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<TITLE>Installer Behaviour</TITLE>
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<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000FF" VLINK="#400080" ALINK="#FF0000">
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<H1><A NAME=S10>TDF Specification, Issue 4.0</A></H1>
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<H3>January 1998</H3>
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<P>
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<HR>
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<DL>
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<DT><A HREF="#S11"><B>4.1</B> - Definition of terms</A><DD>
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<DT><A HREF="#S12"><B>4.2</B> - Properties of Installers</A><DD>
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<H1>4. Installer Behaviour</H1>
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<A NAME=S11>
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<H2>4.1. Definition of terms</H2>
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In this document the behaviour of TDF installers is described in a
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precise manner. Certain words are used with very specific meanings.
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These are:
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<UL>
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<LI>"undefined": means that installers can perform any action,
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including refusing to translate the program. It can produce code with
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any effect, meaningful or meaningless.<P>
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<LI>"shall": when the phrase "<I>P</I> shall be done"
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(or similar phrases involving "shall") is used, every installer
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must perform <I>P</I>.<P>
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<LI>"should": when the phrase "<I>P</I> should be done"
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(or similar phrase involving "should") is used, installers
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are advised to perform <I>P</I>, and producer writers may assume it
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will be done if possible. This usage generally relates to optimisations
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which are recommended.<P>
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<LI>"will": when the phrase "<I>P</I> will be true"
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(or similar phrases involving "will") is used to describe
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the composition of a TDF construct, the installer may assume that
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<I>P</I> holds without having to check it. If, in fact, a producer
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has produced TDF for which <I>P</I> does not hold, the effect is undefined.<P>
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<LI>"target-defined": means that behaviour will be defined,
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but that it varies from one target machine to another. Each target
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installer shall define everything which is said to be "target-defined".
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</UL>
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<P>
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<A NAME=S12>
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<H2>4.2. Properties of Installers</H2>
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All installers must implement all of the constructions of TDF. There
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are some constructions where the installers may impose limits on the
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ranges of values which are implemented. In these cases the description
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of the installer must specify these limits.
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<P>
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Installers are not expected to check that the TDF they are processing
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is well-formed, nor that undefined constructs are absent. If the TDF
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is not well-formed any effect is permitted.
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<P>
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Installers shall only implement optimisations which are correct in
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all circumstances. This correctness can only be shown by demonstrating
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the equivalence of the transformed program, from equivalences deducible
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from this specification or from the ordinary laws of arithmetic. No
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statements are made in this specification of the form "such and
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such an optimisation is permitted".
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<P>
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<I>Fortran90 has a notion of mathematical equivalence which is not
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the same as TDF equivalence. It can be applied to transform programs
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provided parentheses in the text are not crossed. TDF does not acknowledge
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this concept. Such transformations would have to be applied in a context
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where the permitted changes are known.</I>
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<P>
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<HR>
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<P><I>Part of the <A HREF="../index.html">TenDRA Web</A>.<BR>Crown
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Copyright © 1998.</I></P>
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