Interpreter (computing)

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In computer science the term interpreter is sometimes used instead of the term emulator. There are software interpreters and hardware interpreters. We will denote interpreter as a software interpreter. It can also refer to a program that performs compilation as well as emulation. Most interpreters available today generally compile source code when the code is first encountered during program execution, rather than in a separate phase prior to execution.

An interpreter has a number of advantages over a compiler, including:

  • because it can be tailored to a specific programming language making it simpler to implement and more compact (BASIC was supported on many early home computers for this reason).
  • it allows program implementation to be independent of the characteristics of the host cpu (the Java interpreter is a good example of this).

The main disadvantage of interpreters is that when a program is interpreted, it runs slower than if it had been compiled. The difference in speeds could be tiny or great; often an order of magnitude and sometimes more.

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[edit] Interpreter versus compiler

Empirically, it takes longer to run a program under an interpreter than to run the compiled code but it can take less time to interpret it than the total time required to compile and run it. This is especially important when prototyping and testing code when an edit-interpret-debug cycle can often be much shorter than an edit-compile-run-debug cycle.

Interpreting code is slower than running the compiled code because the interpreter must analyze each statement in the program each time it is executed and then perform the desired action whereas the compiled code just performs the action. This run-time analysis is known as "interpretive overhead". Access to variables is also slower in an interpreter because the mapping of identifiers to storage locations must be done repeatedly at run-time rather than at compile time.

There are various compromises between the development speed when using an interpreter and the execution speed when using a compiler. Some systems (e.g., some LISPs) allow interpreted and compiled code to call each other and to share variables. This means that once a routine has been tested and debugged under the interpreter it can be compiled and thus benefit from faster execution while other routines are being developed. Many interpreters do not execute the source code as it stands but convert it into some more compact internal form. For example, some BASIC interpreters replace keywords with single byte tokens which can be used to find the instruction in a jump table. An interpreter might well use the same lexical analyzer and parser as the compiler and then interpret the resulting abstract syntax tree.

[edit] Bytecode interpreter

There is thus a spectrum of possibilities between interpreting and compiling, depending on the amount of analysis performed before the program is executed. For example, Emacs Lisp is compiled to bytecode, which is a highly compressed and optimized representation of the Lisp source, but is not machine code (and therefore not tied to any particular hardware). This "compiled" code is then interpreted by a bytecode interpreter (itself written in C). The compiled code in this case is machine code for a virtual machine, which is implemented not in hardware, but in the bytecode interpreter. The same approach is used with the Forth code used in Open Firmware systems: the source language is compiled into "F code" (a bytecode), which is then interpreted by an architecture-independent virtual machine.

[edit] Just-in-time compilation

Just-in-time compilation, or JIT, refers to a technique where bytecode is compiled to native machine code at runtime; giving the high execution speed of running native code at the cost of increased startup-time as the bytecode is compiled. It has gained attention in recent years, which further blurs the distinction between interpreters, byte-code interpreters and compilation. JIT is available for both the .NET and Java platforms. The JIT technique is a few decades old, appearing in languages such as Smalltalk in the 1980's.

[edit] Example of a simple interpreter

There is an example of a simple program and interpreter in the Literate programming article.

[edit] Punched card interpreter

The term "interpreter" often referred to a piece of unit record equipment that could read punched cards and print the characters in human-readable form along the top edge of the card. The IBM 550 Numeric Interpreter and IBM 557 Alphabetic Interpreter are typical examples from 1930 and 1954, respectively.

[edit] See also

[edit] External link

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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