Bytecode is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code. Since instructions are processed by software, they may be arbitrarily complex, but are nonetheless often akin to traditional hardware instructions; virtual stack machines are the most common, but virtual register machines have also been built. Different parts may often be stored in separate files, similar to object modules, but dynamically loaded during execution.
The name bytecode stems from instruction sets which have one-byte opcodes followed by optional parameters. Intermediate representations such as bytecode may be output by programming language implementations to ease interpretation, or it may be used to reduce hardware and operating system dependence by allowing the same code to run on different platforms. Bytecode may often be either directly executed on a virtual machine (i.e. interpreter), or it may be further compiled into machine code for better performance.
Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) which encode the result of parsing and semantic analysis of things like type, scope, and nesting depths of program objects. They therefore allow much better performance than direct interpretation of source code.
A bytecode program may be executed by parsing and directly executing the instructions, one at a time. This kind of bytecode interpreter is very portable. Some systems, called dynamic translators, or "just-in-time" (JIT) compilers, translate bytecode into machine language as necessary at runtime: this makes the virtual machine unportable, but doesn't lose the portability of the bytecode itself. For example, Java and Smalltalk code is typically stored in bytecoded format, which is typically then JIT compiled to translate the bytecode to machine code before execution. This introduces a delay before a program is run, when bytecode is compiled to native machine code, but improves execution speed considerably compared to direct interpretation of the source code—normally by several times.
Because of its performance advantage, today many language implementations execute a program in two phases, first compiling the source code into bytecode, and then passing the bytecode to the virtual machine. Therefore, there are virtual machines for Java, Python, PHP, Forth, and Tcl. The implementation of Perl and Ruby 1.8 instead work by walking an abstract syntax tree representation derived from the source code.
Full article ▸