Aurora is a prototype Or-Parallel implementation of the full Prolog language fot shared-memory multiprocessors, developed as part of an informal research collaboration known as the "Gigalips Project". This report describes the abstract machine of the implementation, expressed in terms of a Prolog engine adapted for Or-Parallel execution by means of the SRI model. An algorithms interface defining the communication between the engine and sheduler components of each Aurora process is described, and enables different engine and scheduler components to be used interchangeably. Both the interface and the engine are Fundamentally revised versions of those used in previous generations of the implementation.