Java Bytecode - Instructions

Instructions

See also: Java bytecode instruction listings

As each byte has 256 potential values, there are 256 possible opcodes. Of these, 0x00 through 0xca, 0xfe, and 0xff are assigned values. 0xca is reserved as a breakpoint instruction for debuggers and is not used by the language. Similarly, 0xfe and 0xff are not used by the language, and are reserved for internal use by the virtual machine.

Instructions fall into a number of broad groups:

• Load and store (e.g. aload_0,istore)
• Arithmetic and logic (e.g. ladd,fcmpl)
• Type conversion (e.g. i2b,d2i)
• Object creation and manipulation (new,putfield)
• Operand stack management (e.g. swap,dup2)
• Control transfer (e.g. ifeq,goto)
• Method invocation and return (e.g. invokespecial,areturn)

There are also a few instructions for a number of more specialized tasks such as exception throwing, synchronization, etc.

Many instructions have prefixes and/or suffixes referring to the types of operands they operate on. These are as follows:

For example, "iadd" will add two integers, while "dadd" will add two doubles. The "const", "load", and "store" instructions may also take a suffix of the form "_n", where n is a number from 0-3 for "load" and "store". The maximum n for "const" differs by type.

The "const" instructions push a value of the specified type onto the stack. For example "iconst_5" will push an integer 5, while "dconst_1" will push a double 1. There is also an "aconst_null", which pushes "null". The n for the "load" and "store" instructions specifies the location in the variable table to load from or store to. The "aload_0" instruction pushes the object in variable 0 onto the stack (this is usually the "this" object). "istore_1" stores the integer on the top of the stack into variable 1. For variables with higher numbers the suffix is dropped and operators must be used .