An Operating Systems Vade Mecum

chapter 1INTRODUCTIONThe development of operating systems has progressed enormously in the last fewdecades. Open shop computing has given way to batch processing, which in turn hasyielded to interactive multiprogramming as emphasis shifted first to efficient use ofexpensive machines and then to effective shared use. The recent trend to personalworkstations has turned full circle back to the earliest beginnings: a single user associatedwith a single machine.The long and complex journey has not been pointless. As computer use hasexpanded, so has the sophistication with which we deal with all of its aspects. Ad hocmechanisms have become standardized, then formalized, then analyzed. Standing at thepresent and looking back, we have the advantage of seeing the larger picture and ofknowing how originally unconnected ideas fit together and reinforce each other. Werealize that the proper software for a personal workstation has more in common withlarge operating systems for interactive multiprogramming than it has with open shopcomputing.The future holds the promise of increased use of networking to interconnect computersin various ways. Multiple-user machines will soon be built of communities ofcommunicating computers. This new theme has already begun to be played. It is anexciting time to be studying operating systems.1 PHILOSOPHY OF THE BOOKIn this book, I have carefully tried to distinguish mechanisms from policies. Mechanismsare techniques that perform activities. Policies are rules that decide which activitiesto perform. The mechanisms are the ‘‘nuts and bolts’’ of operating systems and oftendepend to some extent on the hardware on which the operating system runs.1