  This paper describes an abstract machine for linguistic formalisms that are
based on typed feature structures, such as HPSG. The core design of the
abstract machine is given in detail, including the compilation process from a
high-level language to the abstract machine language and the implementation of
the abstract instructions. The machine's engine supports the unification of
typed, possibly cyclic, feature structures. A separate module deals with
control structures and instructions to accommodate parsing for phrase structure
grammars. We treat the linguistic formalism as a high-level declarative
programming language, applying methods that were proved useful in computer
science to the study of natural languages: a grammar specified using the
formalism is endowed with an operational semantics.
