ARM's Jazelle RCT (Runtime Compilation Target) is part of the company's new processor architecture (see "Mobile Systems Get Performance Boost From Next-Gen ARM Core," p. 28). It replaces the existing Jazelle DBX (for Direct Bytecode eXecution) architecture, which executes Java bytecodes directly from memory (see the figure).
RCT requires an ahead-oftime (AOT) or an in-memory just-in-time (JIT) compiler that generates 16-bit RCT instructions. RCT is a superset of 16-bit Thumb2 instructions. Enter and exit instructions switch between RCT mode, also known as Thumb-2EE mode, and Thumb2 mode.
In Thumb-2EE mode, some Thumb2 instructions operate in a slightly different fashion that makes compilation more efficient because the instructions more closely match the execution implied by the bytecode instructions. This approach can result in only a 10% overhead when converting from byte to 16-bit Thumb-2EE codes. Unlike Jazelle DBX, Jazelle RCT is applicable to other virtual machines, such as .NET MSIL (Microsoft Intermediate Language), Python, and Perl.
The Thumb-2EE mode also offers new instructions that provide access to registers R8 through R15, which normally are inaccessible except by 32-bit ARM instructions. These registers usually are used to support things like Java's stack pointer. Implicit null pointer tests and fast array range checking additionally enhance speed.
Typical processor configurations include ARM, Thumb2, and Jazelle RCT support in place of ARM, Thumb2, and Jazelle DBX support. RCT overhead is significantly less than DBX support. Use of AOT means Java could even be used in interrupt routines without any startup delay that JIT support would entail.
Things are looking good for ARMing Java.