Automatic conversion of files from one code set to another is controlled globally by the AUTOCVT(ON|OFF) parameter in the BPXPRMxx parmlib member. AUTOCVT can be overridden by individual programs at a thread level, and therefore is a controlling switch only for programs that do not explicitly establish their own conversion options. The default setting for AUTOCVT is OFF.

Although the value of AUTOCVT can be changed using the SETOMVS command, changing the conversion mode does not affect conversion of opened files for which I/O has already started.

Guideline: When AUTOCVT(ON) is set, every read or write operation for a file is checked to see if conversion is necessary. A performance penalty is therefore involved, even if no conversion occurs. It is recommended that AUTOCVT be left off and each program be enabled for conversion.

For information on commands that allow or disallow automatic code set conversion by default, see Appendix L. Automatic Codeset Conversion: Default Status for Specific Commands in z/OS UNIX System Services Command Reference.