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WHAT IS A COMPUTER?
A computer is an electronic device, operating under the control of instructions stored in its own memory unit, that accepts input or data, processes data
arithmetically and logically, produces output from the processing, and stores the results for future use. All computers perform basically the same four operations:
- Input operations, by which data is entered into the computer for processing.
- Processing operations, which manipulate data by arithmetic and logical operations. Arithmetic operations are addition, subtraction, multiplication, and
division. Logical operations are those that compare data to determine if one value is less than, equal to, or greater than another value.
- Output operations, which make the information generated from processing avail-able for use.
- Storage operations, which store data electronically for future reference.
WHAT ARE DATA AND INFORMATION?
The four operations that can be performed using a computer all require data. Data refers to raw facts, including numbers and words, given to a computer
during the input operation. Examples of data include the hours posted to a payroll time card or the words comprising a memo to the sales staff. A computer accepts data, processes data and, as a result of
the processing, produces output in the form of useful information. Information can therefore be defined as data that has been processed into a form that has meaning and is useful.
WHAT ARE THE COMPONENTS OF A COMPUTER?
To understand how computers process data into information, you need to examine the primary components of the computer. The four primary components of a
computer are:
- input devices
- processor unit
- output devices
- auxiliary storage units
Input devices
Input devices enter data into main memory. Many input devices exist. The two most commonly used are the keyboard and the mouse.
Keyboard
The input device  you will most commonly use on computers is the keyboard, on
which you manually key in or type the data. The keyboard on most com-puters is laid out in much the same manner as a typewriter.
Mouse 
An alternative input device you might encounter is a mouse. A mouse is a pointing device that you can use instead of the cursor control keys. You
lay the palm of your hand over the mouse and move it across the surface of a table or desk. The mouse detects the direction of your movement and sends this information to the screen to move
the cursor. You push buttons on top of the mouse to indicate your choices of actions from lists displayed on the screen.
The Processor
The processor unit is composed of the central processing unit (CPU) and the main memory. The central processing unit contains the electronic circuits that
actually cause processing to occur. The CPU interprets instructions to the computer, performs the logical and arithmetic processing operations, and causes the input and output operations to occur.
Main memory consists of electronic components that store numbers, letters of
the alphabet, and characters such as decimal points or dollar signs. Any data to be processed must be stored in main memory. The amount of main memory in computers is typically measured in kilobytes (K or KB), which equal 1,024
memory locations. A memory location, or byte, usually stores one character. Therefore, a computer with 640K can store approximately 640,000 characters. The amount of main memory for computers may range
from 64K to several million characters, or more. One million characters is called a megabyte (MB).
Output Devices
Output devices make the information resulting from processing available for use. The output from computers can be presented in many forms, such as a
printed report or color graphics. When a computer is used for processing tasks, such as word processing, spreadsheets, or database management, the two
output devices most commonly used are the printer and the televisionlike display device called a screen, monitor, or CRT (cathode ray tube).
Printers
Printers used with computers can be either impact printers or nonimpact printers. An impact printer prints by striking an inked ribbon against the paper.
One type of impact printer often used with microcomputers is the dot matrix printer. To print a character, a dot matrix printer generates a dot pattern
representing a particular character. The printer then activates vertical wires in a print head contained on the printer, so that selected wires press against the
ribbon and paper, creating a character. Dot matrix printers vary in the speed with which they can print characters. These speeds range from 50 characters
per second to over 400 characters per second. Generally, the higher the speed, the higher the cost of the printer. Many dot matrix printers also allow you to
choose two or more sizes and densities of character. Typical sizes include condensed print, standard print, and enlarged print. In addition, each of the
three print sizes can be printed with increased density, or darkness. Another useful feature of dot matrix printers is their capability to print graphics. The dots
are printed not to form characters, but rather to form graphic images. This feature can be especially useful when you are working with a spreadsheet
contained on the worksheet program to produce graphs of the numeric values. Non-impact printers, such as ink jet printers and page printers, form characters
means other than striking a ribbon against paper. An ink jet printer forms a character by using a nozzle that sprays drops of ink onto the page. Ink jet
printers produce relatively high-quality images and print between 150 and 270 characters per second.
Computer Screens (Monitors)
The computer you use probably has a screen sometimes called a monitor or CRT (cathode ray tube). The screen displays the data entered on the keyboard
and messages from the computer. Two general types of screens are used on computers. A mono-chrome screen uses a single color (green, amber, white,
or black) to display text against a contrasting background. Some mono-chrome screens are designed to display only characters; others can display both
characters and graphics. Although they cannot display multiple colors, some monochrome screens simulate full-color output by using up to 64 shades of the screen's single color.
The second type of screen is a color display. These devices are generally capable of displaying 256 colors at once from a range of more than 256,000
choices. Computer graphics, charts, graphs, or pictures, can also be displayed on a screen so that the information can be easily and quickly understood.
Graphics are often used to present information to others, for example, to help people make business decisions.
Auxiliary Storage
Main memory is not large enough to store the instructions and data for all your
applications at one time, so data not in use must be stored elsewhere. Auxiliary storage devices are used to store instructions and data when they are not being used in main memory.
Diskettes
One type of auxiliary storage you will use often with your computer is the diskette. A diskette is a circular piece of oxide-coated plastic that stores data
as magnetic spots. Diskettes are available in various sizes. Microcomputers most commonly use diskettes that are 5 ¼ inches or 3 ½ inches in diameter. To
read data stored on a diskette or to store data on a diskette, you insert the diskette in a disk drive You can tell that the computer is reading data on the
diskette or writing data on it because a light on the disk drive will come on while read/write operations are taking place. Do not try to insert or remove a diskette
when the light is on as you could cause permanent dam-age to the data stored on it. The storage capacities of disk drives and the related diskettes can vary
widely The number of characters that can be stored on a diskette by a disk drive depends on three factors: (1) the number of sides of the diskette used; (2)
the recording density of the bits on a track; and (3) the number of tracks on the diskette. The most commonly used diskettes for personal computers are 51A
inch (left) and 3½ inch (right). An advantage of the 3½-inch size is its rigid plastic housing, which helps prevent damage to the diskette.
Early diskettes and disk drives were designed so that data could be recorded on only one side of the diskette. These drives are called single-sided drives.
Double-sided diskettes, the typical type of diskette used now, provide increased storage capacity because data can be recorded on both sides of the
diskette. Disk drives found on many microcomputers are 5 ¼-inch, double-sided disk drives that can store from 360,000 bytes to 1.25 million bytes on the
diskette. Another popular type is the 3 ½-inch diskette, which, although physically smaller, stores from 720,000 to 1.44 million bytes. An added benefit
of the 3 ½-inch diskette is its rigid plastic housing, which protects the magnetic surface of the diskette. The second factor affecting diskette storage capacity is
the recording density provided by the disk drive. (The recording density is stated in technical literature as the bpi the number of bits that can be recorded
on a diskette in a one-inch circumference of the innermost track on the diskette.) For the user, the diskettes and disk drives are identified as being single
density, double density, or high density. You need to be aware of the density of diskettes used by your system because data stored on high-density diskettes,
for example, cannot be processed by a computer that has only double-density disk drives The third factor that influences the number of characters that can be
stored on a diskette is the number of tracks on the diskette. A track is a very narrow recording band forming a full circle around the diskette. The width of this
recording band depends on the number of tracks on the diskette. The tracks are separated from each other by a very narrow blank gap. Each track on a
diskette is divided into sectors. Sectors are the basic units, for diskette storage. When data is read from a diskette, it reads a minimum of one full sector. When
data is stored on a diskette, it writes one full sector at one time. The tracks and sectors on the diskette and the number of characters that can be stored in each
sector are defined by a special formatting program that is used with the computer. Data stored in sectors on a diskette must be retrieved and placed
into main memory to be processed. The time required to access and retrieve data, called the access time, can be important in some applications. The
access time for diskettes varies from about 175 milliseconds (one millisecond equals 1/1000 of a second) to approximately 300 milliseconds. On average,
data stored in a single sector on a diskette can be retrieved in approximately 1/5 to 1/3 of a second.
Diskette care is important to preserve stored data. Properly handled, diskettes can store data indefinitely. However, the surface of the diskette can be
damaged and the data stored can be lost if the diskette is handled improperly. A diskette will give you very good service if you follow a few simple procedures
- Store a 5 ¼-inch diskette in its protective envelope when you are not using it. This procedure is necessary because the 5 ¼ -inch diskette has
an oval opening, the access window, which permits the read/write heads to access the diskette but also allows the diskette to be easily damaged or soiled.
- Keep diskettes in their original box or in a special diskette storage box to protect them from dirt and dust and prevent them from being accidentally
bent. Store the container away from heat and disk to excessive direct sunlight. Magnetic and electrical equipment, including telephones, radios,
and televisions, can erase the data on a diskette so do not place diskettes near such devices. Do not place heavy objects on a diskette,
because the weight can pinch the covering, causing damage when the disk drive attempts to rotate the diskette.
- To affix one of the self-adhesive labels supplied with most diskettes, write or type the information on the label before you place the label on the
diskette. If the label is already on the diskette, do not use an eraser to change the label. If you must write on the label after it is on the diskette,
use only a felt tip pen, not a pen or pencil, and press lightly.
- To use the diskette, grasp the diskette on the side away from the side to be inserted into the disk drive. Slide the diskette carefully into the slot on
the disk drive. If the disk drive has a latch or door, close it. If it is difficult to close the disk drive door, do not force it-the diskette may not be inserted
fully, and forcing the door closed may damage the diskette. Reinsert the diskette if necessary, and try again to close the door.
- The diskette write-protect feature prevents the accidental erasure
 of the data stored on a diskette by preventing the disk drive from writing new data or erasing existing data. On a 5 ¼-inch
diskette, a write-protect notch is located on the side of the diskette. A special write-protect label is placed over this notch whenever you want to protect the data. On the 3 ½-inch
diskette, a small switch can slide to cover and uncover the write protection window. On a 3½-inch diskette, when the window is uncovered the data is protected.
Hard Disk
Another form of auxiliary storage is a hard disk. A hard disk consists of one or
more rigid metal platters coated with a metal oxide material that allows data to be magnetically recorded on the surface of the platters Although hard disks are
available in read-write head cartridge form, most hard disks cannot be removed fom the computer. As with diskettes, the data is recorded on hard disks on a series of tracks.  The tracks are divided into sectors when the disc is
formatted. The hard disk platters spin at a high rate of speed, typically 3,600 revolutions per minute. When reading data from the disk, the read head senses the
magnetic spots that are recorded on the platter and records that information into main memory. When writing, the data is transferred from main memory and is stored as
magnetic spots on the tracks on the recording surface of one or more of the disk platters. Unlike diskette drives, the read/write heads on a hard disk drive do not actually touch the surface of the disk.
The number of platters permanently mounted on the spindle of a hard disk varies from one to four. On most drives, each surface of the platter can be used
to store data. Thus, if a hard disk drive uses one platter, two surfaces are available for data. If the drive uses two platters, four sets of read/write heads
read and record data from the four surfaces. Storage capacities of fixed disks for microcomputers ranged from 100 Megabytes (1 byte = 1 character) to well over twenty Gigabytes (100 Megabytes = 1/10th Gigabyte).
Booting the Computer: When a computer's power is turned on, the operating
system is loaded into main memory by a set of instructions contained internally within the hardware of the computer. This process is called booting the
computer. When the operating system is loaded into main memory, it is stored in a portion of main memory.
Interface with Users: To communicate with the operating system, the user
must enter commands that the operating system can interpret and upon which it can act. The commands can vary from copying a file from one diskette to
another, to loading and executing application software.
Coordinating System Devices: Computer hardware is constructed with
electrical connections from one device to another The operating system translates a program's requirements to access a specific hardware device,
such as a printer. The operating system can also sense whether the devices are ready for use, or if there is some problem in using a device, such as a
printer not being turned on and, therefore, not ready to receive output.
Interrupts
An interrupt is a signal sent from a hardware device (such as a disk controller board, a network interface board, the keyboard, and so on) to the CPU telling
the CPU that the device requires immediate attention. The device that wants the CPU's attention sends the interrupt signal over a special data line, called
the interrupt request line (IRQ), that connects the device to the CPU.
When the CPU receives the interrupt signal, it stops what it is doing and immediately retrieves from memory special program instructions (called the
interrupt service routine) and performs the functions dictated by the routine. After the CPU completes the interrupt service routine, it returns to what it was doing before it received the interrupt.
The CPU knows which device sent the interrupt signal, because each expansion board and device has its own dedicated IRQ line. The interrupt line
is set on the board at the time it is installed in the computer. Two devices cannot have the same interrupt. If they do, the result is that at least one, if not both, devices will malfunction..
Memory Addresses
The second method of expansion board communication is the memory address. The memory address refers to a range of computer memory to which the
device has exclusive rights.
The reason for the exclusive use of this memory is similar to the exclusive use
of an IRQ line discussed previously. If two boards were trying to write to and read from the same memory location, information and instructions would become confused quickly.
Input/0utput (IO) Addresses
The third method of board communication is the I/0 address. The 1/0 address is of equal or greater importance than IRQ lines and memory addresses when
configuring boards. The I/0 address corresponds to a port (or door) to the CPU. The I/0 addresses are not to be confused with the COM and LPT ports used for
printers or other external communication. The I/0 address port is for internal communication between the CPU and the hardware device. Take care to
ensure that each device is assigned its own port I/0 address.
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