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13.8 Machine Code Insertions

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

Syntax

2
code_statement ::= qualified_expression;
3
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

4
The qualified_expression is expected to be of any type. 

Legality Rules

5
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. 
6
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

7
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

8
An implementation may place restrictions on code_statements. An implementation is not required to provide package System.Machine_Code.
NOTES
9
18  An implementation may provide implementation-defined pragmas specifying register conventions and calling conventions.
10/2
19  {AI95-00318-02} 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.” 
11
20  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

12
Example of a code statement: 
13/3
{AI05-0229-1} M : Mask;
procedure Set_Mask
  with Inline;
14
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
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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