The exit routines are given control in task mode and protect key zero with no locks held and 31-bit addressing mode. They must not operate in cross-memory mode.
When writing an ACS installation exit routine, you should only reference data that is explicitly passed, because too many different environments can invoke an ACS installation exit. An ACS routine should not issue a dynamic allocation request, because a dynamic allocation request can invoke an ACS exit. During the course of allocation, the ACS routine or exit can be entered multiple times.
Linkage is with standard MVS linkage conventions.