Installation exits enable an installation to tailor JES3 without having to modify JES3 code. By not modifying JES3 code, you reduce the amount of work necessary to install JES3 maintenance or new JES3 function. Thus, installation exit routines can be very useful in helping you develop an operating system tailored to your needs.

IBM supplies a module for each JES3 installation exit. (Some exits perform a default function, but most are dummy modules). You may, of course, write your own installation exit routine in place of any provided by IBM. For each exit, either the IBM-supplied routine or an installation exit routine must reside in the JES3 module library, or JES3 issues warning messages IAT3020 and IAT3102.

See z/OS JES3 Initialization and Tuning Reference for information on how the installation exit routines influence JES3 job management.

All of the IBM-supplied exit routines (except exit 26) specify AMODE 31 and RMODE=ANY. (IATUX26 specifies AMODE 31 but RMODE=24 meaning that it is loaded below 16 megabytes.) All exit routines are entered in 31-bit addressing mode. The exit modules are loaded above the 16-megabyte address.

As with DSPs, you must assemble your installation exit routine using the High Level Assembler with the z/OS® macro libraries at the current release. Then you must link-edit your exit routine into a JES3 library replacing the exit routine provided by IBM. Any program that you write that has explicit AMODE or RMODE attributes must be link-edited with the z/OS Binder.