interpreter
<programming> A program which executes other programs. This is in
contrast to a compiler which does not execute its input program (the "source
code") but translates it into executable "machine code" (also called "object
code") which is output to a file for later execution. It may be possible to
execute the same source code either directly by an interpreter or by compiling
it and then executing the machine code produced.
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.
(1995-01-30)
Nearby terms:
interpolation « Interpress « interpreted «
interpreter
» Interpretive Menu Processor » Inter-process
Communication » interrupt
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