Abstract: Operational and denotational semantics are defined in connection with different paradigms of programming languages, including a simple functional programming language, a simple procedural programming language and a simple object oriented language. A visual teaching assistant system was designed to demonstrate the visual procedure of running programs of different programming languages. With the assistance of the system students can easily understand and grasp the formal semantics of programming languages and their implementation.

Key words: formal semantics, visual instruction assistant system, incremental design, practice driven instruction