Overview

Multiuser systems like OpenVMS have several features not typically found on single-user operating systems. The following sections describe some of the OpenVMS features that provide for a multiuser environment.

Many operating systems are designed primarily for the use of only one person at a time, even though they may include most of the features required to support multiple users. The evolution of Microsoft Windows and the Apple Macintosh operating systems are examples of these. These systems have been retrofitted to some extent to allow access by more than one user simultaneously.

OpenVMS, like UNIX, evolved from the opposite direction. It was designed first as an interactive system to be used by many people at once, each using a command line interface (CLI). Single-user graphical user interfaces (GUIs) were added later. An OpenVMS system can support the simultaneous use of multiple terminal-based users and X Window System clients.

Both types of OpenVMS interfaces, CLI and GUI, are described later in this book.

Concurrent Access by Many Users

As mentioned earlier, OpenVMS can support many simultaneous users while also running other tasks. This can be accomplished on computers that may have only a single processor.

How does a computer system with only one processor perform many tasks at once? The answer is that it presents a clever illusion. In reality, a processor can be working on only one task at a given instant. The operating system switches rapidly between tasks, giving each a very brief amount of attention, just milliseconds, before...