Upon the initial entry to your exec, REXX provides a default ADDRESS. This is used for any commands until the addressing is changed. For a normal EXEC file, the default addressing supplied by REXX is to the "operating system" so that commands issued within the exec are by default routed to the underlying operating system for execution.

For IDF macros, the default addressing environment is supplied by IDF when it invokes REXX to execute the macro.

z/VM

The default ADDRESS is in the form of a PSW, telling REXX that it should use its "LPSW Fastpath" mechanism to directly enter IDF command handling.

z/OS

The default ADDRESS is ASM"

z/VSE

The default ADDRESS is "ASM"