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<H1> 13.2 Pragma Pack</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [A <FONT FACE="Arial, Helvetica">pragma</FONT>
Pack specifies that storage minimization should be the main criterion
when selecting the representation of a composite type.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented" Style="margin-bottom: 0.2em">The form of
a <FONT FACE="Arial, Helvetica">pragma</FONT> Pack is as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I4370"></A>Pack(<I>first_subtype_</I><A NAME="I4371"></A><FONT FACE="Arial, Helvetica">local_name</FONT>);
</DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> The <I>first_subtype_</I><FONT FACE="Arial, Helvetica">local_name</FONT>
of a <FONT FACE="Arial, Helvetica">pragma</FONT> Pack shall denote a
composite subtype. </DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4372"></A><A NAME="I4373"></A><A NAME="I4374"></A><A NAME="I4375"></A><A NAME="I4376"></A>A
<FONT FACE="Arial, Helvetica">pragma</FONT> Pack specifies the <I>packing</I>
aspect of representation; the type (or the extension part) is said to
be <I>packed</I>. For a type extension, the parent part is packed as
for the parent type, and a <FONT FACE="Arial, Helvetica">pragma</FONT>
Pack causes packing only of the extension part. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The only
high level semantic effect of a <FONT FACE="Arial, Helvetica">pragma</FONT>
Pack is independent addressability (see <A HREF="AA-9-10.html">9.10</A>,
``<A HREF="AA-9-10.html">Shared Variables</A>''). </FONT></DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> If a type is packed, then the implementation should
try to minimize storage allocated to objects of the type, possibly at
the expense of speed of accessing components, subject to reasonable complexity
in addressing calculations. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>A <FONT FACE="Arial, Helvetica">pragma</FONT>
Pack is for gaining space efficiency, possibly at the expense of time.
If more explicit control over representation is desired, then a <FONT FACE="Arial, Helvetica">record_representation_clause</FONT>,
a Component_Size clause, or a Size clause should be used instead of,
or in addition to, a <FONT FACE="Arial, Helvetica">pragma</FONT> Pack.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I4377"></A>The
recommended level of support for pragma Pack is: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>For a packed record type, the components should be packed
as tightly as possible subject to the Sizes of the component subtypes,
and subject to any <FONT FACE="Arial, Helvetica">record_representation_clause</FONT>
that applies to the type; the implementation may, but need not, reorder
components or cross aligned word boundaries to improve the packing. A
component whose Size is greater than the word size may be allocated an
integral number of words. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The implementation
can always allocate an integral number of words for a component that
will not fit in a word. The rule also allows small component sizes to
be rounded up if such rounding does not waste space. For example, if
Storage_Unit = 8, then a component of size 8 is probably more efficient
than a component of size 7 plus a 1-bit gap (assuming the gap is needed
anyway). </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>For a packed array type, if the component subtype's Size
is less than or equal to the word size, and Component_Size is not specified
for the type, Component_Size should be less than or equal to the Size
of the component subtype, rounded up to the nearest factor of the word
size. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>9.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>If a component
subtype is aliased, its Size will generally be a multiple of Storage_Unit,
so it probably won't get packed very tightly. </FONT></DIV>
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