine.

Software solutions

Traditional methods of improving Java execution speed include software solutions - such as optimized JVMs, just-in-time (JIT) or ahead-of-time (AOT) compilers - and hardware solutions - such as dedicated Java processors and Java co-processors.

Depending on the system, high speed levels can be achieved using these methods. However, delivering this performance on an embedded platform has typically involved power, memory or platform cost. JIT and AOT compilers compile code for immediate execution on the target device. An AOT compiler compiles all code after application download, some of which may not even run during execution.

A JIT compiler, meanwhile, compiles code "on sight" - i.e. just prior to execution. On an embedded device, JIT compilation causes a delay between an application's launch and its actual run. Research has likewise shown that dynamically compiled code expands four to six times. So, in addition to slow application startup with a JIT, extra memory is required for the code compiled by JIT and AOT solutions.

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