Jim Mohr's SCO Companion


Copyright 1996-1998 by James Mohr. All rights reserved. Used by permission of the author.

Be sure to visit Jim's great Linux Tutorial web site at https://www.linux-tutorial.info/

Introduction to Operating Systems

What is an operating system?

Previous: Introduction to Operating Systems

In simple terms, the operating system is a manager. It manages all the resources available on a computer. These resources can be the hard disk, a printer, or the monitor screen. Even memory is a resource that needs to be managed. Within an operating system are the management functions that determines who gets to read data from the hard disk, what file is going to be printed next, what characters appear on the screen and how much memory a certain program gets.

Once upon a time, there was no such thing as an operating system. The computers of 40 years ago ran one program at a time. The computer programmer would load the program he (they were almost universally male at that time) had written and run it. If there was a mistake that causes the program to stop sooner than expected, the programmer had to start over. Since there were many other people waiting for their turn to try their program, it may have been several days before the first programmer got a chance to run his deck of cards through the machine again (what they had to do to run a program). Even if the program did run correctly, the programmer probably never got to work on the machine directly. The program (punched cards) was fed into the computer by an operator who then passed the printed output back to the programmer several hours later.

As technology advanced, many such programs, or jobs, were all loaded onto a single tape. This tape was then loaded and manipulated by an other program, which was the ancestor of today's operating systems. This program would monitor the behavior of the running program and if it misbehaved (crashed) the monitor could then immediately load and run another. Such programs were called (logically) monitors.

In the sixties, technology and operating system theory advanced to the point that many different programs could be held in memory at once. This was the concept of 'multi-programming.' If one program needed to wait for some external event such as the tape to rewind to the right spot, another program could have access to the CPU. This improved performance dramatically and allowed the CPU to be busy almost 100% of the time.

By the end of the sixties, something wonderful happened: UNIX was born. It actually began as a one man project by Ken Thompson of Bell Labs and has grown to become the most widely used operating system. In the time since UNIX was first developed, it has gone through many different generations and even mutations. Some differ substantially from the original version, like BSD (Berkeley Software Distribution) UNIX and others, such as SCO UNIX, still contain major portions that are based on the original source code. (A friend of mine described UNIX as the only operating system where you can throw the manual onto the keyboard and get a real command.)

SCO UNIX is an operating system like many others, such a DOS, VMS, OS/360 or CP/M. It performs many of the same tasks in very similar manners. It is the manager and administrator of all the system resources and facilities. Without it, nothing works. Despite this, most users can go on indefinitely without knowing even which operating system they are on, let alone the basics of how an operating system works.

For example, if you own a car, you don't really need to know the details of the internal combustion engine to understand that this is what makes the car move forward. You don't need to know the principles of hydraulics to understand what isn't happening when pressing the brake pedal has no effect.

An operating system is like that. You can work productively for years without even knowing what operating system you're running on, let alone how it works. Sometimes things go wrong. If you work in a company like many, you are given a number to call when problems arise, you tell them what happened and they deal with it.

If the computer is not back up within a few minutes, you get upset and call back, demanding to know when "that damned thing will be up and running again." When the technician (or whoever has to deal with the problem) tries to explain what is happening and what is being done to correct the problem, the response is usually along the lines of, "Well, ya, I need it back up now."

The problem is that many people hear the explanation, but didn't understand it. It is not unexpected for people to not want to acknowledge they didn't understand the answer. Instead, they try to deflect the other person's attention away from that fact. Had they understood the explanation, they would be in a better position to understand that what the technician is doing and that they are actually working on the problem.

By having a working knowledge of the principles of an operating system you are in a better position to understand not only the problems that can arise, but also what steps are necessary to find a solution. There is also the attitude that you have a better relationship with things you understand. Like in a car, if you see steam pouring out from under the hood, you know that you need to add water. This also applies to the operating system.

In this section, that's what we're going to talk about. What goes into an operating system and what does it do? How does it do it? How are you, the user, affected by all this?

Because of advances is both hardware design and performance, computers are able to process increasingly larger amounts of information. The speed at which computer transactions occur is often talked about in terms of billionths of a second. Because of this speed, today's computers can give the appearance of doing many things simultaneously by actually switching back and forth between each task extremely fast. This is the concept of multi-tasking. That is, the computer is working on multiple tasks "at the same time.”

Another function of the operating system is to keep track of what each program is doing. That is, the operating system whose program, or task, is currently writing its file to the printer or which program needs to read a certain spot on the hard disk. This is the concept of multi-user, as multiple users have access to the same resources.

Next: Processes

Next Chapter: Basics of SCO UNIX


Copyright 1996-1998 by James Mohr. All rights reserved. Used by permission of the author.

Be sure to visit Jim's great Linux Tutorial web site at https://www.linux-tutorial.info/