Operating systems are an essential part of any computer system. Similarly, a course on operating systems is an essential part of any computer-science education. The course is intended as an introduction to operating systems at undergraduate level or postgraduate level. Students are expected to familiar with basic data structures before they begin the course, as well as computer organisation, and a high-level language, such as C. The fundamental concepts and algorithms covered in the lessons are often based on those used in the existing commercial operating systems. The aim of the course is to present these concepts and algorithms in a general setting not connected to one particular operating system only. The lessons show and explain what operating systems are, what they do, and how they are designed and constructed. Also dealt with is the development of the concept of an operating system, as well as the the common features of an operating system. The course shows, what an operating system can do for the user, and what it does for the computer-system operator. Some lessons will describe the process concept and concurrency as the heart of modern operating systems and process in main memory during execution. To improve both the utilisation of CPU and the speed of its response to its users, the computer has to keep several processes in memory. There are many different memory-management schemes. These schemes reflect various approaches to memory management, and the effectiveness of the different algorithms depends on the situation. Large number of examples will be with respect to the most popular operating systems including Unix, Irix, Linux and various types of Microsoft Windows.