At the heart of digital photography is the computer, yet when you open a computer you find mostly air and a board with a- few tiny items attached. The most remarkable fact about this assembly of items is that if there is the slightest fault in any one of them, then the computer will probably not work at all. This gives us an insight into the way computers work: a strict hierarchy of control and instructions so interrelated that the failure of one exchange can bring the whole system down.
At the top of the hierarchy is you: by turning on the computer and entering commands via the mouse or keyboard, you tell components in the computer what to" do. Data goes into a controller, which turns it into the appropriate type for specific parts of the computer - the key component being the central processor unit (CPU).
One way to visualize the CPU is to imagine that a city has a traffic system in which every junction is controlled by a set of lights, and that at each junction the lights can order a car to turn left, right, stop, and so on. When you load a program, such as an image-manipulation application, into the CPU, you are programming the way the traffic lights work. Now if you speed up the vision, so that the cars are electrical pulses and the lights change hundreds of millions of times a second, you are close to seeing how a CPU works.
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