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The traditional Mac OS Device Manager controls the exchange of information between applications and hardware devices by providing a common programming interface for applications and other software to use when communicating with generic device drivers. Normally, applications don't communicate directly with generic drivers; instead, they call Device Manager functions or call the functions of another manager that calls the Device Manager.
In the second generation of Power Macintosh, two significant additions have been made to the Device Manager. First, drivers can now process more than one request simultaneously. Such drivers are called concurrent drivers. Second, a new entry point has been added, similar to IODone. It is called IOCommandIsComplete. Details on concurrent drivers and their use of IOCommandIsComplete are given in Completing an I/O Request.