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<H1> 13.7.1 The Package System.Storage_Elements</H1>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The following language-defined
library package exists: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Examples"><TT><A NAME="I4554"></A><A NAME="I4555"></A><A NAME="I4556"></A><B>package</B> System.Storage_Elements <B>is</B><BR>
<B>pragma</B> Preelaborate(System.Storage_Elements);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> <A NAME="I4557"></A><A NAME="I4558"></A>Storage_Offset <B>is</B> <B>range</B> <I>implementation-defined</I>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT> <B>subtype</B> <A NAME="I4559"></A>Storage_Count <B>is</B> Storage_Offset <B>range</B> 0..Storage_Offset'Last;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> <A NAME="I4560"></A><A NAME="I4561"></A>Storage_Element <B>is</B> <B>mod</B> <I>implementation-defined</I>;<BR>
<B>for</B> Storage_Element'Size <B>use</B> Storage_Unit;<BR>
<B>type</B> <A NAME="I4562"></A><A NAME="I4563"></A>Storage_Array <B>is</B> <B>array</B><BR>
(Storage_Offset <B>range</B> <>) <B>of</B> <B>aliased</B> Storage_Element;<BR>
<B>for</B> Storage_Array'Component_Size <B>use</B> Storage_Unit;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT> --<I> <A NAME="I4564"></A>Address Arithmetic:</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> "+"(Left : Address; Right : Storage_Offset)<BR>
<B>return</B> Address;<BR>
<B>function</B> "+"(Left : Storage_Offset; Right : Address)<BR>
<B>return</B> Address;<BR>
<B>function</B> "-"(Left : Address; Right : Storage_Offset)<BR>
<B>return</B> Address;<BR>
<B>function</B> "-"(Left, Right : Address)<BR>
<B>return</B> Storage_Offset;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> "<B>mod</B>"(Left : Address; Right : Storage_Offset)<BR>
<B>return</B> Storage_Offset;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Examples"><TT> --<I> Conversion to/from integers:</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> <A NAME="I4565"></A><A NAME="I4566"></A>Integer_Address <B>is</B> <I>implementation-defined</I>;<BR>
<B>function</B> <A NAME="I4567"></A><A NAME="I4568"></A>To_Address(Value : Integer_Address) <B>return</B> Address;<BR>
<B>function</B> <A NAME="I4569"></A><A NAME="I4570"></A>To_Integer(Value : Address) <B>return</B> Integer_Address;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Examples"><TT> <B>pragma</B> Convention(Intrinsic, "+");<BR>
<I>-- ...and so on for all language-defined subprograms declared in this package.</I><BR>
<B>end</B> System.Storage_Elements;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>The Convention
<FONT FACE="Arial, Helvetica">pragma</FONT>s imply that the attribute
Access is not allowed for those operations.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The <B>mod</B> function is needed
so that the definition of Alignment makes sense. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> Storage_Element represents a storage element.
Storage_Offset represents an offset in storage elements. Storage_Count
represents a number of storage elements. <A NAME="I4571"></A><A NAME="I4572"></A>Storage_Array
represents a contiguous sequence of storage elements. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>The index subtype
of Storage_Array is Storage_Offset because we wish to allow maximum flexibility.
Most Storage_Arrays will probably have a lower bound of 0 or 1, but other
lower bounds, including negative ones, make sense in some situations.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Note that there are some language-defined
subprograms that fill part of a Storage_Array, and return the index of
the last element filled as a Storage_Offset. The Read procedures in Streams
(see <A HREF="AA-13-13-1.html">13.13.1</A>), Streams.Stream_IO (see <A HREF="AA-A-12-1.html">A.12.1</A>),
and System.RPC (see <A HREF="AA-E-5.html">E.5</A>) behave in this manner.
These will raise Constraint_Error if the resulting Last value is not
in Storage_Offset. This implies that the Storage_Array passed to these
subprograms should not have a lower bound of Storage_Offset'First, because
then a read of 0 elements would always raise Constraint_Error. A better
choice of lower bound is 1. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> Integer_Address is a [(signed or modular)] integer
subtype. To_Address and To_Integer convert back and forth between this
type and Address. </DIV>
<H4 ALIGN=CENTER>Implementation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal"> Storage_Offset'Last shall be greater than or
equal to Integer'Last or the largest possible storage offset, whichever
is smaller. Storage_Offset'First shall be <= (-Storage_Offset'Last).
</DIV>
<H4 ALIGN=CENTER>Implementation Permissions</H4>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> Package System.Storage_Elements may be declared
pure. </DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal"> Operations in System and its children should
reflect the target environment semantics as closely as is reasonable.
For example, on most machines, it makes sense for address arithmetic
to ``wrap around.'' <A NAME="I4573"></A>Operations that do not make sense
should raise Program_Error. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>For example,
on a segmented architecture, X < Y might raise Program_Error if X
and Y do not point at the same segment (assuming segments are unordered).
Similarly, on a segmented architecture, the conversions between Integer_Address
and Address might not make sense for some values, and so might raise
Program_Error. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>We considered making
Storage_Element a private type. However, it is better to declare it as
a modular type in the visible part, since code that uses it is already
low level, and might as well have access to the underlying representation.
We also considered allowing Storage_Element to be any integer type, signed
integer or modular, but it is better to have uniformity across implementations
in this regard, and viewing storage elements as unsigned seemed to make
the most sense. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>To_Address
is intended for use in Address clauses. Implementations should overload
To_Address if appropriate. For example, on a segmented architecture,
it might make sense to have a record type representing a segment/offset
pair, and have a To_Address conversion that converts from that record
type to type Address. </FONT></DIV>
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