A program interacts with its environment through accepting and delivering information from and to its environment respectively. As such, the functionality in a program is achieved through its input/output statements. Based on this hypothesis, this paper proposes a novel approach for the extraction of code fragments for implementing each functionality or a given group of related functionalities from program source code. This helps software maintainer to identify affected code fragments for making changes to functionalities. The code fragments extracted are also reusable for implementing the same functionalities in the same hardware and software platform. At least, the designs of the code fragments are reusable.