Session variables on MPE/iX can be either predefined or user-defined. A user-defined variable is one that you define yourself, and it is valid for the duration of the session in which you defined it. A predefined variable is defined for you by the system when you start a session. Some predefined variables are READ-only; you cannot change their value. (These are sometimes referred to as "system-reserved" variables.) Other predefined variables are READ-WRITE, and you can change their value within a session.
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