VTAM® schedules the ATTN exit routine with "CNOS" in word 4 of the parameter list when a CNOS request is received for the application program and is processed by VTAM. In other words, a negotiation occurs. A negotiation cannot take place for a single session. However, an ATTN(LOSS) exit can be returned for a single session that is deactivated.

The RPLAREA field in the read-only RPL points to a CNOS session limits data structure that contains the new session limits and the application program's security acceptance level. The application program can use the ISTSLCNS DSECT to map the storage. For details on the CNOS session limits data structure, see Table 1.

For more information about the parameter list that is pointed to in register 1 of the ATTN exit and for an explanation of entry procedures, refer to z/OS Communications Server: SNA Programming.

When the VTAM operator issues a MODIFY CNOS command, the ATTN(CNOS) exit is used to notify both application programs in a parallel session environment. The ATTN(CNOS) exit also is used to notify only the local application program in a single session environment. When the VTAM operator issues a MODIFY DEFINE, the ATTN(CNOS) exit is used to notify the specified application program. The application program can use this information to manage CNOS requirements more effectively.

This exit will also be scheduled when VTAM issues an internal MODIFY CNOS (see Synchronizing end points after session activation failure for more information).