Figure 1 shows the processing of a REXX exec in any MVS™ address space.

Figure 1. Overview of REXX Exec processing in any address space

As shown in the figure, before the language processor executes a REXX exec, a language processor environment must exist. After an environment is located or initialized, the exec is loaded into storage and is then executed. While an exec is executing, the language processor may need to access different system services, for example, to handle data stack requests or for I/O processing. The system services are handled by routines that are known as replaceable routines. The following topics describe the initialization and termination of language processor environments, the loading and freeing of an exec, and the replaceable routines. In addition, there are several exits you can provide to customize REXX processing. See REXX exit routines where the exits are summarized.