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13.5.1 Record Representation Clauses

A record_representation_clause specifies the storage representation of records and record extensions, that is, the order, position, and size of components (including discriminants, if any).


record_representation_clause ::= 
    for first_subtype_local_name use
       record [mod_clause]
       end record [local_name];
component_clause ::= 
    component_local_name at position range first_bit .. last_bit;
position ::= static_expression
first_bit ::= static_simple_expression
last_bit ::= static_simple_expression
If a local_name appears at the end of the record_representation_clause, it shall repeat the first_subtype_local_name.

Name Resolution Rules

Each position, first_bit, and last_bit is expected to be of any integer type. 

Legality Rules

The first_subtype_local_name of a record_representation_clause shall denote a specific record or record extension subtype. 
If the component_local_name is a direct_name, the local_name shall denote a component of the type. For a record extension, the component shall not be inherited, and shall not be a discriminant that corresponds to a discriminant of the parent type. If the component_local_name has an attribute_designator, the direct_name of the local_name shall denote either the declaration of the type or a component of the type, and the attribute_designator shall denote an implementation-defined implicit component of the type.
The position, first_bit, and last_bit shall be static expressions. The value of position and first_bit shall be nonnegative. The value of last_bit shall be no less than first_bit – 1. 
  If the nondefault bit ordering applies to the type, then either: 
the value of last_bit shall be less than the size of the largest machine scalar; or
the value of first_bit shall be zero and the value of last_bit + 1 shall be a multiple of System.Storage_Unit. 
At most one component_clause is allowed for each component of the type, including for each discriminant (component_clauses may be given for some, all, or none of the components). Storage places within a component_list shall not overlap, unless they are for components in distinct variants of the same variant_part.
A name that denotes a component of a type is not allowed within a record_representation_clause for the type, except as the component_local_name of a component_clause.

Static Semantics

A record_representation_clause (without the mod_clause) specifies the layout.
  If the default bit ordering applies to the type, the position, first_bit, and last_bit of each component_clause directly specify the position and size of the corresponding component.
  If the nondefault bit ordering applies to the type, then the layout is determined as follows:
the component_clauses for which the value of last_bit is greater than or equal to the size of the largest machine scalar directly specify the position and size of the corresponding component;
for other component_clauses, all of the components having the same value of position are considered to be part of a single machine scalar, located at that position; this machine scalar has a size which is the smallest machine scalar size larger than the largest last_bit for all component_clauses at that position; the first_bit and last_bit of each component_clause are then interpreted as bit offsets in this machine scalar. 
A record_representation_clause for a record extension does not override the layout of the parent part; if the layout was specified for the parent type, it is inherited by the record extension. 

Implementation Permissions

An implementation may generate implementation-defined components (for example, one containing the offset of another component). An implementation may generate names that denote such implementation-defined components; such names shall be implementation-defined attribute_references. An implementation may allow such implementation-defined names to be used in record_representation_clauses. An implementation can restrict such component_clauses in any manner it sees fit. 
If a record_representation_clause is given for an untagged derived type, the storage place attributes for all of the components of the derived type may differ from those of the corresponding components of the parent type, even for components whose storage place is not specified explicitly in the record_representation_clause.

Implementation Advice

The recommended level of support for record_representation_clauses is: 
An implementation should support machine scalars that correspond to all of the integer, floating point, and address formats supported by the machine.
An implementation should support storage places that can be extracted with a load, mask, shift sequence of machine code, and set with a load, shift, mask, store sequence, given the available machine instructions and run-time model.
A storage place should be supported if its size is equal to the Size of the component subtype, and it starts and ends on a boundary that obeys the Alignment of the component subtype.
For a component with a subtype whose Size is less than the word size, any storage place that does not cross an aligned word boundary should be supported.
An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place. 
An implementation is not required to support a component_clause for a component of an extension part if the storage place is not after the storage places of all components of the parent type, whether or not those storage places had been specified. 
NOTE   If no component_clause is given for a component, then the choice of the storage place for the component is left to the implementation. If component_clauses are given for all components, the record_representation_clause completely specifies the representation of the type and will be obeyed exactly by the implementation. 


Example of specifying the layout of a record type: 
Word : constant := 4;  --  storage element is byte, 4 bytes per word
type State         is (A,M,W,P);
type Mode          is (Fix, Dec, Exp, Signif);
type Byte_Mask     is array (0..7)  of Boolean with Component_Size => 1;
type State_Mask    is array (State) of Boolean with Component_Size => 1;
type Mode_Mask     is array (Mode)  of Boolean with Component_Size => 1;
type Program_Status_Word is
      System_Mask        : Byte_Mask;
      Protection_Key     : Integer range 0 .. 3;
      Machine_State      : State_Mask;
      Interrupt_Cause    : Interruption_Code;
      Ilc                : Integer range 0 .. 3;
      Cc                 : Integer range 0 .. 3;
      Program_Mask       : Mode_Mask;
      Inst_Address       : Address;
end record;
for Program_Status_Word use
      System_Mask      at 0*Word range 0  .. 7;
      Protection_Key   at 0*Word range 10 .. 11; -- bits 8,9 unused
      Machine_State    at 0*Word range 12 .. 15;
      Interrupt_Cause  at 0*Word range 16 .. 31;
      Ilc              at 1*Word range 0  .. 1;  -- second word
      Cc               at 1*Word range 2  .. 3;
      Program_Mask     at 1*Word range 4  .. 7;
      Inst_Address     at 1*Word range 8  .. 31;
  end record;
for Program_Status_Word'Size use 8*System.Storage_Unit;
for Program_Status_Word'Alignment use 8;
The record_representation_clause defines the record layout. The Size clause guarantees that (at least) eight storage elements are used for objects of the type. The Alignment clause guarantees that aliased, imported, or exported objects of the type will have addresses divisible by eight.

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