4) Device drivers

One of the functions generally performed by an operating system is to convert an ugly I-O device into a more tractable virtual device. A typical example is to convert a line printer, with its many special functions, into a write only sequential text file. A problem with this approach is that some of the flexibility of the device may be lost. As far as possible, we wished to avoid making such conversions in the basic system. In fact, we desired to permit a user to make his own conversion, if he wished. Thus, our system would provide an interface to each I-O device which gave a using program direct control over that device. For example, all of the special functions of the printer would be directly available. Finally, we would supply a user level program which converted the I-O device into a generally useful virtual device, but would not attempt to handle all conceivable user desires.