Bytecode

"Portable code" redirects here. For other uses, see software portability.

Bytecode, also known as p-code (portable code), is a form of instruction set designed for efficient execution by a software interpreter. 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.

Look up bytecode in Wiktionary, the free dictionary.

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.

Since bytecode 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.[1][2] Different parts may often be stored in separate files, similar to object modules, but dynamically loaded during execution.

Execution

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 hardware-specific, 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 orders of magnitude.

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. There are bytecode based virtual machines of this sort for Java, Python, PHP,[3] Tcl, and Forth (however, Forth is not ordinarily compiled via bytecodes in this way, and its virtual machine is more generic instead). The implementation of Perl and Ruby 1.8 instead work by walking an abstract syntax tree representation derived from the source code.

More recently, the authors of V8[4] and Dart[5] have challenged the notion that intermediate bytecode is a necessity for fast and efficient VM implementation. Both of these language implementations currently do direct JIT compilation from source code to machine code without any bytecode intermediary.[6]

Examples

Notes

  1. ↑ The Implementation of Lua 5.0 involves a register-based virtual machine.
  2. ↑ "Dalvik VM". is register based
  3. ↑ Although PHP opcodes are generated each time the program is launched, and are always interpreted and not Just-In-Time compiled
  4. ↑ "Dynamic Machine Code Generation". Google.
  5. ↑ Loitsch, Florian. "Why Not a Bytecode VM?". Google.
  6. ↑ "JavaScript myth: JavaScript needs a standard bytecode".
  7. ↑ The Implementation of the Icon Programming Language
  8. ↑ The Implementation of Icon and Unicon a Compendium
  9. ↑ For the details refer to "United States Patent 6,973,644".
  10. ↑ For the details refer to "R Installation and Administration".
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