13.7 The Package System
For each implementation there is a library package called System which includes the definitions of certain configuration-dependent characteristics.
The following language-defined library package exists:
package System is pragma Preelaborate(System);
type Name is implementation-defined-enumeration-type; System_Name : constant Name := implementation-defined;
-- System-Dependent Named Numbers:
Min_Int : constant := root_integer'First; Max_Int : constant := root_integer'Last;
Max_Binary_Modulus : constant := implementation-defined; Max_Nonbinary_Modulus : constant := implementation-defined;
Max_Base_Digits : constant := root_real'Digits; Max_Digits : constant := implementation-defined;
Max_Mantissa : constant := implementation-defined; Fine_Delta : constant := implementation-defined;
Tick : constant := implementation-defined;
-- Storage-related Declarations:
type Address is implementation-defined; Null_Address : constant Address;
Storage_Unit : constant := implementation-defined; Word_Size : constant := implementation-defined * Storage_Unit; Memory_Size : constant := implementation-defined;
-- 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
-- Other System-Dependent Declarations: type Bit_Order is (High_Order_First, Low_Order_First); Default_Bit_Order : constant Bit_Order;
-- 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;
Default_Priority : constant Priority := (Priority'First + Priority'Last)/2;
private ... -- not specified by the language end System;
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.
The named numbers Fine_Delta and Tick are of the type universal_real; the others are of the type universal_integer.
The meanings of the named numbers are:
- The smallest (most negative) value allowed for the expressions of a signed_integer_type_definition.
- The largest (most positive) value allowed for the expressions of a signed_integer_type_definition.
- A power of two such that it, and all lesser positive powers of two, are allowed as the modulus of a modular_type_definition.
- A value such that it, and all lesser positive integers, are allowed as the modulus of a modular_type_definition.
- The largest value allowed for the requested decimal precision in a floating_point_definition.
- 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.
- 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.)
- The smallest delta allowed in an ordinary_fixed_point_definition that has the real_range_specification range -1.0 .. 1.0.
- A period in seconds approximating the real time interval during which the value of Calendar.Clock remains constant.
- The number of bits per storage element.
- The number of bits per word.
- An implementation-defined value that is intended to reflect the memory size of the configuration in storage elements.
Address is of a definite, nonlimited type. 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.
See 13.5.3 for an explanation of Bit_Order and Default_Bit_Order.
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. Package System may be declared pure.
Address should be a private type.
14 There are also some language-defined child packages of System defined elsewhere.
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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual