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<H1> B.2 The Package Interfaces</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> Package Interfaces is the parent of several library
packages that declare types and other entities useful for interfacing
to foreign languages. It also contains some implementation-defined types
that are useful across more than one language (in particular for interfacing
to assembly language). </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>The
contents of the visible part of package Interfaces and its language-defined
descendants.</FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The library package
Interfaces has the following skeletal declaration: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT><A NAME="I6343"></A><A NAME="I6344"></A><BR>
<B>package</B> Interfaces <B>is</B><BR>
<B>pragma</B> Pure(Interfaces);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> Integer_<I>n</I> <B>is</B> <B>range</B> -2**(<I>n</I>-1) .. 2**(<I>n</I>-1) - 1; --<I>2's complement</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> Unsigned_<I>n</I> <B>is</B> <B>mod</B> 2**<I>n</I>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> Shift_Left (Value : Unsigned_<I>n</I>; Amount : Natural)<BR>
<B>return</B> Unsigned_<I>n</I>;<BR>
<B>function</B> Shift_Right (Value : Unsigned_<I>n</I>; Amount : Natural)<BR>
<B>return</B> Unsigned_<I>n</I>;<BR>
<B>function</B> Shift_Right_Arithmetic (Value : Unsigned_<I>n</I>; Amount : Natural)<BR>
<B>return</B> Unsigned_<I>n</I>;<BR>
<B>function</B> Rotate_Left (Value : Unsigned_<I>n</I>; Amount : Natural)<BR>
<B>return</B> Unsigned_<I>n</I>;<BR>
<B>function</B> Rotate_Right (Value : Unsigned_<I>n</I>; Amount : Natural)<BR>
<B>return</B> Unsigned_<I>n</I>;<BR>
...<BR>
<B>end</B> Interfaces;</TT></DIV>
<H4 ALIGN=CENTER>Implementation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> An implementation
shall provide the following declarations in the visible part of package
Interfaces: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>Signed and modular integer types of <I>n</I> bits, if supported
by the target architecture, for each <I>n</I> that is at least the size
of a storage element and that is a factor of the word size. The names
of these types are of the form Integer_<I>n</I> for the signed types,
and Unsigned_<I>n</I> for the modular types; </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>For example,
for a typical 32-bit machine the corresponding types might be Integer_8,
Unsigned_8, Integer_16, Unsigned_16, Integer_32, and Unsigned_32.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The wording above implies, for
example, that Integer_16'Size = Unsigned_16'Size = 16. Unchecked conversions
between same-Sized types will work as expected. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I6345"></A><A NAME="I6346"></A>For each such modular
type in Interfaces, shifting and rotating subprograms as specified in
the declaration of Interfaces above. These subprograms are Intrinsic.
They operate on a bit-by-bit basis, using the binary representation of
the value of the operands to yield a binary representation for the result.
The Amount parameter gives the number of bits by which to shift or rotate.
For shifting, zero bits are shifted in, except in the case of Shift_Right_Arithmetic,
where one bits are shifted in if Value is at least half the modulus.
</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>9.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>We considered making
shifting and rotating be primitive operations of all modular types. However,
it is a design principle of Ada that all predefined operations should
be operators (not functions named by identifiers). (Note that an early
version of Ada had "<B>abs</B>" as an identifier, but it was
changed to a reserved word operator before standardization of Ada 83.)
This is important because the implicit declarations would hide non-overloadable
declarations with the same name, whereas operators are always overloadable.
Therefore, we would have had to make shift and rotate into reserved words,
which would have been upward incompatible, or else invent new operator
symbols, which seemed like too much mechanism. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>Floating point types corresponding to each floating point
format fully supported by the hardware. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>The
names for these floating point types are not specified. <A NAME="I6347"></A>However,
if IEEE arithmetic is supported, then the names should be IEEE_Float_32
and IEEE_Float_64 for single and double precision, respectively.</FONT></DIV>
<H4 ALIGN=CENTER>Implementation Permissions</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> An implementation may provide implementation-defined
library units that are children of Interfaces, and may add declarations
to the visible part of Interfaces in addition to the ones defined above.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Implementation-defined
children of package Interfaces. The contents of the visible part of package
Interfaces.</FONT></DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> For each implementation-defined convention identifier,
there should be a child package of package Interfaces with the corresponding
name. This package should contain any declarations that would be useful
for interfacing to the language (implementation) represented by the convention.
Any declarations useful for interfacing to any language on the given
hardware architecture should be provided directly in Interfaces. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>For example,
package Interfaces.XYZ_Pascal might contain declarations of types that
match the data types provided by the XYZ implementation of Pascal, so
that it will be more convenient to pass parameters to a subprogram whose
convention is XYZ_Pascal. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> An implementation supporting an interface to
C, COBOL, or Fortran should provide the corresponding package or packages
described in the following clauses. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13.a</FONT></DIV>
<DIV Class="Annotations" Style="margin-bottom: 0.4em"><FONT SIZE=-1><B>Implementation
Note: </B>The intention is that an implementation might support several
implementations of the foreign language: Interfaces.This_Fortran and
Interfaces.That_Fortran might both exist. The ``default'' implementation,
overridable by the user, should be declared as a renaming: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13.b</FONT></DIV>
<DIV Class="SmallExamples"><TT><B>package</B> Interfaces.Fortran <B>renames</B> Interfaces.This_Fortran;</TT></DIV>
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