This article briefly describes how screen painting hangs together.

Screens can be used with 2 different forms/types of ABAP's. The differences can be explained like this:

• An ABAP screen application: an ABAP of type "M" (Modulepool, set in with program attributes), has to be called via a transaction code that holds the first screen number. When the transaction is started, the first PBO module on the screen that is defined on the transaction is called. Applications have a screen flow which is set up with the statements SET SCREEN nnn and LEAVE SCREEN, but also CALL SCREEN nnn (for popups). Module pools can thus be set up to cater for multiple transaction codes, with each their own screenflow. For larger module pools, the actual screen flow can be determined at run-time.
• Expanding an ABAP report with one or more screens: a report that has been constructed with (possibly) WRITE statements can use screens that can be called with the CALL SCREEN nnn statement. ABAP reports can be started without transaction code as well.

The explanation that follows starts from a screen application; there are no real differences between both forms of ABAP, except for the ones mentioned above..

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