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Before the introduction of ABAP Objects, subroutines were mainly used for the local modularization of programs, however, they can also be called externally. Their functions are implemented between the statements FORM and ENDFORM. A subroutine is declared immediately when implemented.
Subroutines should no longer be created in new programs for the following reasons:
In those places where subroutines cannot yet be replaced by methods (PERFORM ON COMMIT|ROLLBACK, GENERATE SUBROUTINE POOL), they should be used purely as wrappers for method calls and must not contain any other functional code.
The syntax allows the definition of a subroutine to be split between a declaration part and an implementation part using the statements FORM ... DEFINITION and FORM ... IMPLEMENTATION, but this is not supported by all tools and should be avoided.