1.1 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Lecture 2: OS Structures (Chapter 2.7)

1.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Operating System Design and Implementation Start by defining goals and specifications Affected by choice of hardware, type of system User goals and System goals User goals – convenient to use, easy to learn, reliable, safe, and fast System goals –easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient Important principle to separate Policy: What will be done? Mechanism: How to do it? Mechanisms determine how to do something, policies decide what will be done The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later

1.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition MS-DOS Monolithic Structure

1.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Traditional UNIX System Structure

1.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Layered Operating System

1.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Microkernel System Structure Moves as much from the kernel into “user” space The exokernel seems to move even more of the traditional kernel into user space! Communication takes place between user modules using message passing Benefits: Easier to extend Easier to port the operating system to new architectures More reliable (less code is running in kernel mode) More secure Disadvantages: Claimed: Performance overhead of user space to kernel space communication The paper proves performance penalties due to implementation, not concepts.

1.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Solaris Modular Approach Somewhat similar with a layered approach, yet more flexible.

1.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Mac OS X Structure Mach: microkernel. Provides memory management, RPCs, IPC, thread scheduling BSD (Unix) provides: command line interfaec, support for networking and file systems, POSIX API (including Pthreads)

1.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Exokernel Structure