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<H1> 13.5.2 Storage Place Attributes</H1>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I4499"></A>For
a component C of a composite, non-array object R, the <I>storage place
attributes</I> are defined: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The storage
place attributes are not (individually) specifiable, but the user may
control their values by giving a <FONT FACE="Arial, Helvetica">record_representation_clause</FONT>.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DL Class="Hanging"><DT> R.C'Position<DD Class="Hanging">
<A NAME="I4500"></A><A NAME="I4501"></A>Denotes the same value as R.C'Address
- R'Address. The value of this attribute is of the type <I>universal_integer</I>.
</DL>
<DIV Class="Paranum"><FONT SIZE=-2>2.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Thus, R.C'Position
is the offset of C in storage elements from the beginning of the object,
where the first storage element of an object is numbered zero. R'Address
+ R.C'Position = R.C'Address. For record extensions, the offset is not
measured from the beginning of the extension part, but from the beginning
of the whole object, as usual.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>In ``R.C'Address - R'Address'',
the "-" operator is the one in System.Storage_Elements that
takes two Addresses and returns a Storage_Offset. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DL Class="Hanging"><DT> R.C'First_Bit<DD Class="Hanging">
<A NAME="I4502"></A><A NAME="I4503"></A>Denotes the offset, from the
start of the first of the storage elements occupied by C, of the first
bit occupied by C. This offset is measured in bits. The first bit of
a storage element is numbered zero. The value of this attribute is of
the type <I>universal_integer</I>.</DL>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DL Class="Hanging"><DT> R.C'Last_Bit<DD Class="Hanging">
<A NAME="I4504"></A><A NAME="I4505"></A>Denotes the offset, from the
start of the first of the storage elements occupied by C, of the last
bit occupied by C. This offset is measured in bits. The value of this
attribute is of the type <I>universal_integer</I>. </DL>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The ordering
of bits in a storage element is is defined in <A HREF="AA-13-5-3.html">13.5.3</A>,
``<A HREF="AA-13-5-3.html">Bit Ordering</A>''.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>R.C'Size = R.C'Last_Bit - R.C'First_Bit
+ 1. (Unless the implementation chooses an indirection representation.)</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>If a <FONT FACE="Arial, Helvetica">component_clause</FONT>
applies to a component, then that component will be at the same relative
storage place in all objects of the type. Otherwise, there is no such
requirement. </FONT></DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4506"></A><A NAME="I4507"></A>If a component
is represented using some form of pointer (such as an offset) to the
actual data of the component, and this data is contiguous with the rest
of the object, then the storage place attributes should reflect the place
of the actual data, not the pointer. If a component is allocated discontiguously
from the rest of the object, then a warning should be generated upon
reference to one of its storage place attributes. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>For discontiguous
components, these attributes make no sense. For example, an implementation
might allocate dynamic-sized components on the heap. For another example,
an implementation might allocate the discriminants separately from the
other components, so that multiple objects of the same subtype can share
discriminants. Such representations cannot happen if there is a <FONT FACE="Arial, Helvetica">component_clause</FONT>
for that component. </FONT></DIV>
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