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interestingly enough he selected a marvelous example of its usefulness. He suggested that a machine that could recognize a face would be of immense value; not just recognize a picture, but recognize a face as seen from all angles and from varying distances. The human mind, he noted, was fully capable of seeing a face in these various guises and still recognizing the person. How useful it would be if we had a machine that could accomplish the same thing. But, it needed to be a machine of a small size, not something the size of a large building, if its full utility was to be realized. It was a presentation that lit the way for computer systems to become truly ubiquitous extensions of the human species.

Personal Electronic Devices

Personal electronic devices are computers that have all the characteristics described above. They have a central processing unit, various input/output devices, and different forms of memories. Your cellular phone has a small keyboard, a microphone, speakers, most likely a screen and a camera. All this is just like your laptop computer, only smaller. The keyboard connects to your fingers, the microphone to your mouth, the speakers to your ears, and the screen and camera are for your eyes. A big difference today between your phone and your laptop, however, is that inside your cellular phone there is another computer, one that contains your private information, that which makes the cellular phone truly yours. That computer is typically located in a small encasing under the battery, from which it can be removed. That computer inside the computer is called the Subscriber Identity Module or SIM. If you take your SIM out, your phone doesn’t work anymore because it can no longer be recognized by the cellular phone network as yours. If you take your SIM out and transfer it to another cellular phone, then that new phone becomes yours and can be used to make calls just like the older one could. This is because the SIM contains all the information pertaining to you, which the phone consults before connecting you to someone else.

At the time of this writing, there are more than two billion cellular phones containing a Subscriber Identity Module operating in the world. What differentiates the Subscriber Identity Module from other computers is defined by its role. Since the information it contains must be, and remain, confidential, this computer must be protected from outside intrusion of all sorts. Therefore, it is a very special kind of computer, with many specific features, all oriented toward protecting your information inside. Physically, it presents itself in the form a plastic substrate into which a small chip is embedded.

The most salient feature of the Subscriber Identity Module is the monolithic computer architecture it presents. By monolithic, we mean that the complete computer is constructed as a single integrated circuit chip. This includes the central processing unit, memories and means of communicating with the world outside this computer. By virtue of a single integrated circuit chip structure, the interconnections between the components of the computer are embedded within the chip. Consequently, it is very difficult for these interconnections to be accessed by external monitoring equipment, which is a desired property for a computer whose main function is to protect its content against unauthorized access of all kinds. As a result, the chip provides a computing platform in which information can be stored, while the only means of accessing that information is through the input/output facilities provided by the chip itself. If the computer within the chip can adequately safeguard this input/output channel, then we can achieve a secure information storage platform as well as a secure processing platform.

Also embedded within the integrated circuitry of the Subscriber Identity Module are several types of memory. One type, termed read-only memory, is actually populated with binary information at

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