It is possible to specify the convention
Ada in a pragma
Import or pragma
Export. However this refers to
the calling conventions used by GNAT, which may or may not be
similar enough to those used by some other Ada 83 or Ada 95
compiler to allow interoperation.
If arguments types are kept simple, and if the foreign compiler generally follows system calling conventions, then it may be possible to integrate files compiled by other Ada compilers, provided that the elaboration issues are adequately addressed (for example by eliminating the need for any load time elaboration).
In particular, GNAT running on VMS is designed to be highly compatible with the DEC Ada 83 compiler, so this is one case in which it is possible to import foreign units of this type, provided that the data items passed are restricted to simple scalar values or simple record types without variants, or simple array types with fixed bounds.