A cyber-physical system (CPS) is an interactive system of continuous plants and real-time controller programs. These systems usually feature a tight relationship between the physical and computational components and exhibit strict true-concurrency with respect to time. These communication and concurrency issues have been well investigated in event-based synchronous languages but only for discrete systems. In this paper, we present an imperative-style programming language for CPS and explore an observation-oriented denotational semantics for the language. Furthermore, a set of algebraic laws that could facilitate the transformation of programs are investigated and consistency of the algebraic laws can be ensured with respect to the denotational semantics. The algebraic laws which have been established in the framework of our semantic model could greatly enhance the reliability of algebraic transformation.