80 Carlos Inchaurralde
metaphor in the explanation of linguistic activity, which clearly is cogni-
tive in nature. Functional Grammar has always followed the computer
metaphor, or rather, the computer-language metaphor, with its Prolog-like
formalisms which make it so easy to implement in computer programs.
Nevertheless, Functional Grammar has always had the same operational
problem as Prolog: the logical relations are clearly expressed, and unifica-
tion and deduction are easy; but the actual procedure, the real machinery
that works behind the scenes, is hidden from us and has no specification –
not even a tentative one – of any kind.
What I suggest here is that we again resort to the computer metaphor
when looking at the processes that are covered in Hengeveld’s model. If
we take the two sides of communicative interaction, the speaker’s and the
addressee’s, as the two poles of a chain of actions, then the entire encod-
ing-decoding process would follow a process similar to that which is
currently used when two operating systems are connected by means of a
protocol. The layer model is customary in the architecture of present-day
operating systems. In the OSI Standard (Open Systems Interconnection
Reference Model),^4 for instance, there are several different layers: the
physical layer, the data link layer, the network layer, the transport layer,
the session layer, the presentation layer, the application layer, and the user
program. The different layers may generate requests for those below them
(the lowest of all being the physical layer). For instance, a word processor
(the user program layer) may generate a request for a certain routine that
opens windows with some given colours and a given size (the presentation
layer). This means that the user program layer generates a request which is
passed on to the application layer and then goes on down to the presenta-
tion layer. This layer performs the routine and sends an acknowledgement
primitive that goes up to the user layer again to acknowledge receipt of the
request and the performance of the action requested (see Figure 1). In this
respect, the Functional Grammar model proposed assumes this kind of
communication moving up and down the different layers.
However, the interesting point in our use of the operating systems
metaphor is how we can employ the basic model for communication across
local area networks as a model for natural language communication be-
tween individuals in a Functional Discourse Grammar framework. In
modern information systems, the communication is arranged in fixed-
length chunks of data called packets. These packets, as well as handshak-
ing messages for regulating the communication, are transmitted between
two interconnected systems as ‘frames’ created by a data-link layer and
then transmitted by the physical layer. A request from the user program