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ABAP Keyword Documentation → ABAP Programming Guidelines → Robust ABAP →System Fields
An ABAP program can request the status of the ABAP runtime environment using the system fields. From a technical viewpoint, these system fields are a set of predefined variables - the components of the predefined sy structure of data type SYST from the ABAP Dictionary - that are always available while the program is running.
The system field sy-repid is an exception. This field indicates the name of the program currently running. In this case, we are dealing with a predefined constant and not a component of the predefined sy structure.