The Operating System

The job of the OS is to manage the computer. Everything you do on the computer is run through the OS. However, you almost never actually work directly WITH the OS. As any good manager should, the OS needs to know what resources it has available. The OS then accepts requests to use those resources and determines the best way to apply the resources to fill the requests.

Imagine you run a home improvement company. You have a carpenter, plumber, electrician, painter, and a heating and air conditioning specialist, on staff. You take calls from building contractors and other customers requesting some combination of these services for projects they are working on. You then schedule your workers (resources) to make the best use of them across each of the requests.

In the above example, you fill the roll of the OS. A person who hired a contractor to built them a house, who in turn uses some of your staff, never actually works with you. That contractor fills a roll that would be served by Application software. When you surf the web, you use an application (Internet Explorer, Firefox, etc..). You make requests of that application software, and the application software makes requests of the OS to accomplish what you want. Why not have the Internet Browser Application go direct to the Internet rather than through the OS? You might be running multiple programs that all use the Internet at the same time. Maybe you are running I-Tunes, or some other software that gets information over the Internet. Each of these programs would be fighting to get control over the Internet. With as OS, each program can make requests for Internet service, and the OS can provide access to all programs.

Booting The Operating System

Once the system BIOS has finished POST, and located the Operating System, it reads the 'bootstrap' for the OS into memory and turns control over to it. The bootstrap is part of the OS that is in charge of setting up the computer the way the OS needs it. Starting an OS has a few stages that need to be completed before it is ready to do any work for the user. Some of these stages will be similar to what the BIOS has already done. The reason the OS does these things again is that each BIOS may do things a little differently. The only way an OS can be sure that everything it needs is done, is to do it itself.

There are many different OS options for a computer. I am going to be general in my description here. Windows, for the most part, follows this process, as do most other modern OS's.

The bootstrap takes control from the BIOS, and locates the core of the OS, generally referred to as the kernel.

The kernal is the actual manager, but the kernel does not know what resources are available, or how to use them. Once the kernel is loaded, a list of devices, or resources is loaded. The OS needs instructions on how to use each of the devices available to it. Those instructions are contained in small programs called 'Drivers'. These are loaded next. Once the kernel and the drivers are loaded, the OS may do a scan to see if there are any new devices that it did not know about. If it finds any new devices, it will attempt to locate drivers. The OS might ask you where it should look. Once the device is properly loaded, it will be added to the list of devices so the OS knows about it next time it starts.

Finally, once all devices are ready, the OS looks for any software that is set up to start when the OS is started. This might be defensive (anti-virus) software, or it might just be a program that customizes the system for you. Once all of this is done, the Computer is finally ready to work for you.