IBM SC33-1683-02 - Manual (Page 98)

The correspondence between command parameters (such as PROGRAM) and their
positions and values in the parameter list (in this case, argument 1, ‘MYPROG’) can
be deduced from the translated code for the particular command.
Important
Modifying CICS commands by tampering with argument 0 is not supported,
and leads to unexpected errors or results.
For example, if an application program is written in assembler or PL/I and you
modify argument 0, you will be writing to program storage (that is, storage
occupied by the program itself), which could cause 0C4 abends. Furthermore,
modifying argument 0 not only alters the CICS command for
this execution of
the command in the application program, it changes the CICS command in the
virtual storage copy of the application program. This means that the next task
to invoke the same copy of the program will also execute the modified
command.
This particular example of the danger of tampering with argument 0 does not
apply to COBOL or C/370™ application programs, but nevertheless you
should not modify CICS commands for application programs written in any
supported language.
Bypassing commands
An XEIIN or XEISPIN exit program can bypass execution of a command by setting
the UERCBYP return code. If it does this, EDF is not invoked, but XEISPOUT,
XEIOUT, and exit trace are invoked if they are active.
Bypassing an EXEC CICS command allows an exit program to replace the CICS
function with its own processing, for example.
Before setting UERCBYP, your program should check the value pointed to by
UEPPGM, to ensure that it is not bypassing an EXEC CICS command issued by
CICS.
Exit XEIIN
When invoked
Before the execution of any EXEC CICS API or SPI command.
Exit-specific parameters
UEPARG
Address of the EXEC command parameter list.
UEPEXECB
Address of the system EIB.
UEPUSID
Address of the 8-character userid.
UEPPGM
Address of the 8-character application program name.
UEPLOAD
Address of the application program’s load-point.
EXEC interface program exits
66 CICS TS for OS/390: CICS Customization Guide