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a machine that reads the information in a system of punched holes on a punch card and prints it in the form of letters and numbers on the same card. The decoded and printed information on the card facilitates retrieval of cards in files and visual control over the correct recording of data. The interpreter is part of the punch-card computer unit.
There are interpreters for single entries of decoded information on the blank, top margin of punch cards and for periodic printing of data in columns between the card’s punching positions. The interpreter is a fairly complex device, equipped with mechanisms for card feeding and punch reading and a printer. The fastest interpreters can process 80–120 cards/min.
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 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 analyse 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 index into a jump table. An interpreter might well use the same lexical analyser and parser as the compiler and then interpret the resulting abstract syntax tree.
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 "byte-code" which is a highly compressed and optimised representation of the Lisp source but is not machine code (and therefore not tied to any particular hardware). This "compiled" code is then executed (interpreted) by a byte code 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 byte-code interpreter.
See also partial evaluation.
interpreterA high-level programming language translator that translates and runs the program at the same time. It converts one program statement into machine language, executes it, and then proceeds to the next statement. This differs from regular executable programs that are presented to the computer as binary-coded instructions. Interpreted programs remain in the source language the programmer wrote in, which is human readable text.
Multiplatform Runtime Modules
A major advantage of an interpreted language is that it is generally able to run on more than one hardware platform. The source code is the same, but the actual interpreter software ("runtime module") converts the source into machine language. The interpreter must itself be in the native machine language of the hardware it runs in, which means changes in the language require updated interpreters for each hardware platform.
Slower, But Easier to Test
Interpreted programs run slower than their compiler counterparts. Whereas the compiler translates the entire program before it is run, interpreters translate a line at a time while the program is being run. However, it is very convenient to write an interpreted program, since a single line of code can be tested interactively.
Some languages can be both interpreted and compiled, in which case a program may be developed with the interpreter for ease of testing and debugging and later compiled for production use. See JIT compiler.
Intermediate Languages - Compiled and Interpreted
Languages such as Java and Visual Basic are compiled into an intermediate bytecode language that still requires a runtime module (see illustration below). See Java and Java virtual machine.
|Interpreters vs. Compilers|
|Unlike compiled languages that are translated entirely into machine language ahead of time (right), interpreted languages are translated at runtime. Java and Visual Basic interpreters (center) translate "bytecode," which is an intermediate language compiled from the original source code.|