1.2 The four categories of operating systems

Within the family of operating systems, we can categorize them into four types:

I. Real-time operating system (RTOS)
Real-time operating systems are used to control machinery, scientific instruments and industrial systems. They are designed to allow computers to process and respond to input instantly. They are usually used when computers must react to the consistent input of information without delay. For example, real-time operating systems may be used in navigation. A very important part of an RTOS is managing the resources of the computer so that a particular operation executes in precisely the same amount of time, every time it occurs.

II. Single-user, single task
This operating system is designed to manage the computer so that one user can effectively do one thing at a time. The Palm OS for Palm handheld computers is a good example of a modern single-user, single-task operating system.

1.2 The four categories of operating systems

Within the family of operating systems, we can categorize them into four types:

III. Single-user, multi-tasking
This operating system is mostly used on desktop and laptop computers. Microsoft's Windows and Apple's MacOS platforms are both examples of operating systems that will let a single user have several programs in operation at the same time. For example, it is entirely possible for a Windows user to be writing a note in a word processor while downloading a file from the Internet and at the same time printing the text of an e-mail message.

IV. Multi-user
A multi-user operating system allows many different users to take advantage of the computer's resources simultaneously. The operating system must first make sure that the requirements of the various users are balanced, and that each of the programs they are using has sufficient and separate resources so that a problem with one user will not affect the entire community of users. Unix, VMS and mainframe operating systems, such as MVS, are examples of multi-user operating systems.