We focus on the part of a compiler between syntax analysis and code generation. A language is specified by adding semantic rules in a functional notation to the syntax of the language. Starting with a small sublanguage of while statements, the semantics of the statement constructs of C is built up incrementally. Using a small ad hoc code generator, a compiler has automatically been constructed from the semantics. The semantic description is analogous to a syntax directed construction of a flow diagram for a program. In analogy with grammars and parser generators, minimal knowledge of the underlying theory is required. For the control flow aspects of languages, efficient compilers can quickly be generated.