Supported Native and Nonnative Numeric Formats

Data storage in different computers uses a convention of either little endian or big endian storage. The storage convention generally applies to numeric values that span multiple bytes, as follows:

Little endian storage occurs when:

Big endian storage occurs when:

Intel® Fortran expects numeric data to be in native little endian order, in which the least-significant, right-most zero bit (bit 0) or byte has a lower address than the most-significant, left-most bit (or byte).

The following figures show the difference between the two byte-ordering schemes:

Little and Big Endian Storage of an INTEGER Value

The following figure illustrates the difference between the two conventions for the case of addressing bytes within words.

Byte Order Within Words: (a) Big Endian, (b) Little Endian

Data types stored as subcomponents (bytes stored in words) end up in different locations within corresponding words of the two conventions. The following figure illustrates the difference between the representation of several data types in the two conventions. Letters represent 8-bit character data, while numbers represent the 8-bit partial contribution to 32-bit integer data.

Character and Integer Data in Words: (a) Big Endian, (b) Little Endian



If you serially transfer bytes now from the big endian words to the little endian words (BE byte 0 to LE byte 0, BE byte 1 to LE byte 1, ...), the left half of the figure shows how the data ends up in the little endian words. Note that data of size one byte (characters in this case) is ordered correctly, but that integer data no longer correctly represents the original binary values. The right half of the figure shows that you need to swap bytes around the middle of the word to reconstitute the correct 32-bit integer values. After swapping bytes, the two preceding figures are identical.

Data Sent from Big to Little: (a) After Transfer, (b) After Byte Swaps

You can generalize the previous example to include floating-point data types and to include multiple-word data types.

Moving unformatted data files between big endian and little endian computers requires that the data be converted.

Intel Fortran provides the capability for programs to read and write unformatted data (originally written using unformatted I/O statements) in several nonnative floating-point formats and in big endian INTEGER or floating-point format. Supported nonnative floating-point formats include Compaq* VAX* little endian floating-point formats supported by Digital* FORTRAN for OpenVMS* VAX Systems, standard IEEE big endian floating-point format found on most Sun Microsystems* systems and IBM RISC* System/6000 systems, IBM floating-point formats (associated with the IBM's System/370 and similar systems), and CRAY* floating-point formats.

Converting unformatted data instead of formatted data is generally faster and is less likely to lose precision of floating-point numbers.

The native memory format includes little endian integers and little endian IEEE floating-point formats, S_floating for REAL(KIND=4) and COMPLEX(KIND=4) declarations, T_floating for REAL(KIND=8) and COMPLEX(KIND=8) declarations, and X_floating for REAL(KIND=16) and COMPLEX(KIND=16) declarations.

The keywords for supported nonnative unformatted file formats and their data types are listed in the following table:

Nonnative Numeric Formats, Keywords, and Supported Data Types

Keyword

Description

BIG_ENDIAN

Big endian integer data of the appropriate size (one, two, four, or eight bytes) and big endian IEEE floating-point formats for REAL and COMPLEX single- and double- and extended-precision numbers. INTEGER(KIND=1) data is the same for little endian and big endian.

CRAY

Big endian integer data of the appropriate size (one, two, four, or eight bytes) and big endian CRAY proprietary floating-point format for REAL and COMPLEX single- and double-precision numbers.

FDX

Little endian integer data of the appropriate size (one, two, four, or eight bytes) and the following little endian proprietary floating-point formats:

  • VAX F_float for REAL (KIND=4) and COMPLEX (KIND=4)

  • VAX D_float for REAL (KIND=8) and COMPLEX (KIND=8)

  • IEEE style X_float for REAL (KIND=16) and COMPLEX (KIND=16)

FGX

Little endian integer data of the appropriate size (one, two, four, or eight bytes) and and the following little endian proprietary floating-point formats:

  • VAX F_float for REAL (KIND=4) and COMPLEX (KIND=4)

  • VAX G_float for REAL (KIND=8) and COMPLEX (KIND=8)

  • IEEE style X_float for REAL (KIND=16) and COMPLEX (KIND=16)

IBM

Big endian integer data of the appropriate INTEGER size (one, two, or four bytes) and big endian IBM proprietary (System\370 and similar) floating-point format for REAL and COMPLEX single- and double-precision numbers.

LITTLE_ENDIAN

Native little endian integers of the appropriate INTEGER size (one, two, four, or eight bytes) and the following native little endian IEEE floating-point formats:

  • S_float for REAL (KIND=4) and COMPLEX (KIND=4)

  • T_float for REAL (KIND=8) and COMPLEX (KIND=8)

  • IEEE style X_float for REAL (KIND=16) and COMPLEX (KIND=16)

For additional information on supported ranges for these data types, see Native IEEE Floating-Point Representations.

NATIVE

No conversion occurs between memory and disk. This is the default for unformatted files.

VAXD

Native little endian integers of the appropriate INTEGER size (one, two, four, or eight bytes) and the following little endian VAX proprietary floating-point formats:

  • VAX F_float for REAL (KIND=4) and COMPLEX (KIND=4)

  • VAX D_float for REAL (KIND=8) and COMPLEX (KIND=8)

  • VAX H_float for REAL (KIND=16) and COMPLEX (KIND=16)

VAXG

Native little endian integers of the appropriate INTEGER size (one, two, four, or eight bytes) and the following little endian VAX proprietary floating-point formats:

  • VAX F_float for REAL (KIND=4) and COMPLEX (KIND=4)

  • VAX G_float for REAL (KIND=8) and COMPLEX (KIND=8)

  • VAX H_float for REAL (KIND=16) and COMPLEX (KIND=16)

When reading a nonnative format, the nonnative format on disk is converted to native format in memory. If a converted nonnative value is outside the range of the native data type, a run-time message is displayed.

See also:

Environment Variable F_UFMTENDIAN Method