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13.7.1 The Package System.Storage_Elements

Static Semantics

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The following language-defined library package exists: 
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package System.Storage_Elements is
   pragma Pure(Storage_Elements);
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   type Storage_Offset is range implementation-defined;
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   subtype Storage_Count is Storage_Offset range 0..Storage_Offset'Last;
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   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;
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   -- Address Arithmetic:
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   function "+"(Left : Address; Right : Storage_Offset)
     return Address;
   function "+"(Left : Storage_Offset; Right : Address)
     return Address;
   function "-"(Left : Address; Right : Storage_Offset)
     return Address;
   function "-"(Left, Right : Address)
     return Storage_Offset;
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   function "mod"(Left : Address; Right : Storage_Offset)
     return Storage_Offset;
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   -- Conversion to/from integers:
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   type Integer_Address is implementation-defined;
   function To_Address(Value : Integer_Address) return Address;
   function To_Integer(Value : Address) return Integer_Address;
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   pragma Convention(Intrinsic, "+");
      -- ...and so on for all language-defined subprograms declared in this package.
end System.Storage_Elements;
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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. 
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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

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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

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This paragraph was deleted.

Implementation Advice

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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. 
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