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C.1 Access to Machine Operations

This subclause specifies rules regarding access to machine instructions from within an Ada program. 

Implementation Requirements

The implementation shall support machine code insertions (see 13.8) or intrinsic subprograms (see 6.3.1) (or both). Implementation-defined attributes shall be provided to allow the use of Ada entities as operands.

Implementation Advice

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.
The support for interfacing aspects (see Annex B) should include interface to assembler; the default assembler should be associated with the convention identifier Assembler. 
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. 

Documentation Requirements

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. 
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. 
The implementation shall document the subprogram calling conventions associated with the convention identifiers available for use with the Convention aspect (Ada and Assembler, at a minimum), including register saving, exception propagation, parameter passing, and function value returning. 
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. 

Implementation Advice

The implementation should ensure that little or no overhead is associated with calling intrinsic and machine-code subprograms.
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: 
Atomic read-modify-write operations — e.g., test and set, compare and swap, decrement and test, enqueue/dequeue.
Standard numeric functions — e.g., sin, log.
String manipulation operations — e.g., translate and test.
Vector operations — e.g., compare vector against thresholds.
Direct operations on I/O ports.

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