Dennis's `The UNIX I/O System' paper in Volume 2 of the 7/e manual is basically about how drivers work. Is that near enough, possibly as augmented by Ken's `UNIX Implementation' paper in the same book? Those were my own starting point, long ago, for understanding how to write device drivers. Along with existing source code as examples, of course, but (unlikely many who hack on device drivers, I'm afraid) I have always preferred to have a proper statement of rules, conventions, and interfaces rather than just reading code and guessing. Norman Wilson Toronto ON