In compiler-based Prolog systems

programs exist in the form of object code. Unlike conventional languages

standard Prolog provides programmers with facilities for updating their programs during runtime by using built-in predicates such as assert and retract. Hence

Prolog code should be modifiable

and a powerful code manager is needed. In this paper

a scheme for managing Prolog code database in the framework of the Warren Abstract Machine （WAM） is presented. The form of the object code is first described. Then the methods for managing dynamic code and semi-static code are discussed. This scheme has been used for the implementation of a Prolog code database manager written in C on VAX-11/780.