A software system for use with a Digital Television set-top box. The system uses a software bus application programming interface (API) that is specially designed to make the set-top box compatible with two standard DASE architectures: (1) the architecture wherein the procedural application engine contains the declarative application engine, and (2) the architecture wherein the declarative application engine contains the procedural. An application engine manager uses a function call router to intercept all DASE infrastructure APIs incoming from software applications downloaded by the set-top box from a DTV broadcast. All APIs are routed through the application manager, and all application engines are required to implement the software bus API. A specially-designed software interface also enables the convenient installation of additional applications by simply changing one line of software code.