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+.. _About_This_Guide:
+
+****************
+About This Guide
+****************
+
+.. only:: PRO
+
+   For ease of exposition, 'GNAT Pro' will be referred to simply as
+   'GNAT' in the remainder of this document.
+
+This manual contains useful information in writing programs using the
+GNAT compiler.  It includes information on implementation dependent
+characteristics of GNAT, including all the information required by
+Annex M of the Ada language standard.
+
+GNAT implements Ada 95, Ada 2005 and Ada 2012, and it may also be
+invoked in Ada 83 compatibility mode.
+By default, GNAT assumes Ada 2012,
+but you can override with a compiler switch
+to explicitly specify the language version.
+(Please refer to the *GNAT User's Guide* for details on these switches.)
+Throughout this manual, references to 'Ada' without a year suffix
+apply to all the Ada versions of the language.
+
+Ada is designed to be highly portable.
+In general, a program will have the same effect even when compiled by
+different compilers on different platforms.
+However, since Ada is designed to be used in a
+wide variety of applications, it also contains a number of system
+dependent features to be used in interfacing to the external world.
+
+.. index:: Implementation-dependent features
+
+.. index:: Portability
+
+Note: Any program that makes use of implementation-dependent features
+may be non-portable.  You should follow good programming practice and
+isolate and clearly document any sections of your program that make use
+of these features in a non-portable manner.
+
+What This Reference Manual Contains
+===================================
+
+This reference manual contains the following chapters:
+
+* :ref:`Implementation_Defined_Pragmas`, lists GNAT implementation-dependent
+  pragmas, which can be used to extend and enhance the functionality of the
+  compiler.
+
+* :ref:`Implementation_Defined_Attributes`, lists GNAT
+  implementation-dependent attributes, which can be used to extend and
+  enhance the functionality of the compiler.
+
+* :ref:`Standard_and_Implementation_Defined_Restrictions`, lists GNAT
+  implementation-dependent restrictions, which can be used to extend and
+  enhance the functionality of the compiler.
+
+* :ref:`Implementation_Advice`, provides information on generally
+  desirable behavior which are not requirements that all compilers must
+  follow since it cannot be provided on all systems, or which may be
+  undesirable on some systems.
+
+* :ref:`Implementation_Defined_Characteristics`, provides a guide to
+  minimizing implementation dependent features.
+
+* :ref:`Intrinsic_Subprograms`, describes the intrinsic subprograms
+  implemented by GNAT, and how they can be imported into user
+  application programs.
+
+* :ref:`Representation_Clauses_and_Pragmas`, describes in detail the
+  way that GNAT represents data, and in particular the exact set
+  of representation clauses and pragmas that is accepted.
+
+* :ref:`Standard_Library_Routines`, provides a listing of packages and a
+  brief description of the functionality that is provided by Ada's
+  extensive set of standard library routines as implemented by GNAT.
+
+* :ref:`The_Implementation_of_Standard_I/O`, details how the GNAT
+  implementation of the input-output facilities.
+
+* :ref:`The_GNAT_Library`, is a catalog of packages that complement
+  the Ada predefined library.
+
+* :ref:`Interfacing_to_Other_Languages`, describes how programs
+  written in Ada using GNAT can be interfaced to other programming
+  languages.
+
+* :ref:`Specialized_Needs_Annexes`, describes the GNAT implementation of all
+  of the specialized needs annexes.
+
+* :ref:`Implementation_of_Specific_Ada_Features`, discusses issues related
+  to GNAT's implementation of machine code insertions, tasking, and several
+  other features.
+
+* :ref:`Implementation_of_Ada_2012_Features`, describes the status of the
+  GNAT implementation of the Ada 2012 language standard.
+
+* :ref:`Obsolescent_Features` documents implementation dependent features,
+  including pragmas and attributes, which are considered obsolescent, since
+  there are other preferred ways of achieving the same results. These
+  obsolescent forms are retained for backwards compatibility.
+
+* :ref:`Compatibility_and_Porting_Guide` presents some guidelines for
+  developing portable Ada code, describes the compatibility issues that
+  may arise between GNAT and other Ada compilation systems (including those
+  for Ada 83), and shows how GNAT can expedite porting applications
+  developed in other Ada environments.
+
+* :ref:`gnu_fdl` contains the license for this document.
+
+.. index:: Ada 95 Language Reference Manual
+
+.. index:: Ada 2005 Language Reference Manual
+
+This reference manual assumes a basic familiarity with the Ada 95 language, as
+described in the
+:title:`International Standard ANSI/ISO/IEC-8652:1995`.
+It does not require knowledge of the new features introduced by Ada 2005 or
+Ada 2012.
+All three reference manuals are included in the GNAT documentation
+package.
+
+Conventions
+===========
+.. index:: Conventions, typographical
+
+.. index:: Typographical conventions
+
+Following are examples of the typographical and graphic conventions used
+in this guide:
+
+* ``Functions``, ``utility program names``, ``standard names``,
+  and ``classes``.
+
+* ``Option flags``
+
+* :file:`File names`
+
+* ``Variables``
+
+* *Emphasis*
+
+* [optional information or parameters]
+
+* Examples are described by text
+
+  ::
+
+    and then shown this way.
+
+* Commands that are entered by the user are shown as preceded by a prompt string
+  comprising the ``$`` character followed by a space.
+
+Related Information
+===================
+
+See the following documents for further information on GNAT:
+
+* :title:`GNAT User's Guide for Native Platforms`,
+  which provides information on how to use the
+  GNAT development environment.
+
+* :title:`Ada 95 Reference Manual`, the Ada 95 programming language standard.
+
+* :title:`Ada 95 Annotated Reference Manual`, which is an annotated version
+  of the Ada 95 standard.  The annotations describe
+  detailed aspects of the design decision, and in particular contain useful
+  sections on Ada 83 compatibility.
+
+* :title:`Ada 2005 Reference Manual`, the Ada 2005 programming language standard.
+
+* :title:`Ada 2005 Annotated Reference Manual`, which is an annotated version
+  of the Ada 2005 standard.  The annotations describe
+  detailed aspects of the design decision.
+
+* :title:`Ada 2012 Reference Manual`, the Ada 2012 programming language standard.
+
+* :title:`DEC Ada, Technical Overview and Comparison on DIGITAL Platforms`,
+  which contains specific information on compatibility between GNAT and
+  DEC Ada 83 systems.
+
+* :title:`DEC Ada, Language Reference Manual`, part number AA-PYZAB-TK, which
+  describes in detail the pragmas and attributes provided by the DEC Ada 83
+  compiler system.