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13.7.1 The Package System.Storage_Elements

Static Semantics

1
The following language-defined library package exists: 
2/2
{AI95-00362-01} package System.Storage_Elements is
   pragma Pure(Storage_Elements);
3
   type Storage_Offset is range implementation-defined;
4
   subtype Storage_Count is Storage_Offset range 0..Storage_Offset'Last;
5
   type Storage_Element is mod implementation-defined;
   for Storage_Element'Size use Storage_Unit;
   type Storage_Array is array
     (Storage_Offset range <>) of aliased Storage_Element;
   for Storage_Array'Component_Size use Storage_Unit;
6
   -- Address Arithmetic:
7/3
{AI05-0229-1}    function "+"(Left : Address; Right : Storage_Offset) return Address
      with Convention => Intrinsic;
   function "+"(Left : Storage_Offset; Right : Address) return Address
      with Convention => Intrinsic;
   function "-"(Left : Address; Right : Storage_Offset) return Address
      with Convention => Intrinsic;
   function "-"(Left, Right : Address) return Storage_Offset
      with Convention => Intrinsic;
8/3
{AI05-0229-1}    function "mod"(Left : Address; Right : Storage_Offset)
      return Storage_Offset
         with Convention => Intrinsic;
9
   -- Conversion to/from integers:
10/3
{AI05-0229-1}    type Integer_Address is implementation-defined;
   function To_Address(Value : Integer_Address) return Address
      with Convention => Intrinsic;
   function To_Integer(Value : Address) return Integer_Address
      with Convention => Intrinsic;
11/3
{AI05-0229-1} end System.Storage_Elements;
11.a/3
Reason: {AI05-0229-1} The Convention aspects imply that the attribute Access is not allowed for those operations.
11.b
The mod function is needed so that the definition of Alignment makes sense. 
11.c/2
Implementation defined: The range of Storage_Elements.Storage_Offset, the modulus of Storage_Elements.Storage_Element, and the declaration of Storage_Elements.Integer_Address..
12
Storage_Element represents a storage element. Storage_Offset represents an offset in storage elements. Storage_Count represents a number of storage elements. Storage_Array represents a contiguous sequence of storage elements. 
12.a
Reason: The index subtype of Storage_Array is Storage_Offset because we wish to allow maximum flexibility. Most Storage_Arrays will probably have a lower bound of 0 or 1, but other lower bounds, including negative ones, make sense in some situations.
12.b/2
This paragraph was deleted.{AI95-00114-01}
13
Integer_Address is a [(signed or modular)] integer subtype. To_Address and To_Integer convert back and forth between this type and Address. 

Implementation Requirements

14
Storage_Offset'Last shall be greater than or equal to Integer'Last or the largest possible storage offset, whichever is smaller. Storage_Offset'First shall be <= (–Storage_Offset'Last).

Implementation Permissions

15/2
 {AI95-00362-01}
Paragraph 15 was deleted. 

Implementation Advice

16
Operations in System and its children should reflect the target environment semantics as closely as is reasonable. For example, on most machines, it makes sense for address arithmetic to “wrap around.” Operations that do not make sense should raise Program_Error. 
16.a.1/2
Implementation Advice: Operations in System and its children should reflect the target environment; operations that do not make sense should raise Program_Error.
16.a
Discussion: For example, on a segmented architecture, X < Y might raise Program_Error if X and Y do not point at the same segment (assuming segments are unordered). Similarly, on a segmented architecture, the conversions between Integer_Address and Address might not make sense for some values, and so might raise Program_Error.
16.b
Reason: We considered making Storage_Element a private type. However, it is better to declare it as a modular type in the visible part, since code that uses it is already low level, and might as well have access to the underlying representation. We also considered allowing Storage_Element to be any integer type, signed integer or modular, but it is better to have uniformity across implementations in this regard, and viewing storage elements as unsigned seemed to make the most sense.
16.c
Implementation Note: To_Address is intended for use in Address clauses. Implementations should overload To_Address if appropriate. For example, on a segmented architecture, it might make sense to have a record type representing a segment/offset pair, and have a To_Address conversion that converts from that record type to type Address.

Extensions to Ada 95

16.d/2
{AI95-00362-01} Package System.Storage_Elements is now Pure, so it can be portably used in more places. (Ada 95 allowed it to be Pure, but did not require that.) 

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