Bytecode, also known as p-code (portable code), is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readablesource 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.
The name bytecode stems from instruction sets which have one-byteopcodes 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.
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 magnitudes[citation needed]. Please note that bytecode as mandatory step for performance improvement is currently challenged, ex: Google's V8[3] and DartVM[4] do direct translation of source code to JITed machine code without bytecode intermediary. [5]
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,[6]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.
ActionScript executes in the ActionScript Virtual Machine (AVM), which is part of Flash Player and AIR. ActionScript code is typically transformed into bytecode format by a compiler. Examples of compilers include the one built into Adobe Flash Professional and the one that is built into Adobe Flash Builder and available in the Adobe Flex SDK.
Emacs is a text editor with a majority of its functionality implemented by its specific dialectofLisp. These features are compiled into bytecode. This architecture allows users to customize the editor with a high level language, which after compilation into bytecode yields reasonable performance.
The R environment for statistical computing offers a byte code compiler through the compiler package, now standard with R version 2.13.0. It is possible to compile this version of R so that the base and recommended packages take advantage of this.[10]
Scheme 48 implementation of Scheme using bytecode interpreter
Bytecodes of many implementations of the Smalltalk programming language
