D_FLOAT,G_FLOAT,IEEE_FLOAT

2.3.22 /FLOAT --- Specify Floating-Point Format in Memory

The /FLOAT qualifier specifies the floating-point data format to be used in memory for REAL or COMPLEX data. For performance reasons, consider specifying the same floating-point memory format as the floating-point format used by unformatted files the program will access if the data falls within the acceptable range.

Figure 2-1 shows the FORTRAN command qualifiers used to specify the floating-point format used in memory and in an unformatted file. Because REAL (KIND=16) (same as REAL*16) data is always in X_float format on OpenVMS I64 and Alpha systems, the /FLOAT keyword specifies the format for REAL (KIND=4), COMPLEX (KIND=4), REAL (KIND=8), and COMPLEX (KIND=8) data (or equivalent declarations).

Figure 2-1 Specifying the Format of Numeric Data for Unformatted Files

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To specify the floating-point format (such as big endian) for all unformatted files opened by the program, use the /CONVERT qualifier. To allow the same program to use different floating-point formats, you must use the predefined logical names or the OPEN CONVERT keyword to specify the format for specific unit numbers, as described in Chapter 9.

The qualifier has the following form:

 

/FLOAT=

• D_FLOAT
• G_FLOAT
• IEEE_FLOAT

 

 

Note

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The OpenVMS Alpha instruction set does not support D_float computations, and the OpenVMS I64 instruction set does not support D_float, F_float or G_float computations. As a result, any data stored in those formats is converted to a native format for arithmetic computations and then converted back to its original format. On Alpha systems, the native format used for D_float is G_float. On I64 systems, S_float is used for F_float data, and T_float is used for D_float and G_float data.

This means that for programs that perform many floating-point computations, using D_float data on Alpha systems is slower than using G_float or T_float data. Similarly, using D_float, F_float, or G_float data on I64 systems is slower than using S_float or T_float data. Additionally, due to the conversions involved, the results might differ from native VAX D_float, F_float, and G_float computations and results.

You should not mix floating data type formats in routines that pass single-precision or double-precision quantities among themselves.

D_FLOAT

Specifies that the memory format for REAL (KIND=4) and COMPLEX (KIND=4) data is VAX F_float and that the memory format for REAL (KIND=8) and COMPLEX (KIND=8) data is VAX D_float. Same as the obsolete qualifier /NOG_FLOATING.

Due to the considerations noted above, we do not recommend use of the /FLOAT=D_FLOAT qualifier unless a program must use unformatted data files in D_float format. If range and accuracy constraints permit the use of the other REAL (KIND=8) data types, consider converting existing unformatted files that contain D_float data to another format, such as G_float on Alpha systems, or T_float on Alpha or I64 systems (see Chapter 9).

G_FLOAT

Specifies that the memory format for single precision REAL (KIND=4) and COMPLEX (KIND=4) data is VAX F_float and that the memory format for double precision REAL (KIND=8) and COMPLEX (KIND=8) data is VAX G_float. Same as the obsolete qualifier /G_FLOATING.

The default on Alpha systems is /FLOAT=G_FLOAT.

Due to the considerations noted above, on I64 systems we do not recommend use of the /FLOAT=G_FLOAT qualifier unless a program must use unformatted data files in G_float format. If range and accuracy constraints permit it, consider converting existing unformatted files that contain G_float data to T_float (see Chapter 9).

IEEE_FLOAT

Specifies that the memory format for single precision REAL (KIND=4) and COMPLEX (KIND=4) data is IEEE S_float and the memory format for double precision REAL (KIND=8) and COMPLEX (KIND=8) is IEEE T_float.

The default on I64 systems is /FLOAT=IEEE_FLOAT. If possible, this default should be used, because it provides the greatest performance and accuracy on I64.

Specifying /FLOAT=IEEE_FLOAT allows the use of certain IEEE exceptional values. When you specify /FLOAT=IEEE_FLOAT, you should be aware of the /CHECK=FP_EXCEPTIONS, /CHECK=FP_MODE, /IEEE_MODE, and /ROUNDING_MODE qualifiers.

Because REAL (KIND=16) (same as REAL*16) and COMPLEX (KIND=16) (same as COMPLEX*32) data is always in X_float format on I64 and Alpha systems, operations that use REAL (KIND=16) and COMPLEX (KIND=16) data may encounter certain exceptional values even when /FLOAT=IEEE_FLOAT is not used.

For More Information:

• On intrinsic floating-point data types, see Chapter 8.
• On converting unformatted files, see Section 2.3.12.
• On qualifiers of interest when you specify /FLOAT=IEEE_FLOAT, see:

  Section 2.3.11 (/CHECK=FP_EXCEPTIONS)
  Section 2.3.11 (/CHECK=FP_MODE) (I64 only)
  Section 2.3.24 (/IEEE_MODE)
  Section 2.3.40 (/ROUNDING_MODE)