OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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also has the ability to contain several element types that divide a single
slot into two halves, where each half represents the index of another entry
in the constant pool. A relatively small number of instructions access
the constant pool, but it is an absolutely critical component of the virtual
machine. Examples of such instructions include the field access bytecodes,
the invoke* bytecodes and the ldc, ldc w and ldc2 w bytecodes.
2.3 The Java Instruction Set
The Java Virtual Machine instruction set includes a total of 201 distinct bytecodes,
each of which performs a specific task. These bytecodes are used to represent the
functionality of a Java application within a class file. The java compiler, javac,
generates the class files and the bytecodes that they contain.
In many ways, Java bytecodes are similar to assembly language instructions. Each
bytecode performs a relatively small amount of work, with a typical Java language
statement being translated into several bytecodes. The bytecodes defined by the Java
Virtual Machine Specification can be categorized into broad families. These families
are discussed in the following sections.
2.3.1 Loads and Stores
Load and store bytecodes are used to move values between the operand stack and
the local variable array. Load bytecodes copy a value from the local variable array,
pushing it onto the stack. Store bytecodes pop the top value from the operand stack
and store it into a slot in the local variable array. Of the 201 bytecodes defined by
astore dstore fstore istore lstore
astore 0 dstore 0 fstore 0 istore 0 lstore 0
astore 1 dstore 1 fstore 1 istore 1 lstore 1
astore 2 dstore 2 fstore 2 istore 2 lstore 2
astore 3 dstore 3 fstore 3 istore 3 lstore 3
aload dload fload iload lload
aload 0 dload 0 fload 0 iload 0 lload 0
aload 1 dload 1 fload 1 iload 1 lload 1
aload 2 dload 2 fload 2 iload 2 lload 2
aload 3 dload 3 fload 3 iload 3 lload 3
Table 2.1: Load and Store Bytecodes