SWI-Prolog 7.4.1

A comprehensive Prolog environment

SWI-Prolog is a tool based on a very simple Prolog virtual machine called ZIP that defines 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. 

SWI-Prolog has the possibility of extending the minimal set of instructions in order to enhance the 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 (;/2, |/2), if-then (->/2) and negation-by-failure (\+/1).