WHAT IS AN OPERATING SYSTEM?

An operating system – commonly referred to as an OS – is a set of computer programs that manage the hardware and software resources of a computer.  The OS processes electronic devices with a rational response to commands that are approved by the system.

At the foundation of all system software, an operating system performs basic tasks like controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating the network, and managing files.  The OS can also provide a graphical user interface for higher functions.  Essentially, the OS forms a platform for other system software as well as application software.

The operating system is the most important program that runs on a computer.  Without an operating system, your computer would not work.  It would not be able to process requests for print, simple calculations, or any other function.  It is really the brain that runs the equipment.

For larger system, the OS has great responsibilities than with a PC.  In larger systems, the operating system is kind of like a traffic cop.  It makes sure that different users and programs running at the same time on different systems don’t interfere with each other.  It also acts as a security guard making sure that unauthorized users are not able to access the system.

There are four classifications of a computer operating system.  They are:

• Multi-User:  Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users
  
  
• Multi-Processing:  Supports running a program on more than one CPU
  
  
• Multi-Tasking: Allows more than one program to run concurrently
  
  
• Multi-Threading: Allows different parts of a single program to run concurrently
  
  
• Real Time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.

Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system.

Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux.

In any device that has an operating system, there's usually a way to make changes to how the device works. This is far from a happy accident; one of the reasons operating systems are made out of portable code rather than permanent physical circuits is so that they can be changed or modified without having to scrap the whole device.

For a desktop computer user, this means you can add a new security update, system patch, new application or often even a new operating system entirely rather than junk your computer and start again with a new one when you need to make a change.

As long as you understand how an operating system works and know how to get at it, you can in many cases change some of the ways it behaves. And, it's as true of your cell phone as it is of your computer.

So, essentially, when you turn on your computer, the first program is a set of instructions kept in the computer’s read only memory.  These instructions examine the system hardware to make sure everything is functioning properly.  This power-on self test check the CPU, the memory, and the basic input/output systems (BIOS) for errors and stores the result in a special memory location.

Once the test has successfully completed, the software loaded in ROM (sometimes called the BIOS or firmware) will begin to activate the computer's disk drives. In most modern computers, when the computer activates the hard disk drive, it finds the first piece of the operating system: the bootstrap loader.

The bootstrap loader is a small program that has a single function: It loads the operating system into memory and allows it to begin operation. In the most basic form, the bootstrap loader sets up the small driver programs that interface with and control the various hardware subsystems of the computer.

It sets up the divisions of memory that hold the operating system, user information and applications. It establishes the data structures that will hold the myriad signals, flags and semaphores that are used to communicate within and between the subsystems and applications of the computer. Then it turns control of the computer over to the operating system.