22C:116, Lecture Notes, Lecture 1, Fall 1998

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.

• A course on elementary computer architecture.
  You are expected to have a fair understanding of computer architecture at two levels. First, you should be familiar with processor-memory-switch level descriptions of computers, including questions of basic bus structure, direct memory access I/O, and memory management units. Second, you should be familiar with the instruction-set-processor level, specifically with issues of memory addressing, register utilization, and typical input/output instructions.

• A course on elementary operating systems.
  You are expected to have a fair understanding of process, memory and input-output management from the user's perspective. File system organization and security, input-output buffering, process creation and synchronization issues should be familiar.

• An introductory course on analysis of algorithms.
  Graph algorithms play a major part in this course. Specifically, algorithms for finding connected components of a graph, algorithms for depth first and breadth first traversal of a graph, and algorithms for finding shortest paths in a graph. Knowledge of data structures and recursive programming sufficient to understand such algorithms is essential.

• Significant programming skills.
  We will illustrate a number of concepts from operating systems with exercises involving the Unix system and with exercises involving a low-level portable thread package written in C and usable in C, C++ and possibly in other languages. While we do not specify the particular programming language you should use, use of the required thread package is likely to be very difficult in languages other than C or C++; it is possible that the routines in the package may operate correctly if called using external linkage from languages such as Pascal or Ada, but there are no guarantees of this!