Understanding Software Layers of a Computer

By Stephen Bucaro

You're typing in a word processor and you decide it would be nice to have a hard copy. You select File | Print in menu and printer comes to life, feeding out a copy of your document on paper. It seems like a simple process, but in reality your request passed through many layers of software before reaching your printer.

Although you requested print through a menu of your word processing application, that application did not contact printer directly. Instead, aplication made a request to computer's operating system. Remember, today's computers are multi-tasking. That means they can perform more than one task at a time. The operating system mediates priority of multiple tasks requesting to use a hardware device.

The operating system did not contact printer directly. Remember, today's computers can have a multitude of different hardware attached. There are thousands of different kinds of printers, and a computer may have several printers attached. Instead, operating system communicates with a piece of software called a "device driver", specific to printer that you selected.

The device driver did not contact printer directly. Instead, device driver contacted a program called BIOS (Basic Input/Output System). Whereas all programs up to this point had been loaded from computers hard drive, BIOS was loaded from a memory chip. Finally, BIOS communicates directly with electronic circuits of printer.

[Application]--[Operating System]--[Device Driver]-- [BIOS]--[Hardware]

The above explanation describes common software layers of a computer. Each of software layers described above may itself consist of many layers. An application may communicate with operating system through a piece of software called a DLL (Dynamic Link Library).

The operating system especially is constructed of many layers. Sometimes operating system is described to be like an onion. At center of onion is operating systems kernal. Only other layers of operating system are allowed to communicate directly with kernal. The outer layer of operating system consists of programs called services which applications can communicate with directly.

Sadly, there are always people out there ready to spoil party by abusing this ability. These are people with malicious intent and those who see this access to your computer as a commercial opportunity. Controlling who can access your computer has therefore become a major concern and Firewalls provide us with answer.