A process of re-engineering large and complicated systems

For very large and old systems the program source code is often the only reliable representation. Supporting documentation for system maintenance can be extracted from the source code by a reverse engineering process, which is similar to that used in electrical engineering. Program schematics alone are enough to perform the re-engineering process of decomposition. The example shows that there is no need for semantic description of all units and circuits while decomposing the system. To identify all program statements which are involved in a particular application, the formal documentation reverse engineered from the source code is often enough-the majority of functional names are not needed, just a flow of transfers of control. It means that irrespective of the size and complexity of a system, it can formally be described without the intensive involvement of a maintainer. Separate applications identified from this formal documentation can then be enriched by gradually adding a semantic content within each application until this application becomes easy to read and understand