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<H2><A NAME="sec:1.1"><SPAN class="sec-nr">1.1</SPAN> <SPAN class="sec-title">SWI-Prolog</SPAN></A></H2>

<A NAME="sec:swiprolog"></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. Those days Prolog systems 
were very aware of its 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 very 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 when in debug mode. 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">-&gt;/2</A>) 
and negation-by-failure (<A class="pred" href="control.html#\+/1">\+/1</A>).

<H3><A NAME="sec:1.1.1"><SPAN class="sec-nr">1.1.1</SPAN> <SPAN class="sec-title">Books 
about Prolog</SPAN></A></H3>

<A NAME="sec:books"></A>

<P>This manual does not describe the full syntax and semantics of 
Prolog, nor how one should write a program in Prolog. These subjects 
have been described extensively in the literature. See <CITE><A class="cite" href="Bibliography.html#Bratko:86">Bratko, 
1986</A></CITE>,
<CITE><A class="cite" href="Bibliography.html#Sterling:86">Sterling &amp; 
Shapiro, 1986</A></CITE>, and <CITE><A class="cite" href="Bibliography.html#Clocksin:87">Clocksin &amp; 
Melish, 1987</A></CITE>. For more advanced Prolog material see <CITE><A class="cite" href="Bibliography.html#Keefe:90">O'Keefe, 
1990</A></CITE>. Syntax and standard operator declarations confirm to 
the `Edinburgh standard'. Most built in predicates are compatible with 
those described in <CITE><A class="cite" href="Bibliography.html#Clocksin:87">Clocksin &amp; 
Melish, 1987</A></CITE>. SWI-Prolog also offers a number of primitive 
predicates compatible with Quintus Prolog<SUP class="fn">1<SPAN class="fn-text">Quintus 
is a trademark of Quintus Computer Systems Inc., USA</SPAN></SUP>
<CITE><A class="cite" href="Bibliography.html#QUINTUS:manual">Qui, 1997</A></CITE> 
and BIM_Prolog<SUP class="fn">2<SPAN class="fn-text">BIM is a trademark 
of BIM sa/nv., Belgium</SPAN></SUP>
<CITE><A class="cite" href="Bibliography.html#BIMPROLOG:manual">BIM, 
1989</A></CITE>.

<P>ISO compliant predicates are based on ``Prolog: The Standard'',
<CITE><A class="cite" href="Bibliography.html#Deransart:96">Deransart <EM>et 
al.</EM>, 1996</A></CITE>, validated using <CITE><A class="cite" href="Bibliography.html#stdprolog:98">Hodgson, 
1998</A></CITE>.

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