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<H1> C.1 Access to Machine Operations</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [This clause specifies rules regarding access
to machine instructions from within an Ada program.] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Support
for access to machine instructions.</FONT></DIV>
<H4 ALIGN=CENTER>Implementation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6624"></A>The implementation shall support
machine code insertions (see <A HREF="AA-13-8.html">13.8</A>) or intrinsic
subprograms (see <A HREF="AA-6-3-1.html">6.3.1</A>) (or both). Implementation-defined
attributes shall be provided to allow the use of Ada entities as operands.
</DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> The machine code or intrinsics support should
allow access to all operations normally available to assembly language
programmers for the target environment, including privileged instructions,
if any. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Of course,
on a machine with protection, an attempt to execute a privileged instruction
in user mode will probably trap. Nonetheless, we want implementations
to provide access to them so that Ada can be used to write systems programs
that run in privileged mode. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6625"></A><A NAME="I6626"></A><A NAME="I6627"></A><A NAME="I6628"></A>The
interfacing pragmas (see <A HREF="AA-B.html">Annex B</A>) should support
interface to assembler; the default assembler should be associated with
the convention identifier Assembler.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> If an entity is exported to assembly language,
then the implementation should allocate it at an addressable location,
and should ensure that it is retained by the linking process, even if
not otherwise referenced from the Ada code. The implementation should
assume that any call to a machine code or assembler subprogram is allowed
to read or update every object that is specified as exported. </DIV>
<H4 ALIGN=CENTER>Documentation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> The implementation shall document the overhead
associated with calling machine-code or intrinsic subprograms, as compared
to a fully-inlined call, and to a regular out-of-line call.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> The implementation shall document the types of
the package System.Machine_Code usable for machine code insertions, and
the attributes to be used in machine code insertions for references to
Ada entities.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> The implementation shall document the subprogram
calling conventions associated with the convention identifiers available
for use with the interfacing pragmas (Ada and Assembler, at a minimum),
including register saving, exception propagation, parameter passing,
and function value returning.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> For exported and imported subprograms, the implementation
shall document the mapping between the Link_Name string, if specified,
or the Ada designator, if not, and the external link name used for such
a subprogram. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Implementation-defined
aspects of access to machine operations.</FONT></DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> The implementation should ensure that little
or no overhead is associated with calling intrinsic and machine-code
subprograms.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> It is recommended
that intrinsic subprograms be provided for convenient access to any machine
operations that provide special capabilities or efficiency and that are
not otherwise available through the language constructs. Examples of
such instructions include: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>Atomic read-modify-write operations -- e.g., test and set,
compare and swap, decrement and test, enqueue/dequeue.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>Standard numeric functions -- e.g., <I>sin</I>, <I>log</I>.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>String manipulation operations -- e.g., translate and test.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>Vector operations -- e.g., compare vector against thresholds.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>Direct operations on I/O ports.</LI></UL>
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