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A.5.3 Attributes of Floating Point Types

Static Semantics

  1. The following representation-oriented attributes are defined for every subtype S of a floating point type T.
  2. S'Machine_Radix 
    Yields the radix of the hardware representation of the type
T. The value of this attribute is of the type universal_integer.
  3. The values of other representation-oriented attributes of a floating point subtype, and of the "primitive function" attributes of a floating point subtype described later, are defined in terms of a particular representation of nonzero values called the canonical form. The canonical form (for the type T) is the form 
    +/-mantissa*T'Machine_Radix ** exponent
    where
    1. mantissa is a fraction in the number base T'Machine_Radix, the first digit of which is nonzero, and
    2. exponent is an integer.

  1. S'Machine_Mantissa 
    Yields the largest value of p such that every value
expressible in the canonical form (for the type T), having a
p-digit mantissa and an exponent between T'Machine_Emin and
T'Machine_Emax, is a machine number, See section 3.5.7 Floating Point Types, of the type
T. This attribute yields a value of the type
universal_integer.
  2. S'Machine_Emin 
    Yields the smallest (most negative) value of exponent such
that every value expressible in the canonical form (for the
type T), having a mantissa of T'Machine_Mantissa digits, is a
machine number, See section 3.5.7 Floating Point Types, of the type T. This attribute
yields a value of the type universal_integer.
  3. S'Machine_Emax 
    Yields the largest (most positive) value of exponent such
that every value expressible in the canonical form (for the
type T), having a mantissa of T'Machine_Mantissa digits, is a
machine number See section 3.5.7 Floating Point Types, of the type T. This attribute
yields a value of the type universal_integer.
  4. S'Denorm 
    Yields the value True if every value expressible in the form

    +/-mantissa*T'Machine_Radix ** T'Machine_Emin

where mantissa is a nonzero T'Machine_Mantissa-digit fraction
in the number base T'Machine_Radix, the first digit of which
is zero, is a machine number, See section 3.5.7 Floating Point Types, of the type T;
yields the value False otherwise. The value of this
attribute is of the predefined type Boolean.
  5. The values described by the formula in the definition of S'Denorm are called denormalized numbers. A nonzero machine number that is not a denormalized number is a normalized number. A normalized number x of a given type T is said to be represented in canonical form when it is expressed in the canonical form (for the type T) with a mantissa having T'Machine_Mantissa digits; the resulting form is the canonical-form representation of x.
  6. S'Machine_Rounds 
    Yields the value True if rounding is performed on inexact
results of every predefined operation that yields a result of
the type T; yields the value False otherwise. The value of
this attribute is of the predefined type Boolean.
  7. S'Machine_Overflows 
    Yields the value True if overflow and divide-by-zero are
detected and reported by raising Constraint_Error for every
predefined operation that yields a result of the type T;
yields the value False otherwise. The value of this
attribute is of the predefined type Boolean.
  8. S'Signed_Zeros 
    Yields the value True if the hardware representation for the
type T has the capability of representing both positively and
negatively signed zeros, these being generated and used by
the predefined operations of the type T as specified in IEC
559:1989; yields the value False otherwise. The value of
this attribute is of the predefined type Boolean.
  9. For every value x of a floating point type T, the normalized exponent of x is defined as follows:
    1. the normalized exponent of zero is (by convention) zero;
    2. for nonzero x, the normalized exponent of x is the unique integer k such that T'Machine_Radix ** (k - 1) <=|x|<T'Machine_Radix ** k.

  1. The following primitive function attributes are defined for any subtype S of a floating point type T.
  2. S'Exponent 
    S'Exponent denotes a function with the following specification:

    1. function S'Exponent (X : T)
  return universal_integer
    2. The function yields the normalized exponent of X.

  1. S'Fraction 
    S'Fraction denotes a function with the following specification:

    1. function S'Fraction (X : T)
  return T
    2. The function yields the value X*T'Machine_Radix ** -k, where k is the normalized exponent of X. A zero result, which can only occur when X is zero, has the sign of X.

  1. S'Compose 
    S'Compose denotes a function with the following specification:

    1. function S'Compose (Fraction : T;
                    Exponent : universal_integer)
  return T
    2. Let v be the value Fraction*T'Machine_Radix ** (Exponent-k), where k is the normalized exponent of Fraction. If v is a machine number of the type T, or if |v|>=T'Model_Small, the function yields v; otherwise, it yields either one of the machine numbers of the type T adjacent to v. Constraint_Error is optionally raised if v is outside the base range of S. A zero result has the sign of Fraction when S'Signed_Zeros is True.

  1. S'Scaling 
    1. S'Scaling denotes a function with the following specification:

    2. function S'Scaling (X          : T;
                    Adjustment : universal_integer)
  return T
    3. Let v be the value X*T'Machine_Radix ** (Adjustment). If v is a machine number of the type T, or if |v|>=T'Model_Small, the function yields v; otherwise, it yields either one of the machine numbers of the type T adjacent to v. Constraint_Error is optionally raised if v is outside the base range of S. A zero result has the sign of X when S'Signed_Zeros is True.

  1. S'Floor 
    S'Floor denotes a function with the following specification:

    1. function S'Floor (X : T)
  return T
    2. The function yields the value Floor(X), i.e., the largest (most positive) integral value less than or equal to X. When X is zero, the result has the sign of X; a zero result otherwise has a positive sign.

  1. S'Ceiling 
    S'Ceiling denotes a function with the following specification:

    1. function S'Ceiling (X : T)
  return T
    2. The function yields the value Ceiling(X), i.e., the smallest (most negative) integral value greater than or equal to X. When X is zero, the result has the sign of X; a zero result otherwise has a negative sign when S'Signed_Zeros is True.

  1. S'Rounding 
    S'Rounding denotes a function with the following specification:

    1. function S'Rounding (X : T)
  return T
    2. The function yields the integral value nearest to X, rounding away from zero if X lies exactly halfway between two integers. A zero result has the sign of X when S'Signed_Zeros is True.

  1. S'Unbiased_Rounding 
    S'Unbiased_Rounding denotes a function with the following
specification:

    1. function S'Unbiased_Rounding (X : T)
  return T
    2. The function yields the integral value nearest to X, rounding toward the even integer if X lies exactly halfway between two integers. A zero result has the sign of X when S'Signed_Zeros is True.

  1. S'Truncation 
    S'Truncation denotes a function with the following
specification:

    1. function S'Truncation (X : T)
  return T
    2. The function yields the value Ceiling(X) when X is negative, and Floor(X) otherwise. A zero result has the sign of X when S'Signed_Zeros is True.

  1. S'Remainder 
    S'Remainder denotes a function with the following
specification:

    1. function S'Remainder (X, Y : T)
  return T
    2. For nonzero Y, let v be the value X-n*Y, where n is the integer nearest to the exact value of X/Y; if |n-X/Y|=1/2, then n is chosen to be even. If v is a machine number of the type T, the function yields v; otherwise, it yields zero. Constraint_Error is raised if Y is zero. A zero result has the sign of X when S'Signed_Zeros is True.

  1. S'Adjacent 
    S'Adjacent denotes a function with the following specification:

    1. function S'Adjacent (X, Towards : T)
  return T
    2. If Towards=X, the function yields X; otherwise, it yields the machine number of the type T adjacent to X in the direction of Towards, if that machine number exists. If the result would be outside the base range of S, Constraint_Error is raised. When T'Signed_Zeros is True, a zero result has the sign of X. When Towards is zero, its sign has no bearing on the result.

  1. S'Copy_Sign 
    S'Copy_Sign denotes a function with the following
specification:

    1. function S'Copy_Sign (Value, Sign : T)
  return T
    2. If the value of Value is nonzero, the function yields a result whose magnitude is that of Value and whose sign is that of Sign; otherwise, it yields the value zero. Constraint_Error is optionally raised if the result is outside the base range of S. A zero result has the sign of Sign when S'Signed_Zeros is True.

  1. S'Leading_Part 
    S'Leading_Part denotes a function with the following
specification:

    1. function S'Leading_Part (X : T;
                         Radix_Digits : universal_integer)
  return T
    2. Let v be the value T'Machine_Radix ** (k-Radix_Digits), where k is the normalized exponent of X. The function yields the value
      1. Floor(X/v)*v, when X is nonnegative and Radix_Digits is positive;
      2. Ceiling(X/v)*v, when X is negative and Radix_Digits is positive.

    1. Constraint_Error is raised when Radix_Digits is zero or negative. A zero result, which can only occur when X is zero, has the sign of X.

  1. S'Machine 
    S'Machine denotes a function with the following specification:

    1. function S'Machine (X : T)
  return T
    2. If X is a machine number of the type T, the function yields X; otherwise, it yields the value obtained by rounding or truncating X to either one of the adjacent machine numbers of the type T. Constraint_Error is raised if rounding or truncating X to the precision of the machine numbers results in a value outside the base range of S. A zero result has the sign of X when S'Signed_Zeros is True.

  1. The following model-oriented attributes are defined for any subtype S of a floating point type T.
  2. S'Model_Mantissa 
    If the Numerics Annex is not supported, this attribute yields
an implementation defined value that is greater than or equal
to Ceiling(d*log (10)/log (T'Machine_Radix))+1, where d is
the requested decimal precision of T, and less than or equal
to the value of T'Machine_Mantissa. See section G.2.2 Model-Oriented Attributes of Floating Point Types, for further
requirements that apply to implementations supporting the
Numerics Annex. The value of this attribute is of the type
universal_integer.
  3. S'Model_Emin 
    If the Numerics Annex is not supported, this attribute yields
an implementation defined value that is greater than or equal
to the value of T'Machine_Emin. See section G.2.2 Model-Oriented Attributes of Floating Point Types, for further
requirements that apply to implementations supporting the
Numerics Annex. The value of this attribute is of the type
universal_integer.
  4. S'Model_Epsilon 
    Yields the value T'Machine_Radix ** 1-T'Model_Mantissa. The value
of this attribute is of the type universal_real.
  5. S'Model_Small 
    Yields the value T'Machine_Radix ** T'Model_Emin-1. The value of
this attribute is of the type universal_real.
  6. S'Model 
    S'Model denotes a function with the following specification:

    1. function S'Model (X : T)
  return T
    2. If the Numerics Annex is not supported, the meaning of this attribute is implementation defined; See section G.2.2 Model-Oriented Attributes of Floating Point Types, for the definition that applies to implementations supporting the Numerics Annex.

  1. S'Safe_First 
    Yields the lower bound of the safe range, See section 3.5.7 Floating Point Types, of the
type T. If the Numerics Annex is not supported, the value of
this attribute is implementation defined; See section G.2.2 Model-Oriented Attributes of Floating Point Types, for the
definition that applies to implementations supporting the
Numerics Annex. The value of this attribute is of the type
universal_real.
  2. S'Safe_Last 
    Yields the upper bound of the safe range, See section 3.5.7 Floating Point Types, of the
type T. If the Numerics Annex is not supported, the value of
this attribute is implementation defined; See section G.2.2 Model-Oriented Attributes of Floating Point Types, for the
definition that applies to implementations supporting the
Numerics Annex. The value of this attribute is of the type
universal_real.

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