In the preceding chapters we have described how a large scale general purpose operating system can be constructed. At present our operating system exists somewhat in a vacuum since we have not yet specified how we can inform it about the jobs we want it to do. The next step is to manufacture an interface between the user and the operating system so that the user can tell the system what he requires of it. The interface can also be used to present information, such as resource requirements, about each job so that the system can optimise performance in the ways described earlier. Of course communication between user and system is not all in one direction; we must also construct a complementary interface by which the system can tell the user what it has been doing.
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