To define z/OS® Communications Server under the z/OS operating system, you need to change the message routing codes, define channel-attached devices and determine the ECSA value.

To define z/OS Communications Server, do the following steps:

1. Change the message routing codes by coding a system user exit routine (if there is multiple console support). Message routing codes determine the console at which messages will appear. If the routing codes provided for z/OS Communications Server messages do not meet your needs, you can change the routing codes used on the messages by coding a system user exit routine (if there is multiple console support). The exit routine receives control before messages are routed so it can examine the messages’ routing codes (and descriptor codes) and change them. The system uses the modified routing codes to route these messages. To change routing codes, do the following steps:
   1. Prepare the write-to-operator/write-to-operator-with-response (WTO/WTOR) exit routine, and add it to the control program. The WTO/WTOR exit routine can be inserted into the resident portion (communications task) of the control program either before or after system generation.
   2. See z/OS Communications Server: SNA Messages for the message routing codes, and decide which new routing codes you want to assign to each message.
2. Define channel-attached devices. If you are adding channel-attached devices, you can define these devices using the Hardware Configuration Definition (HCD) to dynamically add the devices.
3. Determine the ECSA value. The ECSA value is defined by the second value on the CSA parameter in member IEASYSxx. It is recommended that you examine the ECSA value to ensure that it is adequate for z/OS Communications Server. You can determine the ECSA value by reviewing Storage estimate worksheets.