The language defined is different from those of previous PL/I standards, although substantially upward compatible, at the source program and semantic level. The differences are summarized in appendix A. Specifies the syntax and semantics of conforming PL/I programs. Defines a conforming processor (or conforming implementation) only in terms of those conforming programs. The definition is accomplished by specifying a conceptual PL/I machine which translates and interprets intended PL/I programs.