What Is an Operating System

In simple terms, an operating system is a manager. It manages all the available resources 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 determine 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 forty 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 caused the program to stop sooner than expected, the programmer had to start over. Because there were many other people waiting for their turn to try their programs, it may have been several days before the first programmer got a chance to run his deck of cards through the machine again. 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 another 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 1960's, 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 "multiprogramming." 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 percent of the time.

By the end of the 1960's, something wonderful happened: UNIX was born. It began as a one-man project designed 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 or Linux. Others, 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.)

Linux is an operating system like many others, such as 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 using, let alone the basics of how the 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. In many companies, you are given a number to call when problems arise, you report what happened, and it is dealt with.

If the computer is not back up within a few minutes, you get upset and call back, demanding to know when "that darned 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, I need it back up now."

The problem is that many people hear the explanation, but don't understand it. It is common for people to be unwilling to acknowledge that 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 what the technician is doing and that he/she is 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, I am going to discuss what goes into an operating system, what it does, how it does it, and how you, the user, are affected by all this.

Because of advances in 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 multitasking. 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 needs to keep track of 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, etc. This is the concept of multi-users, as multiple users have access to the same resources.

In subsequent sections, I will be referring to UNIX as an abstract entity. The concepts we will be discussing are the same for Linux and any other dialect. When necessary, I will specifically reference where Linux differs.