C.1 Access to Machine Operations
[This subclause specifies rules regarding access to machine instructions
from within an Ada program.]
Implementation defined: Implementation-defined
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.
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.
Implementation Advice: The machine code
or intrinsics support should allow access to all operations normally
available to assembly language programmers for the target environment.
Ramification: 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.
support for interfacing aspects (see Annex B
should include interface to assembler; the default assembler should be
associated with the convention identifier Assembler.
Implementation Advice: Interface to assembler
should be supported; 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.
Implementation Advice: If an entity is
exported to assembly language, then the implementation should allocate
it at an addressable location even if not otherwise referenced from the
Ada code. A call to a machine code or assembler subprogram should be
treated as if it could read or update every object that is specified
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.
Documentation Requirement: The overhead
of calling machine-code or intrinsic subprograms.
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
Documentation Requirement: The types
and attributes used in machine code insertions.
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
Documentation Requirement: The subprogram
calling conventions for all supported convention identifiers.
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
Documentation Requirement: The mapping
between the Link_Name or Ada designator and the external link name.
The implementation should ensure that little or no
overhead is associated with calling intrinsic and machine-code subprograms.
Implementation Advice: Little or no overhead
should be 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,
String manipulation operations — e.g., translate
Vector operations — e.g., compare vector
Direct operations on I/O ports.
Implementation Advice: Intrinsic subprograms
should be provided to access any machine operations that provide special
capabilities or efficiency not normally available.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe