We present a comparative study on three software development methods which cover the entire development life cycle for hard real-time systems: (1) Structured Analysis, (2) Computer Aided Prototyping, and (3) Spec formal logic specification method. We use a simple example to demonstrate the software development process using all three approaches. The strengths and weaknesses of each method are discussed. Hard real-time systems, Systematic Software development, Structured analysis, Computer aided prototyping, the Spec language.