This document defines the computer programming language PL/I General Purpose Subset, referred to in most of this definition simply as "PL/I". It is intended to serve as an authoritative reference rather than as a tutorial introduction.

The language defined by this document 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. The text that follows defines standard PL/I in terms of an extended reference PL/I language. The extended reference PL/I language contains operations and portions of operations that are not accessed by PL/I programs that conform to this standard. Those features that are implied by the reference language but for which support is not required of conforming processors are retained for convenience of PL/I implementations containing extensions beyond this standard and for definitional compatibility with earlier PL/I standards (see Appendix A).

As explained in more detail in Section 1.5, this standard specifies the syntax and semantics of conforming PL/I programs. It makes no prescriptive statements about the syntax or semantics of non-conforming programs. It defines a conforming processor (or conforming implementation) only in terms of those conforming programs: a conforming processor is one that, subject to the flexibilities and requirements of Section 1.5, gives all conforming programs the interpretations specified by this definition.

The definition is accomplished by specifying a conceptual PL/I machine which translates and interprets intended PL/I programs. Section 1.4 provides a brief introduction to the statements and data types included in the language, to the structure and use of the document, and to the method of definition. The relationship between an actual implementation and the conceptual machine of this document is described in Section 1.5, and the detailed specification of the notation to be used follows in Section 1.6. The main body of the definition is then begun at Section 1.7, and is completed by Chapters 2 through 9.