In 1983,
David H. D. Warren designed an
abstract machine
In computer science, an abstract machine is a theoretical model that allows for a detailed and precise analysis of how a computer system functions. It is similar to a mathematical function in that it receives inputs and produces outputs based on p ...
for the execution of
Prolog
Prolog is a logic programming language that has its origins in artificial intelligence, automated theorem proving, and computational linguistics.
Prolog has its roots in first-order logic, a formal logic. Unlike many other programming language ...
consisting of a
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
architecture and an
instruction set
In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...
.
This design became known as the Warren Abstract Machine (WAM) and has become the ''de facto'' standard target for Prolog
compiler
In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...
s.
Purpose
The purpose of compiling Prolog code to the more low-level WAM code is to make subsequent interpretation of the Prolog program more efficient. Prolog code is reasonably easy to translate to WAM instructions, which can be more efficiently interpreted. Also, subsequent code improvements and compilations to native code are often easier to perform on the more low-level representation.
In order to write efficient Prolog programs, a basic understanding of how the WAM works can be advantageous. Some of the most important WAM concepts are first argument indexing and its relation to choice-points,
tail call optimization, and memory reclamation on failure.
Memory areas
The WAM has the following memory areas:
* The ''global stack'' or ''heap'', used to store compound terms
* The ''local stack'' for environment frames and choice-points
* The ''trail'' to record which variables bindings ought to be undone on backtracking
Example
Here is a piece of Prolog code:
girl(sally).
girl(jane).
boy(B) :- \+ girl(B).
A WAM-based Prolog compiler will compile this into WAM instructions similar to the following:
predicate(girl/1):
switch_on_term(2,1,fail,fail,fail),
label(1): switch_on_atom( sally,3),(jane,5)
label(2): try_me_else(4)
label(3): get_atom(sally,0)
proceed
label(4): trust_me_else_fail
label(5): get_atom(jane,0)
proceed
predicate(boy/1):
get_variable(x(1),0)
put_structure(girl/1,0)
unify_local_value(x(1))
execute((\+)/1)])
An important characteristic of this code is its ability to cope with the various modes in which the predicates can be evoked: any argument might be a variable, a
ground term
In mathematical logic, a ground term of a formal system is a term that does not contain any variables. Similarly, a ground formula is a formula that does not contain any variables.
In first-order logic with identity with constant symbols a ...
, or a partly instantiated term. The "switch" instructions handle the different cases.
References
{{DEFAULTSORT:Warren Abstract Machine
Logic programming
Abstract machines
Virtual machines
SRI International software