A protection domain is a set of names available to a thread of execution. Can merge domains. Extensibility Register a handler for an event. In most cases, performance is a function call. Can raise any event you can name.
Guards restrict access to handlers.
Guards are essentially predicates. Can have multiple handlers for an event. These are services provided by SPIN with appropriate places to which to attach extensions. Raises events associated with traditional MMU exceptions. Thread Management Provides interfaces for scheduling, concurrency, and synchronization.