64-Bit Fortran Porting Guidelines

64-Bit Fortran Porting Guidelines

This section describes which sections of your Fortran source code you need to modify to port to a 64-bit system.

Standard ANSI Fortran 77 code should have no problems, but the following areas need attention:

Code that uses REAL*16 could get different runtime results due to additional accuracy in the QUAD libraries.
fcom and fef77 are incompatible with regard to real constant folding.
Integer variables which were used to hold addresses need to be changed to INTEGER*8.
C interface issues (Fortran passes by reference).
%LOC returns 64-bit addresses.
%VAL passes 64-bit values.

Source code modifications are best done in phases. In the first phase, try to locate all of the variables affected by the size issues mentioned above. In the second phase, locate variables that depend on the variables changed in the first phase. Depending on your code, you may need to iterate on this phase as you track down long sets of dependencies.

Examples of Fortran Portability Issues