The "Grassroots" series has been designed to neet the students needs. The books address core subjects, and provide the student with enough resources and support for a one semester course. This text introduces the reader to operating systems, and gives an overview of the rest of the book. It covers the traditional material on processes, but with more emphasis than usual on threads and considers interactions between concurrent threads, including semaphores, message queues, and monitors. It goes into concurrency in some detail. Memory management is covered, including segmentation and paging. Input/Output is dealt with over two chapters: one concentrates on the high level, device independent aspects, while the other looks at low level aspects such as the interface with hardware, control of devices, and file organisation on disk. Distributed computer systems are introduced, with detail on communication mechanisms, and various distributed services which can be built on top of these. Finally, the work looks briefly at fault handling and security issues, in both stand alone and distributed systems.