Computers, which range in size from monster to micro, are all constructed from the same kind of components. Here, we will illustrate these components by describing micro -computers, realizing that today’s “micro” is capable of much more than the “monster” of just a few dozen years ago. Actually micro -computers are, nowadays, much more interesting than the larger computers; they are more convenient to use, more flexible and more commonly available.
All micro -computers are built from only a few basic components, some of which are shown in Figure. The main component (shown in the center of the figure) is an electronic “black box” unit that contains the Central Processing Unit, some control circuitry, some internal memory, a power supply, and connectors of various sorts to allow other units to plug in.
A computer’s basic components
At the heart of a computer system is the central processing unit, or CPU, which controls the behavior of the system’s other components, and performs the operations on the data. Connected to the CPU are “peripherals”, which perform a wide variety of functions. These pieces of hardware include printers, memory devices and communication units that can link one computer to another over standard telephone lines. In this chapter, we will briefly describe some of the many computers and peripherals, concentrating only on the most common ones.
The inner building blocks of the CPU are registers consisting of 8, 16 or 32 bits (binary digits), the larger ones usually corresponding to faster machines. When the computer is being used, the registers are mainly used to store the data being manipulated. The programs being executed are stored in the internal memory, as well as the data they manipulate. The amount of data and programs that can be stored in memory is measured in units of Kbytes, where a Kbyte, or kilobyte or simply K, is about one thousand bytes (actually 210 or 1024), and where a byte is 8 bits and is the equivalent of a single character, for example a letter, on a written page.
Very roughly, one Kbyte is sufficient to store a page of text. A Megabyte, or Meg, is one million bytes (actually 1,049,376 or 220). The internal memory ranges from 640K, to 16 Megs and larger.
Other Components and Packaging
Input/Output or “I/O” is usually done through a keyboard and a TV -like display or monitor. One commonly used display screen shows 24 lines (rows) each with 80 characters. Each character consists of a matrix of dots or pixels, usually 8 pixels high by 8 wide, making the screen 640 pixels wide by 200 high. Larger configurations, found in higher quality screens, are 1,000 pixels square. Programs and data may be stored externally in auxiliary memory, which often takes the form of one or several disk drives. In the disk drives, the storage is on thin portable “floppy” magnetic disks, 51 2 inches or 312 inches in diameter. The disks are able to store from 720K to beyond a Meg.
The packaging of these basic components to form a complete computer system takes many forms. The organization ranges from all components being integrated into one box, to the other extreme where all are separate components that have to be plugged together. The peripheral devices are connected to the main processor and perform many different functions, usually involving inputs and outputs. They often take the form of “cards”, plastic boards of electronic components. These cards plug into slots on the main processor unit. The family of input peripheral devices include in particular a rectangular keypad for entering digits, a regular typewriter -like keyboard, function keys for specifying special operations and a “wand” for reading bar -codes.
For output, the range of peripheral devices includes mainly screens to display the output, and printers to provide “hard copy” on paper. A marker (cursor) on the display screen, which shows the current working point, can be moved by a mouse, pen, trackball or joystick. A mouse can be used to point, click, or drag objects on the screen. Printers produce text and graphics of varying quality, ranging from dot -matrix impact -type (where the image is produced by a typehead striking the paper as in a typewriter), to ink -jets, to higher quality laser printers and even typesetting machines. Plotters, which manipulate pens of varying colors are often also used to output high quality drawings.
Where there is a requirement for a large amount of auxiliary memory, this is available on “hard disk” drives, which may provide from 40 Megs to hundreds of Megs, and even Gigabytes (one billion bytes). Communication between computers over telephone lines is possible with a device known as a modem (modulator -demodulator). Modems communicate usually at speeds ranging normally from 2400 bauds (bits per second) to 9600 bauds, and beyond. Computers can also be interconnected in networks, be they local area networks (LANs) or wide area networks (WANs), through simple twisted pair wires or coaxial cables.
Many new types of peripheral have recently become available. These include for instance speech recognition input devices, speech output devices, visual input scanners.
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