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13.7 The Package System

1
For each implementation there is a library package called System which includes the definitions of certain configuration-dependent characteristics. 

Static Semantics

2
The following language-defined library package exists: 
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package System is
   pragma Pure(System);
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   type Name is implementation-defined-enumeration-type;
   System_Name : constant Name := implementation-defined;
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   -- System-Dependent Named Numbers:
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   Min_Int               : constant := root_integer'First;
   Max_Int               : constant := root_integer'Last;
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   Max_Binary_Modulus    : constant := implementation-defined;
   Max_Nonbinary_Modulus : constant := implementation-defined;
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   Max_Base_Digits       : constant := root_real'Digits;
   Max_Digits            : constant := implementation-defined;
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   Max_Mantissa          : constant := implementation-defined;
   Fine_Delta            : constant := implementation-defined;
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   Tick                  : constant := implementation-defined;
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   -- Storage-related Declarations:
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   type Address is implementation-defined;
   Null_Address : constant Address;
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   Storage_Unit : constant := implementation-defined;
   Word_Size    : constant := implementation-defined * Storage_Unit;
   Memory_Size  : constant := implementation-defined;
14
   -- Address Comparison:
   function "<" (Left, Right : Address) return Boolean;
   function "<="(Left, Right : Address) return Boolean;
   function ">" (Left, Right : Address) return Boolean;
   function ">="(Left, Right : Address) return Boolean;
   function "=" (Left, Right : Address) return Boolean;
-- function "/=" (Left, Right : Address) return Boolean;
   -- "/=" is implicitly defined
   pragma Convention(Intrinsic, "<");
   ... -- and so on for all language-defined subprograms in this package
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   -- Other System-Dependent Declarations:
   type Bit_Order is (High_Order_First, Low_Order_First);
   Default_Bit_Order : constant Bit_Order := implementation-defined;
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   -- Priority-related declarations (see D.1):
   subtype Any_Priority is Integer range implementation-defined;
   subtype Priority is Any_Priority range Any_Priority'First ..
             implementation-defined;
   subtype Interrupt_Priority is Any_Priority range Priority'Last+1 ..
             Any_Priority'Last;
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   Default_Priority : constant Priority :=
             (Priority'First + Priority'Last)/2;
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private
   ... -- not specified by the language
end System;
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Name is an enumeration subtype. Values of type Name are the names of alternative machine configurations handled by the implementation. System_Name represents the current machine configuration.
20
The named numbers Fine_Delta and Tick are of the type universal_real; the others are of the type universal_integer.
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The meanings of the named numbers are: 
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Min_Int 
The smallest (most negative) value allowed for the expressions of a signed_integer_type_definition.
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Max_Int 
The largest (most positive) value allowed for the expressions of a signed_integer_type_definition.
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Max_Binary_Modulus 

A power of two such that it, and all lesser positive powers of two, are allowed as the modulus of a modular_type_definition.
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Max_Nonbinary_Modulus 

A value such that it, and all lesser positive integers, are allowed as the modulus of a modular_type_definition.
26
Max_Base_Digits 

The largest value allowed for the requested decimal precision in a floating_point_definition.
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Max_Digits 
The largest value allowed for the requested decimal precision in a floating_point_definition that has no real_range_specification. Max_Digits is less than or equal to Max_Base_Digits.
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Max_Mantissa 
The largest possible number of binary digits in the mantissa of machine numbers of a user-defined ordinary fixed point type. (The mantissa is defined in Annex G.)
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Fine_Delta 
The smallest delta allowed in an ordinary_fixed_point_definition that has the real_range_specification range –1.0 .. 1.0. 
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Tick 
A period in seconds approximating the real time interval during which the value of Calendar.Clock remains constant. 
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Storage_Unit 
The number of bits per storage element.
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Word_Size 
The number of bits per word.
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Memory_Size 
An implementation-defined value that is intended to reflect the memory size of the configuration in storage elements. 
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 Address is a definite, nonlimited type with preelaborable initialization (see 10.2.1). Address represents machine addresses capable of addressing individual storage elements. Null_Address is an address that is distinct from the address of any object or program unit.
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 Default_Bit_Order shall be a static constant. See 13.5.3 for an explanation of Bit_Order and Default_Bit_Order. 

Implementation Permissions

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 An implementation may add additional implementation-defined declarations to package System and its children. However, it is usually better for the implementation to provide additional functionality via implementation-defined children of System. 

Implementation Advice

37
Address should be a private type. 
NOTES
38
14  There are also some language-defined child packages of System defined elsewhere. 

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