LMC + scratch: a recipe to construct a mental model of program execution

Understanding how programs execute is one of the critical activities in the learning journey of a programmer. A novice constructs a mental model of program execution while learning programming. Any misconceptions at this stage lead to the development of a discrepant mental model. If left untreated, learning in advanced subjects like data structures and compiler construction may suffer. One of the ways to prevent the situation is carefully and explicitly unveiling the details of program execution. We employed Little Man Computer (LMC) for this purpose. Its interactive visual interface helped them internalise how software interacted with the hardware to achieve the programmer's objective. After spending a few sessions on the programming of LMC, we moved to Scratch. Scratch is a much higher-level language than the LMC assembly. So, while introducing Scratch programming constructs, we mapped the LMC equivalents of these instructions. The strategy helped evade several misconceptions by developing a deep understanding of the program execution model. It also served as a building block for introducing other concepts like state, abstraction, the need for higher-level languages and the role of compilers etc. We tried this approach in an Introduction to Computer Science module where most students had zero or very minimal exposure to programming. We received positive feedback from students and other fellow teachers teaching in the subsequent semesters.