AioPlugin provides support for asynchronous IO event notification. When an external IO event occurs, the event registration causes a Smalltalk semaphore to be signaled. A process can wait on this semaphore, thereby acting as an event handler for the external event. Currently this works with the Unix VM (probably including Mac OS X). UnixAioPlugin implements the primitives that interact with the aio mechanism in the Unix VM. Other platform-specific subclasses could be written to provide a compatible aio mechanism for other platforms.