Microcomputer Operating Systems


A computer operating system is a collection of programs used by the computer to manage its operations and provide an interface between the user or application program and the computer hardware. Operating systems will be discussed in detail in another unit later this year. Without an operating system, a computer cannot even recognize that a key on the keyboard has been pressed, much less what it means. A number of operating systems and user interface programs have been designed for use with microcomputers. Microsoft Windows and the Macintosh operating systems are two examples of interfaces that co-ordinate the use of the machine.

Operating systems can be loaded into memory from either floppy diskettes or hard disks. This process is what is known as booting. The word boot derives from the expression "to lift yourself up by your own bootstraps," which is essentially what a computer does when it is first turned on. In order to read from and write to a disk, the disk operating system (or DOS) must be in memory. However, the disk operating system itself is stored on disk. Therefore, it seems impossible for the computer to load its operating system into memory when it must have the operating system in memory in order to read from a disk. In actuality, the computer already has a small portion of its operating system built into its ROM (read only memory). This part of the operating system starts the process of reading the remainder of the operating system from the disk.

The first operating system developed for use with microcomputers was Digital Research's CPIM (Control Program for Microprocessors). CPIM was stored on a floppy disk so that it could be loaded into any microcomputer, provided the computer used the Intel 8080 or 8085 or the Zilog Z-80 eight-bit microprocessor.
In 1981 IBM introduced the IBM PC and along with it IBM PC-DOS. IBM PC-DOS was developed for IBM by Microsoft. As a result of its agreement with IBM, Microsoft was able to market its own version of the operating system, called MS-DOS. Today, more than 100 different microcomputers-IBM compatibles-use the MS-DOS operating system. IBM PC-DOS and MS-DOS are designed to run on Intel 8088, 8086, and 80286 sixteen-bit microprocessors and 80386 and 80486 thirty-two-bit microprocessors.

Other first generation microcomputer operating systems include Apple DOS and Apple ProDOS, which are used on the Apple II family of microcomputers; Unix (or Xenix), which is used on microcomputers as well as minicomputers. Newer, transparent user interfaces have been introduced to lessen the amount of operating system knowledge required by the user of the microcomputer. The Macintosh and Microsoft Windows operating systems allow the user to point to icons and use pull-down menus on the screen with the help of a mouse (see Figure below).


An icon is a pictoral representation or graphic image that appears on the computer screen. Icons represent commands or menu choices. On the Apple Macintosh, for example, an icon of a trash can is used to signify that a file is being deleted or thrown away.

These transparent user interfaces are often called graphic user interfaces (GUI's).

The type of user interface provided by IBM PC-DOS and MS-DOS is referred to as a command-line user interface (see below).


The differences between the two are discussed in the following section.

Last Updated Jan.6/99