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13.8 Machine Code Insertions

1
   [{machine code insertion} A machine code insertion can be achieved by a call to a subprogram whose sequence_of_statements contains code_statements.]

Syntax

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code_statement ::= qualified_expression;
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A code_statement is only allowed in the handled_sequence_of_statements of a subprogram_body. If a subprogram_body contains any code_statements, then within this subprogram_body the only allowed form of statement is a code_statement (labeled or not), the only allowed declarative_items are use_clauses, and no exception_handler is allowed (comments and pragmas are allowed as usual).

Name Resolution Rules

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   {expected type (code_statement) [partial]} The qualified_expression is expected to be of any type.

Legality Rules

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   The qualified_expression shall be of a type declared in package System.Machine_Code.
5.a
Ramification: This includes types declared in children of System.Machine_Code.
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   A code_statement shall appear only within the scope of a with_clause that mentions package System.Machine_Code.
6.a
Ramification: Note that this is not a note; without this rule, it would be possible to write machine code in compilation units which depend on System.Machine_Code only indirectly.

Static Semantics

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   {System.Machine_Code} The contents of the library package System.Machine_Code (if provided) are implementation defined. The meaning of code_statements is implementation defined. [Typically, each qualified_expression represents a machine instruction or assembly directive.]
7.a
Discussion: For example, an instruction might be a record with an Op_Code component and other components for the operands.
7.b
Implementation defined: The contents of the visible part of package System.Machine_Code, and the meaning of code_statements.

Implementation Permissions

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   An implementation may place restrictions on code_statements. An implementation is not required to provide package System.Machine_Code.
NOTES
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15  An implementation may provide implementation-defined pragmas specifying register conventions and calling conventions.
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16  Machine code functions are exempt from the rule that a return_statement is required. In fact, return_statements are forbidden, since only code_statements are allowed.
10.a
Discussion: The idea is that the author of a machine code subprogram knows the calling conventions, and refers to parameters and results accordingly. The implementation should document where to put the result of a machine code function, for example, ``Scalar results are returned in register 0.''
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17  Intrinsic subprograms (see 6.3.1, ``Conformance Rules'') can also be used to achieve machine code insertions. Interface to assembly language can be achieved using the features in Annex B, ``Interface to Other Languages''.

Examples

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    Example of a code statement:
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M : Mask;
procedure Set_Mask; pragma Inline(Set_Mask);
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procedure Set_Mask is
  use System.Machine_Code; -- assume ``with System.Machine_Code;'' appears somewhere above
begin
  SI_Format'(Code => SSM, B => M'Base_Reg, D => M'Disp);
  --  Base_Reg and Disp are implementation-defined attributes
end Set_Mask;

Extensions to Ada 83

14.a
{extensions to Ada 83} Machine code functions are allowed in Ada 95; in Ada 83, only procedures were allowed.

Wording Changes from Ada 83

14.b
The syntax for code_statement is changed to say ``qualified_expression'' instead of ``subtype_mark'record_aggregate''. Requiring the type of each instruction to be a record type is overspecification.

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