7) Uninterpreted I-O Devices

As far as possible, we intended to provide a user program with a direct representation of each I-O device. We intended to avoid converting I-O devices into virtual objects, such as files. Such conversion would be provided by ``user'' programs, many of which we would write. However, since it would be possible for users with special needs to write their own, we were released from the obligation to provide for all possible uses of a given device. Furthermore, we wanted the possibility of emulating a device by a process. Thus, any communication with a device must be interceptable by an emulating process. This ruled out special virtual instructions for communicating with I-O devices.