Four installation exits are provided, as defaults, for ISO/ANSI Version 3 and Version 4 volumes:

1. Label validation
2. Label validation suppression
3. Volume access
4. File access.

A fifth installation exit, MPF for WTO/WTOR, can be written (or modified, if one has already been written) by your installation to convert ISO/ANSI pre-Version 3 to Version 3 or Version 4 labels (see WTO/WTOR Message Processing Facility Installation Exit (IEAVMXIT)).

All the default installation exit routines are supplied in a module containing a single CSECT (IFG0193G, alias IFG0553G), in SYS1.LPALIB. A copy of the source code for the module is contained in member ANSIEXIT of SYS1.SAMPLIB.

The default routines, except the validation suppression exit, reject the volume. They run in a privileged (supervisor) state and can be modified or replaced to perform I/O (such as overwriting a label), change system control blocks, and mount or remove volumes.

The return code from the exits can be modified to request continued processing. However, in cases in which the label—validation exit is entered and the routine has not been modified to correct certain errors, the results can be unpredictable The prologue of the source code for the exits, in SYS1.SAMPLIB, gives additional details on modifying the exits.

A parameter list, mapped by the macro IECIEPRM, is passed to the exit routines. The same parameter list is passed to the RACF installation exits if a volume is RACF protected and the VOL1 access code is uppercase A through Z for ISO/ANSI Version 3 tapes. In addition to uppercase A through Z, ISO/ANSI Version 4 tapes also allows special characters !*"%&'()+,-./:;<=>?_ and numeric 0–9.

Return codes from the Version 3 and Version 4 exits are returned in the IECIEXRC field of the parameter list. Return codes from the RACF exits are returned in register 15. Return codes from the Version 3 or Version 4 and RACF exits are not the same.

Neither the Version 3 or Version 4 nor RACF installation exits should alter any of the parameter list fields, except IECIEXRC or IECIEUSR.

An important extension to the parameter list is the UCB tape class extension. It contains such items as the volume access code (UCBCXACC), owner identification (UCBCXOWN), and ISO/ANSI version (UCBCXVER). The address of the appropriate UCB is maintained in the parameter list.

If you replace any of the IBM-supplied exit routines with your own routines, follow the programming conventions described under Processing Nonstandard Labels, except that return must be by way of a BR 14 instruction.

In addition, your routines cannot use the DCB parameter list to process any DCB other than the current entry, because the DCBs are not synchronized during Version 3 exit processing.

MODESET to key 0 to alter protected control blocks (such as the UCB). Always restore the original key at entry immediately after you make any alterations to key 0 storage; this minimizes risk of inadvertent data destruction.