Comparison of the Two Methods

In most cases, starting the IPCS dialog directly from ISPF (without first starting IPCS in line mode) is probably the best method to use. This method makes the most efficient use of system resources, in that the storage needed to run IPCS is not required until the user actually invokes the IPCS dialog.

In contrast, starting IPCS line mode before beginning an ISPF session causes system resources for IPCS to be held for the entire ISPF session. This use of resources is of no real consequence if the user plans to use IPCS for most of the time the ISPF session is active. Most users, however, do not need to use IPCS for a large portion of their ISPF session. For such users, entering the IPCS command before starting ISPF causes unnecessary use of system resources, and a corresponding decrease in performance for other ISPF services.

However, there are certain situations in which it might be desirable to start IPCS in line mode before starting ISPF:

The user plans to use IPCS many different times during an ISPF session.
The IPCS dialog starts more quickly, once invoked, when the IPCS command has been entered to start IPCS before starting the ISPF session. For users who plan to invoke the IPCS dialog many times during the course of an ISPF session, the benefit of improved response time might outweigh the disadvantage of reduced performance for other ISPF services.
The user plans to use IPCS in line mode.
Entering the IPCS command is the only way to start IPCS in line mode.
The user needs the problem management and data set management functions.
These functions are available only by entering the IPCS command with the PARM(nn) parameter. See z/OS MVS IPCS User's Guide and z/OS MVS IPCS Commands for further information on managing problems and data sets.

Even if a customized access procedure has started an ISPF session without entering the IPCS command, users can still enter the IPCS command explicitly. The user can exit ISPF, enter the IPCS command, and either use IPCS in line mode or re-enter ISPF to invoke the IPCS dialog.