In addition to the instructions, functions, and commands for writing a REXX exec and the programming services that interface with REXX and the language processor, TSO/E also provides customizing services for REXX processing. The customizing services let you change how REXX execs are processed and how system services are accessed and used.

The REXX language itself, which consists of instructions and built-in functions, is address space independent. The language processor, which interprets a REXX exec, processes the REXX language instructions and functions in the same manner in any address space. However, when a REXX exec executes, the language processor must interface with different host services, such as I/O and storage. MVS™ address spaces differ in how they access and use system services, for example, how they use and manage I/O and storage. Although these differences exist, the language processor must run in an environment that is not dependent on the address space in which it is executing an exec. The environment must allow REXX execs to execute independently of the way in which an address space handles system services. The TSO/E REXX customizing routines and services provide an interface between the language processor and underlying host services and allow you to customize the environment in which the language processor processes REXX execs.