Each MVS™ component has individual programming conventions. Components that have similar functions have similar requirements. Most requirements, however, are unique. Because of these differences among component requirements, we cannot provide universal rules for coding installation exit routines. The specific guidelines we can give are:

• Whenever a macro is provided to perform a service, use the macro.
• Upon entry to your exit routine, save all register contents and restore them before returning to your calling routine. An exception to this guideline is the use of return and reason codes. In many cases, you must insert a return code in register 15, and reason codes in registers 0 and 1 before returning to the calling program.
• Under no circumstances should you assume an interface (such as contents of a register) that is not specifically documented.
• Exits That Use a z/OS UNIX Callable Service: Any exit that uses a z/OS UNIX callable service needs to be written based on the current environment. If the task is not dubbed on entry to the exit (STCBOTCB=0), the exit must undub (perform a CALL BPX1MPC) at completion if the exit has called any other z/OS UNIX callable service callable service. If the task is already dubbed (STCBOTCB does not equal 0), the exit should leave the environment as it was on entry, which would require closing any files that it opens and leaving signal states unchanged.