22C:116, Lecture Notes, Aug. 21, 1995

1. The course title says something important about this class

   • Advanced - This is an advanced course, it has prerequisites! if you haven't had a prior course on operating systems, you should be very hesitant about taking this course!

   • Operating Systems - This is a course about operating systems.

   • Operating - The use of this word implies a subject (something that operates) and an object (something that is operated). What are these?

     The object is clearly the computer. The operating system operates the computer.

     The indirect subject is clear. The operating system operates on behalf of its human user community.

     The direct subject is less clear. Generally, the human users interact with applications programs of one kind or another, and it is these programs on behalf of which the operating system operates.

   • Systems - This word says quite a bit. A system is composed of components of some kind, frequently not primitive components, but subsystems in their own right. What are these subsystems, and does the organization of the operating system relate to their ogranization?

2. What is an Operating System?

   This question was posed to the class, and the following responses were discussed:

   • It may provide a uniform interface across multiple platforms. This is certainly true of modern UNIX, although UNIX was not designed with this in mind. Other modern operating systems, notably MS-DOS and Mac OS can hardly be described as being platform independent.

   • It provides abstractions such as files, windows, timers and other tools that programmers would like to use.

   • It controls the allocation of resources:
     • One or more CPUs.
     • The main memory.
     • The I/O channels.
     • The secondary memory.
     This resource management model was at the forefront of most thinking about operating systems in the 1970's, when most computer systems were shared between many users who were in competition for the scarce resources of the systems.

     With the advent of personal computers, there was even talk about the irrelevance of operating systems, since with only one user, it was easy to imagine that there were no resource contention problems remaining.

   • It provides mechanisms for enforcing security policies. A good operating system will provide general purpose mechanisms that can be used to enforce a variety of policies; most systems in use today, from UNIX to MS/DOS were designed for environments where security was not a big issue, so to the extent they support security, their mechanisms are generally special purpose, limiting the range of policies that may be enforced.

   • The combination of hardware plus operating system software provides a virtual machine to the users. The resources of this machine are more closely related to the needs of applications programmers than the resources of the bare hardware.

   • But to most users, the operating system remains a mysterious piece of expensive and often quirky software. In one sense, there is no need for an operating system, since in theory, everything done by the system could be done directly by the application software.