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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.