Picture a wealthy English household in the early 1900s. Think of a computer—the hardware—as a big house, the family’s residence. The house consists of plumbing and lighting, bricks and mortar, windows and doors—all manner of physical things and processes.
Next, imagine computer software as the people in the house. The household staff, living downstairs, provide a whole range of services at once. The butler stands by the door, the driver washes the car, the housekeeper presses the linen, the cook provides meals and bakes cakes, the gardener rakes the leaves from the lawn. And this activity, which seemingly happens of its own accord, is coordinated by the head of the household staff. Such is the life of the downstairs dwellers, who in a certain sense exist in the background.
Then consider the people upstairs. They are the whole reason for the toil of the people downstairs. The husband desires a driver not simply for peace of mind but because he wishes to travel. The wife employs a cook, so her family can eat well. The children benefit from the work of the gardener, who clears the yard of debris, enabling them to play outdoors safely.
The picture of the family upstairs and their faithful downstairs servants neatly illustrates the great divide in the world of software. The people upstairs are the applications: the word-processing, electronic ledger, database, publishing and numerous other programs that satisfy human needs and wants. The people downstairs collectively perform the functions of an operating system. Theirs is a realm of services, some automatic, some requiring a special request. These services lay the basis for the good stuff of life.
- G. Zachary, in the book Show Stopper! about the making of Windows NT