C/IL 102
Notes on Software, especially Operating Systems

Acknowledgement: Much of the material on this page is based upon How Operating Systems Work

Recall that a computer program is a set of instructions that can be interpreted and executed by a computer. Software is a collective term for computer programs, although it is also sometimes used to refer to stored data. Software is commonly categorized as follows:

• Application Software: provides you with a means to accomplish something that is not inherently related to computers, but for which a computer serves as a useful tool. Examples of popular applications include document preparation (MS Word), presentation preparation (MS Powerpoint), database management (various Oracle products), accounting/financial management (Quickbooks), web browsing (Mozilla Firefox), and games.
• System Software:
  • Utilities: Examples include programs for managing files on secondary storage (copying, moving (from one folder to another), and deleting files; disk defragmentation), program installing/uninstalling; installing printers (or other hardware); setting time and date; setting up a local network; system configuration, and anti-virus programs.

    More generally, a utility program is one whose purpose is aid a user in making effective use of a computer (and/or to make the experience more pleasant).

  • Operating System: The purpose of an OS is to organize/control/manage a computer's hardware and software resources so as to make it possible for application programs to run, hence making the machine usable and useful.

In recent years, desktop and laptop PC's normally have come pre-loaded with the most recent incarnation of the Microsoft Windows family of operating sytems, the latest of which is Windows Vista, which became available in 2007. However, many people still use its predecessor, Windows XP. Many corporate (and educational institution) computers run some flavor of Unix (possibly Linux), which is also popular among serious computer enthusiasts, who tend to view Windows (and Microsoft products in general) with some disdain. Apple Macintosh computers come pre-loaded with a flavor of Unix called OS X.

Special-purpose computers, such as those that control microwave ovens, have no need for an OS, because the set of functions they perform is narrow and unchanging, and hence these functions are typically "hardwired" into the electronic circuitry. Computational devices that are designed to be more flexible, on the other hand, run under the direction of operating systems. By "flexibile" we mean that their behavior can be changed, either by upgrading (or adding new) application programs or possibly even upgrading (or replacing) the OS itself. Cell phones and personal digital assistants (PDAs) utilize OS's, although they tend to be somewhat more rudimentary than the OS's used on more powerful desktop/laptop computers.

Question: What does an operating system do?

Answer: At least two main things:

1. It manages the hardware and software resources of the system, including the processor, memory, and disk space. (On a cell phone, this list would include the keypad, the screen, the address book, the phone dialer, the battery, and the network connection.)
2. It provides a stable, consistent way for application programs (and users) to communicate with the hardware without having to know all the details of the hardware (such as which brand/model of hard disk or printer is attached).

One can categorize OS's based upon the kinds of computers they control and the kinds of applications they support. Indeed, we can identify four broad classes:

• real-time: used for controlling machinery, scientific instruments, and industrial systems (e.g., factories, nuclear reactors). This kind of OS typcially provides very little user interface support or end-user utilities; rather, its purpose is to ensure that "things happen" (in whatever environment it controls) in the right sequence and at the right times.
• single-user, single-task: designed to manage a computer so that one user can execute one program at a time. The Palm OS for Palm hand-held computers is an example, as was MS-DOS (Microsoft Disk Operating System), which ran on "IBM-compatible" PC's in the early 1980's.
• single-user, multi-tasking: designed to allow a single user to run multiple programs at the same time. MS Windows is an example, This is more complicated than single-tasking because the various executing processes must not be allowed to interfere with each other (for example, by overwriting each other's data in RAM) and each process must be given sufficiently many "processor cycles" (with sufficient frequency) to continue doing its job. (If a media player program is playing a music CD or a video on DVD, the processor must devote some attention to it on a regular basis or else the sound or image will become "choppy".)
• multi-user: designed to allow multiple users to run multiple programs at the same time. This is more complicated than single-user because care must be taken not to allow one user's program to access another user's private data. Examples include any in the Unix family of OS's, including Linux.

Now we answer, in somewhat more detail, the question raised earlier: What does an operating system do? We can identify at least six categories of tasks:

• processor management: schedules processes for execution. Typically, many processes are "in execution" concurrently, but the CPU can be devoted to running (i.e., executing the instructions of) at most one process at any moment in time. (Under Windows XP, press CNTRL-ALT-DEL (at the same time) in order to get the Windows Task Manager, which lists all the processes running, how much RAM they're occupying, and what percentage of CPU time they are consuming.) Each process takes turns getting "time slices" (on the order of thousandths of a second) during which the CPU executes some of its instructions before switching its attention to another process.
• memory management: decides where in RAM to store programs and data currently in use (making sure not to allow one to overwrite another).
• device management: handles input from the keyboard and mouse, makes sure that print jobs are scheduled properly.
• storage management: maintains records about the files resident on secondary storage devices (e.g., hard disk, memory sticks), keeping track of where each file is stored and where "empty" space exists for storing new files. (Such records are stored on the storage devices themselves, of course.)
• application interface: an operating system acts as a layer between the hardware and application programs, allowing the latter to issue commands and requests to the former without the latter having to "know" all the details of how the former works. A good analogy is the "interface" through which one drives an automobile. Over the past fifty years, many advances have been made in the "hardware" at the core of an automobile: the engine, transmission, braking system. Yet the interface that a driver utilizes has changed very little over those years: it consists mainly of gas and brake pedals, a steering wheel, and a lever for changing gears.
• user interface: similar to the item above, except this allows a user (rather than an application program) to interact with the hardware in a structured, consistent way.