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G.2.1 Model of Floating Point Arithmetic

In the strict mode, the predefined operations of a floating point type shall satisfy the accuracy requirements specified here and shall avoid or signal overflow in the situations described. This behavior is presented in terms of a model of floating point arithmetic that builds on the concept of the canonical form (see A.5.3).

Static Semantics

Associated with each floating point type is an infinite set of model numbers. The model numbers of a type are used to define the accuracy requirements that have to be satisfied by certain predefined operations of the type; through certain attributes of the model numbers, they are also used to explain the meaning of a user-declared floating point type declaration. The model numbers of a derived type are those of the parent type; the model numbers of a subtype are those of its type.
The model numbers of a floating point type T are zero and all the values expressible in the canonical form (for the type T), in which mantissa has T'Model_Mantissa digits and exponent has a value greater than or equal to T'Model_Emin. (These attributes are defined in G.2.2.)
A model interval of a floating point type is any interval whose bounds are model numbers of the type. The model interval of a type T associated with a value v is the smallest model interval of T that includes v. (The model interval associated with a model number of a type consists of that number only.) 

Implementation Requirements

The accuracy requirements for the evaluation of certain predefined operations of floating point types are as follows. 
An operand interval is the model interval, of the type specified for the operand of an operation, associated with the value of the operand.
For any predefined arithmetic operation that yields a result of a floating point type T, the required bounds on the result are given by a model interval of T (called the result interval) defined in terms of the operand values as follows: 
The result interval is the smallest model interval of T that includes the minimum and the maximum of all the values obtained by applying the (exact) mathematical operation to values arbitrarily selected from the respective operand intervals.
The result interval of an exponentiation is obtained by applying the above rule to the sequence of multiplications defined by the exponent, assuming arbitrary association of the factors, and to the final division in the case of a negative exponent.
The result interval of a conversion of a numeric value to a floating point type T is the model interval of T associated with the operand value, except when the source expression is of a fixed point type with a small that is not a power of T'Machine_Radix or is a fixed point multiplication or division either of whose operands has a small that is not a power of T'Machine_Radix; in these cases, the result interval is implementation defined. 
For any of the foregoing operations, the implementation shall deliver a value that belongs to the result interval when both bounds of the result interval are in the safe range of the result type T, as determined by the values of T'Safe_First and T'Safe_Last; otherwise, 
if T'Machine_Overflows is True, the implementation shall either deliver a value that belongs to the result interval or raise Constraint_Error;
if T'Machine_Overflows is False, the result is implementation defined. 
For any predefined relation on operands of a floating point type T, the implementation may deliver any value (i.e., either True or False) obtained by applying the (exact) mathematical comparison to values arbitrarily chosen from the respective operand intervals.
The result of a membership test is defined in terms of comparisons of the operand value with the lower and upper bounds of the given range or type mark (the usual rules apply to these comparisons).

Implementation Permissions

If the underlying floating point hardware implements division as multiplication by a reciprocal, the result interval for division (and exponentiation by a negative exponent) is implementation defined.

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