Design considerations

The following steps should be taken when invoking VTAM® exit services:

Check the bit in the function code and related session information to be sure VTAM exit services is available. If this bit is not on, VTAM exit services is not available and cannot be used.
Get the pointer to the VTAM exit services parameter list from the session management exit parameter list. The VTAM exit services parameter list pointer is at displacement 72 (decimal) into the session management exit parameter list. The session management exit parameter list is pointed to by register 1 when the session management exit is called by VTAM.
Check the VTAM exit services parameter list pointer to be sure it is not zero.
Get the pointer to the supported functions bitmap. A pointer to the supported functions bitmap is at displacement 0 into the VTAM exit services parameter list.
Check the supported functions bitmap to be sure the bit is on which represents the correct function.
Build the text for the message to be issued.
Build the VTAM exit services message parameter list (EXMPL).
1. Put a pointer to the message text into the EXMPL.
2. Put the length of the message text into the EXMPL.
Put a pointer to the EXMPL into the VTAM exit services parameter list.
Put the function code into the VTAM exit services parameter list.
Put the address of the VTAM exit services parameter list into register 1.
Note: Be sure to save the pointer to the session management exit parameter list. It might be needed by the session management exit upon return from VTAM exit services.
Load the pointer to the VTAM exit services module (ISTIECXS) from the VTAM exit services parameter list into register 15.
Call the VTAM exit services module (using BALR 14,15).
Check the return code from VTAM exit services. The return code is in register 15 when VTAM exit services returns to the session management exit. If the return code is nonzero, take the appropriate action.