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   Re: Notations

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  • From: Paul Prescod <paul@prescod.net>
  • To: xml-dev@ic.ac.uk
  • Date: Sat, 12 Dec 1998 15:45:25 -0600

James Tauber wrote:
> As I've discovered before on this list, there can be a lot of confusion in
> the use of the term "semantic[s]". I think this is partly because you need
> to know what layer you are talking about.
> At the token level, XML specifies both a syntax and a semantics. (eg it
> tells you where '<' can occur and tells you want it means when it does).
> But the semantics at this level are purely to provide the syntax for the
> next.

This is a very important point! XML does have semantics, just not enough
to solve any problems (by itself). 

I've found that there is a similar layer-specificity about the words
"physical" vs. "virtual." For instance, people refer to files on a file
system as "physical" if they are comparing them to logical constructs such
as the result of an FTP transfer (which could have been
generated on the fly). But files are not physical if you compare them to
blocks on the hard drive. It is not correct to say that file systems have
nothing to do with the "physical organization of data." The definition of
physical depends on the layer you are discussing.

A garbage collection system that used double indirection might speak of
"virtual" memory locations (ones not recognized as addresses by the CPU)
and "physical" memory locations (recognized by the CPU) but some physical
memory locations might live in what we commonly call "virtual memory."

 Paul Prescod  - ISOGEN Consulting Engineer speaking for only himself

"Sports utility vehicles are gated communities on wheels" - Anon

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