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Introduction to SGML(Standard Generalized Markup Language)

Contains no means to mark up presentational aspects of documents

Introduction to SGML

       Standard Generalized Markup Language

       Strictly descriptive

       Contains no means to mark up presentational aspects of documents

       Can be easily interfaced to external procedural markup systems and style sheets

       Like HTML, SGML is a computer language rather than a data format

       SGML files can be created manually, or through SGML editor software tools.


       SGML is a meta-language that defines only the syntax of a standard generalised markup language, i.e. is prescribes how we should specify markup, but not what that markup is, nor what it means.


SGML Parser

       Software that reads and analyzes an SGML document

       Validation or transformation

       Not much use by itself

       Part of a bigger SGML application system or browser

SGML History

       More than 10 years history of use and growth…

       Widelypublishingusedand-aerospace,otherindustriesautomotive, defense, software, semiconductor, pharmaceutical,

       ISO standard (ISO 8879) – adopted by several other standards bodies

SGML: Key Features

       Descriptive markup

       Document types

       Data independence


Descriptive markup

       Use of markup codes (names) to categorize parts of a document

       Example: <para> to identify a paragraph

       Advantage: Same document can be processed by different software for different purposes


Document Type

       Notion of ‗document type‘ (hence DTD)


       Type of a document is formally defined by its constituent parts and their structure – expressed in a tree structure

       Example: Report

                     Title, followed by author (optional), abstract, one or more paragraphs

       If title is absent, it is not a report

       If abstract follows paragraphs, it is not a report


Data Independence

       Document portability across different HW and SW environments

       How to handle character set differences

       Descriptive mapping for non-portable characters


       String substitution mechanism (entities): process time substitution of a particular string of characters by other string of characters

Defining an SGML Application


       From SGML view, a document is a hierarchical structure of nested elements (chapters, sections, paragraphs, etc.)

       SGML does not specify any presentational aspects of these elements


       SGML also does not convey any meaning or role of these elements – meaning is implied by the application


       SGML specifies the contexts and levels of document hierarchy in which an element can or must occur


       All documents that can be marked up with the same hierarchy of elements are said to belong to a certain document type


       SGML defines the structure of a particular type of documents via the DTD (Document Type Definition)


       Some general features of an SGML application are specified in another component called SGML Declaration.

       SGML Syntax:


SGML statements are enclosed in angle brackets (<>) and contain a keyword or name followed by one or more parameters separated by spacesCharacter ‗!‘ is inserted between

‗<‗ and the statement keyword


       Example: <!ELEMENT IMG – O EMPTY – Embedded image -->

       Comments within a <! Statement are enclosed in double hyphens (--)


Components of an SGML Document

       SGML Declaration:

       Character set, syntax (e.g. delimiters), optional features

       Usually a single declaration is used for all documents under a particular system


       Usually a single document type definition (DTD)


       Contains rules to which any document of a given type must conform

       Document instance:


       The document itself marked up following the SGML usage conventions specified in the SGML declaration and the DTD.

Advantages of SGML:


       Capable of dealing with any media type or linking protocol

       Extremely flexible

       Separates content from appearance

       Style sheets make content repurposing easy


       ISO standard - thus well supported with many tools available


Disadvantages of SGML:

       Creating DTD's requires exacting software engineering

       Linking tends to be complex

       Writing SGML software is extremely hard


       SGML tools tend to be expansive


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