Jazelle

Jazelle, in the context of computer architecture, refers to a technology developed by ARM Ltd. (now Arm Holdings). It allows ARM processors to execute Java bytecode directly in hardware, or through a combination of hardware and software execution, rather than relying solely on software-based Java Virtual Machines (JVMs).

The primary goal of Jazelle technology was to improve the performance and power efficiency of Java applications running on ARM-based devices, especially mobile devices and embedded systems where these factors are critical. By directly executing bytecode, Jazelle aimed to reduce the overhead associated with interpreting or just-in-time (JIT) compiling Java code in software.

Jazelle technology came in two main forms:

• Jazelle DBX (Direct Bytecode eXecution): This implemented hardware acceleration for a subset of Java bytecodes. Frequently used bytecodes were directly executed by the ARM processor, while less common bytecodes were handled by a software-based JVM. This provided a balance between hardware complexity and performance improvement.

• Jazelle RCT (Runtime Compilation Target): This allowed the ARM processor to dynamically switch between running code as interpreted Java bytecode or as natively compiled ARM instructions. This required a more sophisticated software runtime environment.

Although Jazelle was intended to provide a significant boost to Java performance on ARM devices, it faced challenges, including the increasing sophistication of software JVMs and the evolution of Java bytecode. Modern ARM processors have generally shifted away from dedicated Jazelle hardware towards optimized software-based JVM implementations that leverage features of the ARM architecture such as NEON SIMD instructions. Consequently, Jazelle technology is largely considered legacy and is not a prominent feature of modern ARM processors. The improvements in general-purpose processing power and software optimization have diminished the need for specialized hardware acceleration of Java.