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1.1.2 Computer Software -- The Operating System

Hardware is called ``hard'' because, once it is built, it is relatively difficult to change. However, the hardware of a computer system, by itself, is useless. It must be given directions as to what to do, i.e. a program. These programs are called software; ``soft'' because it is relatively easy to change both the instructions in a particular program as well as which program is being executed by the hardware at any given time. When a computer system is purchased, the hardware comes with a certain amount of software which facilitates the use of the system. Other software to run on the system may be purchased and/or written by the user. Some major vendors of computer systems include: IBM, DEC, HP, AT&T, Sun, Compaq, and Apple.

The remaining blocks in Figure 1.1 are typical software layers provided on most computing systems. This software may be thought of as having a hierarchical, layered structure, where each layer uses the facilities of layers below it. The four major blocks shown in the figure are the Operating System, Utilities, User Programs and Applications.

The primary responsibility of the Operating System (OS) is to ``manage'' the ``resources'' provided by the hardware. Such management includes assigning areas of memory to different programs which are to be run, assigning one particular program to run on the CPU at a time, and controlling the peripheral devices. When a program is called upon to be executed (its operations performed), it must be loaded, i.e. moved from disk to an assigned area of memory. The OS may then direct the CPU to begin fetching instructions from this area. Other typical responsibilities of the OS include Secondary Storage management (assignment of space on the disk), a piece of software called the file system, and Security (protecting the programs and data of one user from activities of other users that may be on the same system).

Many mainframe machines normally use proprietary operating systems, such as VM and CMS (IBM) and VAX VMS and TOPS 20 (DEC). More recently, there is a move towards a standardized operating system and most workstations and desktops typically use Unix (AT&T and other versions). A widely used operating system for IBM PC and compatible personal computers is DOS (Microsoft). Apple Macintosh machines are distinguished by an easy to use proprietary operating system with graphical icons.