CompCert is a formally verified compiler for C that is specified, programmed and proved correct with the Coq proof assistant. CompCert was used in industry to compile critical embedded software. Its correctness proof states that the compiler does not introduce bugs. This semantic preservation property involves the formal semantics of the source and target languages of the compiler. Reasoning on C semantics to prove compiler correctness is challenging, as C is a real language that was not designed with semantics in mind. This paper presents the operational style that was designed for the C semantics of CompCert in order to facilitate the mechanized reasoning on terminating and diverging programs, and details the semantics of the Clight source language of CompCert.