OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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performs garbage collection, a class file loader, and an execution engine. While all
components of the interpreter are necessary in order to run a Java application, the
execution engine is the component that is of most relevance to the studies conducted
as part of this thesis.
There are four core data structures that are maintained as part of the execution
engine for the interpreter. Two additional variables are needed in order to maintain
the current location within these data structures. Each of these is described in the
following paragraphs. They are also presented graphically in Figure 2.1.
Code: This array of bytes contains the opcodes and operands that represent
the functionality of a method. An index into the array, denoted pc,
is also maintained which represents the current location with the array.
Several implementations are possible including representing the program
counter as integer and dereferencing the array through the use of square
brackets. Alternatively, the program counter can be represented as a byte
pointer, allowing the current byte from the code stream to be acquired by
dereferencing the pointer. The examples used in the remainder of this document
will make use of an integer representation for the program counter
in order to make the examples more clear.
Stack: The operand stack is used to hold temporary values that will
be used in a subsequent computation within the same method. It is commonly
represented as an array of 4-byte slots. When category two values –
values of type long or type double which occupy 8 bytes – are placed on the
stack they occupy two adjacent slots. A stack pointer is also maintained
which points to the element within the array which currently represents
the top of the stack. Like the program counter, the stack pointer can be
represented as an integer or as a pointer type. The examples presented
in this chapter will make use of an integer representation of the stack
pointer.
Local Variables: Like the stack, the local variables are typically represented
as an array of 4-byte slots where category two values are represented
using two adjacent slots. No separate pointer is needed for the local variables
since the local variable being accessed is always specified explicitly
as an operand or known definitively from the opcode being executed.
Constant Pool: The constant pool is also represented as an array of
slots. In addition to containing some elements which occupy two slots, it