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 Ada 83 Programs.
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generally because they have been replaced by other mechanisms.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.