In this paper we develop a formal specification of aspects of the Forth programming language. We describe the operation of the Forth compiler as it translates XSForth control words, dealing in particular with the interpretation of immediate words during compilation. Our goal here is to provide a basis for the study of safety properties of embedded systems, many of which are constructed using Forth or Forth-like languages. To this end we construct a model of the Forth compiler in the -calculus, and have simulated its execution by animating this model using the Pict programming language.