We have defined a direct-execution model dedicated to non-numerical processing which is based upon an internal representation of source programs derived from LISP. This model provides good support for both sophisticated editing (syntactical parsing, tree manipulation, pretty-printing, ...) of conventional languages and artificial intelligence languages. A high level microprogramming language (LEM) was designed to write the interpreters and the editors. A hardware processor was built and a LISP interpreter, microprogrammed in LEM, has been operational since September 1980.

This paper describes the basic principles and the architecture of a general host machine based upon lists processing. Current works in this field are dealing with conventional direct execution schemes which use lineary structured Directly Executable Languages: prefixed languages with varying formats for operators and operands. If these languages are convenient for interpretation and provide an efficient execution scheme, on the other hand, they are very hard to generate.