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One widely accepted definition of modern computer requires three prop- erties of this calculating device:

1. It must be electronic and not exclusively mechanical. 2. It must be digital and not analog.

3. It must employ the stored program concept.

As it happens, even the Analytical Engine invented by Babbage fails to satisfy every one of these three requirements.

To find the first invention that is believed to satisfy at least one of these three properties, we skip to the 1890s. The United States has a long his- tory of taking census every ten years. In 1880 the census was tabulated, like every decade prior, by hand. This process of counting citizenry and categorizing them by geographic region was becoming difficult because of rapid population growth. In fact the 1880 census was barely completed before 1890 when the next census was to begin.

A man named Herman Hollerith invented a calculating device built specifically for tabulating the US census. Hollerith’s machine completed the 1890 census in less than one year. More important for computing, Hollerith’s machine ran on electricity. The Hollerith tabulating machine can fairly be labeled as the first calculating (i.e., computer-like) hardware that satisfies any of the properties that distinguish a modern computer.

FIGURE 1.11 Charles Babbage and Ada Lovelace were among the first computer

Hollerith later founded a Tabulating Machine Company to build these devices, and his company merged to form IBM Corporation in 1924; IBM remains today as one of the world’s largest manufacturers of computers. Hollerith’s tabulating machine also provides convincing evidence of the future capacity of computers to assist in solving human problems.

Before revealing the candidates for the first modern computer, there are two of the preceding properties of a modern computer that have yet to be mentioned. The first issue is that a modern computer must be digital. Prior to the 1930s, machines that stored data typically did so as repre- sented using mechanical gears or electrical signals. Gears can generally be rotated to an infinite number of different angles. Similarly, electrical sig- nals are infinitely variable in terms of voltage, amperage, capacitance, and inductance. This kind of continuous change is called analog. For example, an analog wristwatch often has a sweep second hand and can position the minute hand at an infinite number of positions around the dial.

A digital system, unlike analog systems, is one in which there are not an infinite number of possibilities and change is not continuous. Instead, digital systems restrict values to be one of a few choices. For example, hours and minutes on a digital watch are displayed as numbers. It is not possible for the minute number to display anything between 9:30 and 9:31. Most of our automobile speedometers are analog with a needle that rotates gradually as the car accelerates. However, a few cars have digital speed- ometers that display the current speed as a single number in either whole miles or meters per hour.

An explanation of the stored program concept requires a brief look at the major units of hardware in a modern computer. Figure 1.12 diagrams a simple desktop-style computer with three components: a keyboard, a dis- play, and a system unit. These three components can be used to illustrate

the four essential parts of a computer. Every modern computer must have at least one of each of the following:

1. Input device 2. Output device 3. Memory 4. Processor

An input device, as its names implies, provides a way to get data into the computer. The input device shown in Figure 1.12 is a keyboard. Other kinds of input devices include computer mice, track pads, and micro- phones. Smartphones and tablet computers use the surface of the LCD as an input device.

Output devices provide ways for the computer to share the results of its computation with the user. On personal computers the most common output device is the computer display. Other output devices include print- ers and speakers. Together, input devices and output devices provide the computer hardware that supports the user interface.

Some devices are connected to computers in order to support both input and output. For example, the data (i.e., the images) for viewing your favorite motion picture may be input to your computer from a DVD player/recorder, but that same DVD device can be used for output when you archive the photo collection on your computer. The term I/O, pro- nounced “eye oh,” is commonly used by computer scientists to refer to the combination of input and output. Disk drives, flash memory cards, CD units, and DVD units are all examples of I/O devices.

Data that is input needs to be stored. Such storage occurs within the system unit in a component known as memory. Unlike human memory, the memory chips inside a computer almost never fail to correctly store and retrieve data. Chapter 2 explores more about how computer memories are measured and how they store data in digital form.

You’ve typed in your user name and password, and the computer has stored these data somewhere in its memory. One more thing needs to hap- pen in order for the computer to be of any value—the computer must pro- cess your input. So a processor is the fourth essential part of a computer. Like the mechanical calculators from the seventeenth century, computer processors can perform numeric calculations; but the range of potential computer calculations clearly goes far beyond these early machines. In

addition, today’s computer processors typically perform trillions of opera- tions per second.

Computer memory and computer processors are less visible than I/O devices. Generally, memory and processors consist of small integrated cir- cuits approximately 1 cm2_{. Both memory and processor are located within}

the system unit box of Figure 1.12. However, the fact that integrated cir- cuits are so small also makes it possible to locate them more compactly. For example, the memory and processor(s) of a laptop are positioned in the same box just underneath the keyboard. Some desktop computers place the processor(s) and memory inside the same case as the display. Tablet computers and smartphones package all four components—input device, output device, memory, and processor(s)—in a single case.

You know that computer data is stored in the computer’s memory and is manipulated by the computer’s processor, but how is the proces- sor instructed regarding the particular calculations to perform? In other words, how does computer software (the program and the instructions) work together with the computer hardware components? The answer is that computer processors respond to particular instructions, known as machine instructions. Different processors respond to different machine instruc- tions, just like different human cultures use different natural languages.

Input devices can supply the instructions to the computer. A user who knows the instructions could input them via the keyboard, or the software could be downloaded (input) from the Internet. The program will not be available to the processor until it has been loaded (moved) into computer memory. This is known as the stored program concept—the third, and last, requirement of a modern computer. The point is that computer memo- ries today are used for two things: (1) they store data, and (2) they store the instructions that process that data. The stored program concept also requires circuitry for the computer to transfer instructions from memory to the processor so that they can be executed. Typically, the processor not only executes the instructions but also controls their retrieval from memory.