Emily is planning to take a trip next week and she’s looking for something to read on the airplane. In the past, she would have gone to a bookstore during business hours and browsed through the shelves to select a book from the limited stock at hand. Today, Emily has the option of sitting down at a computer, at any time of the day or night, to select and buy a book or many other types of products.

Although shopping online is now a routine experience for millions, its familiarity and simplicity mask the sophisticated events behind the scenes. Indeed, what Emily does in the next 5 minutes would have been impossible without many discoveries and inventions from computer science.

Emily’s Computer

Emily’s family, like more than half of the households in the United States, owns a personal computer and related software. Her garden-variety home PC, available for roughly $600, is able to run much more sophisticated programs—and run them much faster—than the first computer her employer bought only 20 years ago. Indeed, the very idea of a home computer, outside the professional or aficionado market, is only about 20 years old.

If Emily were to pause and consider the functionality available to her, she might marvel that no other machine is as flexible, general-purpose, or malleable as a computer. The idea that machines can be adapted to completely new situations for which they were not originally designed usually borders on the farcical. Emily’s pencil will not also serve as a stapler if the need arises. Computers, though, are

The cost of computing power has decreased dramatically. A typical 2003 home PC, which had a 2.66 GHz system clock, 256 Mb of RAM, and a 40 Gb hard drive, outperformed the IBM/XT PC, released in 1983, which had a 4.77 MHz system clock, 640 Kb of RAM, and a 10 Mb hard drive, by a factor of roughly 500—at one-tenth the cost. Hill (in Chapter 2) describes the phenomenon of exponential growth in computing power, and he shows how computer science research strives to design computers that sustain this remarkable rate of improvement in the cost/performance ratio of computers.

Emily’s computer and software are remarkable not only for their low cost but also for their high complexity. The hardware comprises billions of transistors, and the software installed on the computer is defined by tens of millions of lines of code; hundreds of millions of lines of additional software is available. The capability of the computer is built up from these tiny elements.

Shaw (in Chapter 4) describes how abstraction hides the complex and oftenmessy details of a piece of hardware or software in favor of a simpler, more focused view of the aspects of immediate

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