1.3 Introduction to operating systems

Operating system is a hard term to define. Silberschatz e.t. defines that “ an operating system is a program that manages the computer hardware. ” However, what you consider an operating system depends on your needs and your view of the system. The first view is that operating system is a scheduler/simulator, on this view, the operating system has resources for which it is in charge, responsible for handing them out as well as recovering them later. Resources here include CPU, memory, I/O devices, and disk space. The second view is that operating system is a virtual machine, it means operating system provides a new machine. This machine could be the same as the underlying machine. Allows many users to believe they have an entire piece of hardware to themselves. This could implement a different, perhaps more powerful machine. Or just a different machine entirely. It may be useful to be able to completely simulate another machine with your current hardware. The other view is that operating system is a multiplexor which allows sharing of resources, provides protection from interference, and provides for a level of cooperation between users. This also for the economic reasons, we can’t afford for all resources.

According to these three views, if we have enough hardware to give anyone too much; the hardware was well defined; the sharing problem is solved then we would not need operating systems. Unfortunatly, they will still be needed as a servant or provider of services: need to provide things like in the above views, but deal with environments that are less than perfect, need to help the users use the computer by: providing commonly used subroutines; providing access to hardware facilities; providing higher-level "abstract" facilities; providing an environment which is easy, pleasant, and productive to use. This view as a provider of services fits well with our modern network view of computing, where most resources are services.

Internally, operating systems vary greatly in their makeup, since they are organized along many different lines. The design of a new operating system is a major task. It is important that the goals of the system be well defined before the design begins. These goals form the basis for choices among various algorithms and strategies.

Because an operating system is large and complex, it must be created piece by piece. Each of these pieces should be well delineated portion of the system, with carefully defined inputs, outputs, and functions.

What are the desires of an operating system?

We can discuss them in term of: Usability, Facilities, Cost, and Adaptability.

Usability:

• Robustness

OS accept all valid inputs withour error, and gracefully handle all invalid inputs. OS would not be crashed in any circumtances, and could be recovered in the case that we suddenly remove hardware while they are running, or lost of power supplied.

• Consistency

For example, if "-" means options flags in one place, it means it in another. The key idea is conventions . We should base on the concept: The Principle of Least Astonishment. This helps us easy to understand and adapt with the new system.