Specifying the address of the environment block

You can explicitly call the initialization routine, IRXINIT, to initialize a language processor environment in an address space. If you explicitly call IRXINIT to initialize an environment, you can optionally specify this environment when you invoke any of the TSO/E REXX routines. The environment block represents the environment in which you want the routine to run. Generally, you can specify the address of the environment block:

Using the environment block address parameter in the routine's parameter list
In register 0.

If you specify the environment block address in the parameter list, TSO/E REXX uses the address you specify and ignores the contents of register 0. However, TSO/E does not validate the address you specify in the parameter list. Therefore, you must ensure that you pass a correct address or unpredictable results may occur. For more information, see Using the environment block address parameter.

If you do not specify an address in the environment block address parameter, the TSO/E REXX routine checks register 0 for the address of an environment block. If register 0 contains the address of a valid environment block, the routine runs in the environment represented by that environment block. If the address is not valid, the routine locates the current non-reentrant environment and runs in that environment. If register 0 contains a 0, the routine immediately searches for the last non-reentrant environment created, thereby eliminating the processing required to check whether register 0 contains a valid environment block address.

If you use IRXINIT to initialize reentrant environments, see Using the environment block for reentrant environments for information about running in reentrant environments.