ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

A2.10.2 Jazelle direct bytecode execution support
Application Level Programmers’ Model
From ARMv5TEJ, the architecture requires every system to include an implementation of the Jazelle
extension. The Jazelle extension provides architectural support for hardware acceleration of bytecode
execution by a Java Virtual Machine (JVM).
In the simplest implementations of the Jazelle extension, the processor does not accelerate the execution of
any bytecodes, and the JVM uses software routines to execute all bytecodes. Such an implementation is
called a trivial implementation of the Jazelle extension, and has minimal additional cost compared with not
implementing the Jazelle extension at all. An implementation that provides hardware acceleration of
bytecode execution is a non-trivial Jazelle implementation.
These requirements for the Jazelle extension mean a JVM can be written to both:
function correctly on all processors that include a Jazelle extension implementation
automatically take advantage of the accelerated bytecode execution provided by a processor that
includes a non-trivial implementation.
Typically, a non-trivial implementation of the Jazelle extension implements a subset of the bytecodes in
hardware, choosing bytecodes that:
can have simple hardware implementations
account for a large percentage of bytecode execution time.
The required features of a non-trivial implementation are:
provision of the Jazelle state
a new instruction, BXJ, to enter Jazelle state
system support that enables an operating system to regulate the use of the Jazelle extension hardware
system support that enables a JVM to configure the Jazelle extension hardware to its specific needs.
The required features of a trivial implementation are:
Normally, the Jazelle instruction set state is never entered. If an incorrect exception return causes
entry to the Jazelle instruction set state, the next instruction executed is treated as UNDEFINED.
The BXJ instruction behaves as a BX instruction.
Configuration support that maintains the interface to the Jazelle extension is permanently disabled.
For more information about trivial implementations see Trivial implementation of the Jazelle extension on
page B1-81.
A JVM that has been written to take advantage automatically of hardware-accelerated bytecode execution
is known as an Enabled JVM (EJVM).
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