Bytecode, also known as p-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 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-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, 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.
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" 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.
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, Tcl, and
Forth.
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 the V8 and Dart languages 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.
Examples ActionScript executes in the ActionScript
Virtual Machine, 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.
Adobe Flash objects BANCStar, originally bytecode for an interface-building
tool but used as a language in its own right.
Byte Code Engineering Library C to Java Virtual Machine compilers
CLISP implementation of Common Lisp used to compile only to bytecode for many years; however,
now it also supports compilation to native code with the help of GNU lightning.
CMUCL and Scieneer Common Lisp implementations of Common Lisp can compile either to bytecode
or to native code; bytecode is much more compact Common Intermediate Language executed by Common
Language Runtime.
Used by Microsoft .NET languages such as C#.
Dalvik bytecode, designed for the Android platform, is executed by the Dalvik virtual
machine.
Dis bytecode, designed for the Inferno, is executed by the Dis virtual machine.
EiffelStudio for the Eiffel programming language EM the Amsterdam Compiler Kit virtual machine
used both as an intermediate compilation language and as moderne bytecode language
Emacs is a text editor with a majority of its functionality implemented by its specific
dialect of Lisp.
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.
Embeddable Common Lisp implementation of Common Lisp can compile to bytecode or C code
Ericsson implementation of Erlang uses BEAM bytecodes
Icon and Unicon programming languages Infocom used the Z-machine to make its software
applications more portable.
Java bytecode, which is executed by the Java Virtual Machine
ASM BCEL
Javassist JMangler
LLVM, a modular bytecode compiler and virtual machine
Lua uses a register-based bytecode virtual machine.
m-code of the MATLAB programming language O-code of the BCPL programming language
OCaml programming language optionally compiles to a compact bytecode form
p-code of UCSD Pascal implementation of the Pascal programming language
Parrot virtual machine Pick BASIC also referred to as Data BASIC
or MultiValue BASIC 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.
Scheme 48 implementation of Scheme using bytecode interpreter
Bytecodes of many implementations of the Smalltalk programming language
The SPIN interpreter built into the Parallax Propeller Microcontroller
SWEET16 Tcl
Visual FoxPro compiles to bytecode YARV and Rubinius for Ruby.
Notes