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E.1.4 Other language compatibility issues

-gnat83 switch
All implementations of GNAT provide a switch that causes GNAT to operate in Ada 83 mode. In this mode, some but not all compatibility problems of the type described above are handled automatically. For example, the new Ada 95 reserved words are treated simply as identifiers as in Ada 83. However, in practice, it is usually advisable to make the necessary modifications to the program to remove the need for using this switch. See Compiling Different Versions of Ada.
Support for removed Ada 83 pragmas and attributes
A number of pragmas and attributes from Ada 83 have been removed from Ada 95, generally because they have been replaced by other mechanisms. Ada 95 compilers are allowed, but not required, to implement these missing elements. In contrast with some other Ada 95 compilers, GNAT implements all such pragmas and attributes, eliminating this compatibility concern. These include pragma Interface and the floating point type attributes (Emax, Mantissa, etc.), among other items.