Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable[1]source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) that encode the result of compiler parsing and performing semantic analysis of things like type, scope, and nesting depths of program objects.
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 been built also.[2][3] 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 code as necessary at runtime. This makes the virtual machine hardware-specific but does not lose the portability of the bytecode. For example, Java and Smalltalk code is typically stored in bytecode 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 the bytecode is compiled to native machine code, but improves execution speed considerably compared to interpreting source code directly, normally by around an order of magnitude (10x).[4]
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, Raku, Python, PHP,[a]Tcl, mawk and Forth (however, Forth is seldom 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[1] and Dart[7] have challenged the notion that intermediate bytecode is needed for fast and efficient VM implementation. Both of these language implementations currently do direct JIT compiling from source code to machine code with no bytecode intermediary.[8]
Examples
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 one built into Adobe Flash Professional and one built into Adobe Flash Builder and available in the Adobe Flex SDK.
CLISP implementation of Common Lisp used to compile only to bytecode for many years; however, now it also supports compiling to native code with the help of GNU lightning
CMUCL and Scieneer Common Lisp implementations of Common Lisp can compile either to native code or to bytecode, which is far more compact
EM, the Amsterdam Compiler Kit virtual machine used as an intermediate compiling language and as a modern bytecode language
Emacs is a text editor with most of its functions implemented by Emacs Lisp, its built-in dialect of Lisp. These features are compiled into bytecode. This architecture allows users to customize the editor with a high level language, which after compiling into bytecode yields reasonable performance.
Common Lisp provides a disassemble function[10] which prints to the standard output the underlying code of a specified function. The result is implementation-dependent and may or may not resolve to bytecode. Its inspection can be utilized for debugging and optimization purposes.[11]Steel Bank Common Lisp, for instance, produces:
KEYB, the MS-DOS/PC DOS keyboard driver with its resource file KEYBOARD.SYS containing layout information and short p-code sequences executed by an interpreter inside the resident driver.[14][15]
LSL, a scripting language used in virtual worlds compiles into bytecode running on a virtual machine. Second Life has the original Mono version, Inworldz developed the Phlox version.
Lua language uses a register-based bytecode virtual machine
The R environment for statistical computing offers a bytecode 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 exploit this.[18]
Python scripts are being compiled on execution to Python's bytecode language, and the compiled files (.pyc) are cached inside the script's folder
Compiled code can be analysed and investigated using a built-in tool for debugging the low-level bytecode. The tool can be initialized from the shell, for example:
^PHP has just-in-time compilation in PHP 8,[5][6] and before while not on in the default version, had options like HHVM. For older versions of PHP: Although PHP opcodes are generated each time the program is launched, and are always interpreted and not just-in-time compiled.
^Paul, Matthias R. (2001-12-30). "KEYBOARD.SYS internal structure". Newsgroup: comp.os.msdos.programmer. Archived from the original on 2017-09-09. Retrieved 2016-09-17. […] In fact, the format is basically the same in MS-DOS 3.3 - 8.0, PC DOS 3.3 - 2000, including Russian, Lithuanian, Chinese and Japanese issues, as well as in Windows NT, 2000, and XP […]. There are minor differences and incompatibilities, but the general format has not changed over the years. […] Some of the data entries contain normal tables […] However, most entries contain executable code interpreted by some kind of p-code interpreter at *runtime*, including conditional branches and the like. This is why the KEYB driver has such a huge memory footprint compared to table-driven keyboard drivers which can be done in 3 - 4 Kb getting the same level of function except for the interpreter. […]
^Mendelson, Edward (2001-07-20). "How to Display the Euro in MS-DOS and Windows DOS". Display the euro symbol in full-screen MS-DOS (including Windows 95 or Windows 98 full-screen DOS). Archived from the original on 2016-09-17. Retrieved 2016-09-17. […] Matthias [R.] Paul […] warns that the IBM PC DOS version of the keyboard driver uses some internal procedures that are not recognized by the Microsoft driver, so, if possible, you should use the IBM versions of both KEYB.COM and KEYBOARD.SYS instead of mixing Microsoft and IBM versions […] (NB. What is meant by "procedures" here are some additional bytecodes in the IBM KEYBOARD.SYS file not supported by the Microsoft version of the KEYB driver.)
^Microsoft C Pcode Specifications. p. 13. Multiplan wasn't compiled to machine code, but to a kind of byte-code which was run by an interpreter, in order to make Multiplan portable across the widely varying hardware of the time. This byte-code distinguished between the machine-specific floating point format to calculate on, and an external (standard) format, which was binary coded decimal (BCD). The PACK and UNPACK instructions converted between the two.
