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 moving on to the next. This gives a very convincing sense that many tasks are proceeding at once, even though the CPU is really working on one at a time.

Suppose you are typing an e-mail message on your OpenVMS system. The system spends a very brief amount of time processing the key you pressed and displaying your keystroke on the screen. Between your keystrokes, the system switches to other tasks while waiting for your next keystroke. Only when you press another key does OpenVMS need to turn its attention back to you, just for long enough to process and display the next character.

In this manner, OpenVMS can manage thousands of users, multiple disk drives, multiple printers, network communications, and more. But, in order to pull off this grand illusion of everything happening at once, some very complicated software has to keep track of everything in progress.

Before OpenVMS can take its attention away from you and your e-mail message, it must first ensure that it can pick up where it left off. Every minute detail of your activities, your open files, your terminal, and the current state of the processor must be exactly preserved, so that the system can switch its attention back to you when required.

Some mechanism must exist to store all of this information for every task, some unit of scheduling has to be saved and another loaded each time the system switches its attention from one task to the next. This unit is called the process.



Getting Started with OpenVMS(c) A Guide for New Users
Getting Started with OpenVMS: A Guide for New Users (HP Technologies)
ISBN: 1555582796
EAN: 2147483647
Year: 2005
Pages: 215

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