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<title>SWI-Prolog 7.3.6 Reference Manual: Section 1.5</title><link rel="home" href="index.html">
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<h2 id="sec:implhistory"><a id="sec:1.5"><span class="sec-nr">1.5</span> <span class="sec-title">Implementation
history</span></a></h2>
<a id="sec:implhistory"></a>
<p>SWI-Prolog started back in 1986 with the requirement for a Prolog
that could handle recursive interaction with the C-language: Prolog
calling C and C calling Prolog recursively. In those days Prolog systems
were not very aware of their environment and we needed such a system to
support interactive applications. Since then, SWI-Prolog's development
has been guided by requests from the user community, especially
focussing on (in arbitrary order) interaction with the environment,
scalability, (I/O) performance, standard compliance, teaching and the
program development environment.
<p>SWI-Prolog is based on a simple Prolog virtual machine called ZIP
<cite><a class="cite" href="Bibliography.html#Bowen:83">Bowen <em>et al.</em>,
1983</a>, <a class="cite" href="Bibliography.html#Neumerkel:93">Neumerkel,
1993</a></cite> which defines only 7 instructions. Prolog can easily be
compiled into this language, and the abstract machine code is easily
decompiled back into Prolog. As it is also possible to wire a standard
4-port debugger in the virtual machine, there is no need for a
distinction between compiled and interpreted code. Besides simplifying
the design of the Prolog system itself, this approach has advantages for
program development: the compiler is simple and fast, the user does not
have to decide in advance whether debugging is required, and the system
only runs slightly slower in debug mode compared to normal execution.
The price we have to pay is some performance degradation (taking out the
debugger from the VM interpreter improves performance by about 20%) and
somewhat additional memory usage to help the decompiler and debugger.
<p>SWI-Prolog extends the minimal set of instructions described in
<cite><a class="cite" href="Bibliography.html#Bowen:83">Bowen <em>et al.</em>,
1983</a></cite> to improve performance. While extending this set, care
has been taken to maintain the advantages of decompilation and tracing
of compiled code. The extensions include specialised instructions for
unification, predicate invocation, some frequently used built-in
predicates, arithmetic, and control (<a class="pred" href="control.html#;/2">;/2</a>, <a class="pred" href="control.html#|/2">|/2</a>),
if-then (<a class="pred" href="control.html#send_arrow/2">->/2</a>)
and negation-by-failure (<a class="pred" href="control.html#\+/1">\+/1</a>).
<p></body></html>
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