amplifiers, summing networks, differentiating, and integrating circuits. The weak link in this type of electrical computation was the vacuum tube. To correct this, transistors which consume less power and last longer than vacuum tubes were used in the amplifiers.">
Next came electronic computers. The early electronic computers' mathematical processes were solved by using electrical voltages only, applied to elements such as amplifiers, summing networks, differentiating, and integrating circuits. The weak link in this type of electrical computation was the vacuum tube. To correct this, transistors which consume less power and last longer than vacuum tubes were used in the amplifiers. Through technological research and development, we have progressed from tubes, to transistors, to miniaturized circuits, to integrated circuitry. These advances have made it possible to reduce the size and weight of our computers. Figure 1-4 is an example of one of our modern electronic digital computers.
Figure 1-4. - Electronic digital computer.
A special-purpose computer, as the name implies, is designed to perform a specific operation and usually satisfies the needs of a particular type of problem. Such a computer system would be useful in weather predictions, satellite tracking, or oil exploration. While a special-purpose computer may have many of the same features found in a general-purpose computer, its applicability to a particular problem is a function of its design rather than to a stored program. The instructions that control it are built directly into the computer, which makes for a more efficient and effective operation. A drawback of this specialization, however, is the computer's lack of versatility. It cannot be used to perform other operations.
General-purpose computers are designed to perform a wide variety of functions and operations. You will probably use this type of computer. A general-purpose computer is able to perform a wide variety of operations because it can store and execute different programs in its internal storage. Unfortunately, having this ability is often achieved at the expense of speed and efficiency. In most situations, however, you will find that having this flexibility makes this compromise a most acceptable one.
All analog computers are special-purpose computers. They are designed to measure continuous electrical or physical conditions, such as current, voltage, flow, temperature, length, or pressure. They then convert these measurements into related mechanical or electrical quantities. The early analog computers were strictly mechanical or electromechanical devices. They did not operate on digits (in binary notation, either of the characters, 0 and 1). If digits were involved at all, they were obtained indirectly. Your wrist watch (if nondigital); your car's speedometer; and oil pressure, temperature, and fuel gauges are also considered analog computers. The output of an analog computer is often an adjustment to the control of a machine; such as, an adjustment to a valve that controls the flow of steam to a turbine generator or a temperature setting to control the ovens in the ship's galley for baking. Analog computers are also used for controlling processes. To do so, they must convert analog data to digital form, process it, and then convert the digital results back to analog form.
You should know that a digital computer can process data with greater accuracy than an analog computer, but an analog computer can process data faster than a digital computer, in some systems. Some computers combine the functions of both analog and digital computers. They are called hybrid computers.
Digital computers perform arithmetic and logic functions on separate discrete data, like numbers, or combinations of discrete data, such as name, rate, and division. This makes them different from analog computers that operate on continuous data, like measuring temperature changes. We generally use digital computers for business and scientific data processing. The following are examples:
Accounting - Computers are ideal for keeping payroll records, printing paychecks, billing customers, preparing tax returns, and taking care of many of the other accounting tasks in an organization.
Recordkeeping - Computers can record information like inventories and personnel files. They can also keep track of books checked out of a library. Airline ticket counters are much more efficient than they used to be, thanks to centralized reservation computers that can be reached over the telephone lines.
Industrial Uses - Industrial computers save considerable time and reduce waste by efficiently performing hundreds of industrial tasks, ranging from filling sales orders and routing parts to various locations on an assembly line, to designing earthquake-resistant structures, and controlling an entire oil refinery.
Science - The research and development applications are the most numerous. Digital computers are being used to do lengthy and complicated mathematical calculations millions of times faster than human beings. They are also used to collect, store, and evaluate data from experiments, analyze weather patterns, forecast crop statistics, and, believe it or not, design other computers.
Word Processing - Remember, these words were typed into a desktop computer! Word processing is among the most common applications for personal computers. If you have not discovered the advantages of computer writing, it's time to visit a computer dealer for a personalized demonstration.
None of this work could be performed by a computer without first instructing the computer how to do it by means of a list of instructions called a program. The instructions in the program must be written in one of the languages the computer understands. The most popular generic term for computer programs is software (this is covered in chapter 3). Hardware (covered in chapter 2), of course, refers to the computer and related equipment. It is easy to say that both computer hardware and software are interdependent because neither can perform useful work without the other. Digital computers may be either special or general purpose.
ACCURACY OF COMPUTERS
The fundamental difference between analog and digital computers is that digital computers deal with discrete quantities such as beads on an abacus, notches on a toothed wheel, or electrical pulses, while analog computers deal with continuous physical variables such as electrical voltages or mechanical shaft rotations. Computation with analog computers depends on the relation of information to a measurement of some physical quantity. For example, you can determine the number of boards in a picket fence by either a digital or an analog system as follows. In the digital method (fig. 1-5 , view A), you use an adding machine and count the boards one by one. In the analog method (fig. 1-5 , view B), you draw a string (marked off in inches for the width of each board including the gap) over the length of the fence, then measure the length of the string.
The number of boards may then be determined by dividing the length of string by the number of inches per board.
Figure 1-5A. - Digital computation.
Figure 1-5B. - Analog computation.
The accuracy of an analog computer is restricted to the accuracy with which physical quantities can be sensed and displayed. This, in turn, is related to the quality of the components used in constructing the computer; for example, the tolerance of electrical resistors or mechanical shafts and the quality of the output equipment. In an analog computer, for example, if the constant is represented by a voltage, it probably could be read only to the third decimal place.
On the other hand, the accuracy of a digital computer is governed by the number of significant figures carried in the computations. This, in turn, is determined by the computer's design. In a digital computer, the number of decimal places in the constant could be many, depending on the design of the computer processing unit. The digital computer is, therefore, capable of higher precision and accuracy. However, a computer, regardless of its accuracy, would do you no good if the wrong one were chosen for a given task.
Most of the computer systems you will work with will be general-purpose digital computers. The remainder of this module will be about general-purpose digital computers.
Q.1 How are computers classified?