We present a case study of evolution (or vertical reuse) in the domain of visual interactive software tools. We introduce an architecture suitable for this purpose, called orthogonal architecture. The paper describes the architecture itself, the reverse engineering process by which it was obtained, and the forward engineering process by which it was evolved.

A brief description of the VIFOR (visual interactive Fortran) environment is given. VIFOR is based on a simple, but effective data model of Fortran programs containing three entity classes and three relation classes only. Programs can be displayed and edited in two forms: the traditional one (i.e., code) and the visual form. VIFOR contains transformation tools for both directions, i.e., from code to visual form and from visual form to skeletons of code. Hence, it is suitable for reverse engineering and maintenance of existing code. Specially designed browsers implement the graphical interface.>