A configuration pragma that establishes Ada 83 mode for the unit to which it applies, regardless of the mode set by the command line switches. In Ada 83 mode, GNAT attempts to be as compatible with the syntax and semantics of Ada 83, as defined in the original Ada 83 Reference Manual as possible. In particular, the keywords added by Ada 95 and Ada 2005 are not recognized, optional package bodies are allowed, and generics may name types with unknown discriminants without using the (<>) notation. In addition, some but not all of the additional restrictions of Ada 83 are enforced.
Ada 83 mode is intended for two purposes. Firstly, it allows existing Ada 83 code to be compiled and adapted to GNAT with less effort. Secondly, it aids in keeping code backwards compatible with Ada 83. However, there is no guarantee that code that is processed correctly by GNAT in Ada 83 mode will in fact compile and execute with an Ada 83 compiler, since GNAT does not enforce all the additional checks required by Ada 83.