For every array or record type S, the representation attribute
Scalar_Storage_Order denotes the order in which storage elements
that make up scalar components are ordered within S. The value given must
be a static expression of type System.Bit_Order. The following is an example
of the use of this feature:
-- Component type definitions subtype Yr_Type is Natural range 0 .. 127; subtype Mo_Type is Natural range 1 .. 12; subtype Da_Type is Natural range 1 .. 31; -- Record declaration type Date is record Years_Since_1980 : Yr_Type; Month : Mo_Type; Day_Of_Month : Da_Type; end record; -- Record representation clause for Date use record Years_Since_1980 at 0 range 0 .. 6; Month at 0 range 7 .. 10; Day_Of_Month at 0 range 11 .. 15; end record; -- Attribute definition clauses for Date'Bit_Order use System.High_Order_First; for Date'Scalar_Storage_Order use System.High_Order_First; -- If Scalar_Storage_Order is specified, it must be consistent with -- Bit_Order, so it's best to always define the latter explicitly if -- the former is used.
Other properties are as for standard representation attribute
as defined by Ada RM 13.5.3(4). The default is
For a record type T, if T
specified explicitly, it shall be equal to T
this means that if a
Scalar_Storage_Order attribute definition
clause is not confirming, then the type's
Bit_Order shall be
specified explicitly and set to the same value.
Derived types inherit an explicitly set scalar storage order from their parent types. This may be overridden for the derived type by giving an explicit scalar storage order for the derived type. For a record extension, the derived type must have the same scalar storage order as the parent type.
If a component of T is of a record or array type, then that type must
also have a
Scalar_Storage_Order attribute definition clause.
A component of a record or array type that is a packed array, or that does not start on a byte boundary, must have the same scalar storage order as the enclosing record or array type.
No component of a type that has an explicit
attribute definition may be aliased.
Scalar_Storage_Order attribute definition clause (i.e.
with a value equal to
System.Default_Bit_Order) has no effect.
If the opposite storage order is specified, then whenever the value of a scalar component of an object of type S is read, the storage elements of the enclosing machine scalar are first reversed (before retrieving the component value, possibly applying some shift and mask operatings on the enclosing machine scalar), and the opposite operation is done for writes.
In that case, the restrictions set forth in 13.5.1(10.3/2) for scalar components are relaxed. Instead, the following rules apply:
(position + first_bit / storage_element_size) .. (position + (last_bit + storage_element_size - 1) / storage_element_size)
position + first_bit / storage_element_sizeand covering storage elements at least up to
position + (last_bit + storage_element_size - 1) / storage_element_size
If no scalar storage order is specified for a type (either directly, or by
inheritance in the case of a derived type), then the default is normally
the native ordering of the target, but this default can be overridden using