Internal Memory RAM and ROM
Because the central processing unit only works on small pieces of information at a time, the computer needs a way to store information while it is not being processed. Two kinds of silicon chips serve as the computer's internal memory. Random Access Memory or RAM chips store information temporarily, and the computer can write, read, and erase information on them. RAM chips are erased every time the computer is turned off, so Read Only Memory or ROM chips act as the computer's permanent memory: they store the instructions the CPU needs when you first turn it on.
External Memory Disks and Disk Drives
External memory devices like floppy disks, hard disks, and CD-ROMs store programs and data when the computer is turned off. When a disk is inserted into a disk drive, the computer can read information from it or write new information onto it. Only certain types of CD-ROM can have information written onto them.
Output: The Printer
Printers receive electrical codes from the computer and then print the corresponding letter or number on paper. The result is called hard copy. Plotters and some printers can even reproduce computer graphics on paper.
Types of Printer
Ink Jet Printers
Ink jet printers work as their name suggests, by squirting jets of ink onto the paper through tiny nozzles. Inkjets provide good quality output (on average 600 dots per inch) at reasonable speed and at relatively low cost.
Laser Printers
A laser printer is essentially a photocopier attached to the computer as an output device. Laser printers are capable of high quality output (around 720 dots per inch), at high speed. They can be bought from as little as around £100.
Output Monitors (Visual Display Units)
Monitors (V.D.U.s) are the most frequently used output device. The display is transmitted from the computer to the surface of a cathode ray tube, which is rather like a television screen. Slim, flat screen monitors are also available. The display consists of very small phosphorescent dots combined in different patterns to form words, numbers, and pictures. Monitors vary in their resolution (clarity), more dots on the screen means a higher resolution but obviously costs more. The range of resolutions are CGA, EGA and the highest resolution VGA.
Software Computer Programs
A program is simply a set of instructions, which the programmer wishes the computer to obey. The computer blindly obeys any instruction which it is given it possess no abilities to determine whether an instruction (so long as it is framed correctly) is the correct instruction for any given situation. Programs tell the computer how to turn the input you provide into the output you want. A program is therefore a detailed set of instructions for performing a particular task. Because you cannot actually touch programs they have become known as "software"; this contrasts with the hardware of a computer system monitors, chips, printers etc. which you can touch!
Software takes many forms the computer system itself is coordinated by a special resident program called an operating system (you can read more about this program below), programs to carry out specific tasks for the user are termed "application programs". Stock control, word processing, payroll and accounting functions are typical areas, which are serviced by application programs.
Input/output Media Mass Storage
Floppy Disks
A floppy disk is 3 ½ inches square. Inside its protective case or sleeve, each floppy disc is a circle of flexible plastic whose surface is covered with magnetic oxide. When you use a disc for the first time, you must use a special program called an operating system to format the disk's surface by creating circular tracks divided into pieshaped sectors. A hole punched near the hub of the disk lets the computer know where the first sector is. A track on each disk stores a directory that tells the computer where to find what information. This means that it can go directly to the correct part of the disk instead of wading through all of its contents to find the information it wants. A typical floppy disk can store around 200 pages of typewritten text, depending on how tightly the information is packed. If pictures are stored on disk, they take up a lot of room.
Care of Floppy Disks
Information is stored on floppy disk by means of magnetism. The information stored on a disk can be lost if the magnetic surface of the disk is damaged. When handling a disk you must be careful not to bend it or touch the exposed portions. Moisture, extreme temperatures, and exposure to magnetic fields or equipment can also ruin disks. You should keep backup copies of all important disks to insure that your data will not be lost if a disk is damaged. You can guard against the possibility of accidentally writing over and erasing important data and programs by moving the small writeprotect notch on the side of the disk. When this notch is closed, a sensor inside the drive prevents the head from writing onto the disk.
Hard Disks
All modern PCs have hard disks, which can store vast amounts of information – this varies depending upon their size. Although they operate on the same basic principles as floppies, hard disks spin many times more quickly allowing the read/write head to pass only fractions of an inch over the disk's surface. Hard disks are hermetically sealed in special chambers to prevent dust particles from ruining the disk's surface.
Disk Drives: Read/Write Heads
To use a disk, you place it into a disk drive so that the circular hole in the centre of the disk fits into a cone in the drive. This cone spins the disk allowing all of its sectors to come under a window cut into the sleeve. As the disk spins around, a read / write head moves back and forth over the exposed portion of the disk. The head works like the head on a tape recorder. It reads information on the disk by translating its magnetic fields into electrical signals that are sent to the computer; information is written onto the disk when the head converts the electrical signals from the computer into magnetic fields on the disk's surface.
Operating Systems
A computer is really a system of interconnected parts: a keyboard or input device; internal memory (RAM and ROM); a processor; disks and disk drives; output devices like a monitor and a printer; and, of course, software. A special kind of program called an operating system controls the flow of information between these different components. One of its most important jobs is to enable your hardware to understand the instructions of your software. When you buy a computer, you must be sure that its design or architecture is compatible with the operating system for which the software you want is written. The operating system manages the files on your disks and offers utilities such as formatting blank disks, listing the directory of files on a disk, as well as copying, renaming, and deleting files. Sometimes operating systems are permanently installed, but if the operating system is pre-packaged on a disk, its contents must either be loaded separately into the computer or written onto every disk used, during the formatting process. Windows 95 and Windows 98 are the most common operating systems used today.
Modems and Networks
With the help of a modem, your computer can link up and communicate with other computers all over the world. The modem translates the computer's language of ons and offs into tones of different frequencies, which are then transmitted across telephone lines. At the receiving end the process is reversed: a modem converts tone variations back into the computer's digital code. Computer networks are systems of computers linked together in this way: they are used to send and receive electronic mail, and to allow small personal computers access to the vast data libraries of larger computers, via the Internet.
COMPUTER APPLICATIONS
Application Packages
These are programs that actually perform the tasks that the user wishes to be done. The tasks are usually so complicated that several programs are required and these are collectively called a 'suite' of programs. Typical examples of applications are Stock Control, Databases, Order Processing, Word Processing, Airline Booking Systems, Spreadsheets, Viewdata Systems and Point of Sale Systems. Each package will now be discussed in turn, to give a general explanation of what they are, when they are used and for what purpose.
Stock Control
A stock control system keeps track of all the stock currently held by the company. The computer is informed through the till when each item is sold. It is then an easy matter to deduct the number sold from the current stock level to give a new stock level. If this level is below a reorder level which again is held by the computer, then the computer can automatically print a reorder letter to the supplier.
The number of each item to be supplied is stored and when the supplier delivers, the number of items delivered is added to the amount in stock. Therefore the company, in theory, should never, through its own efforts, run out of any item of stock and should at all times know how much stock it has, in the warehouse, in storage at a retail outlet and on the shelves. Losses through theft are easily calculated through a manual shelf count, and corrective action to reduce losses can be implemented quickly.
Database
A database is a store of information made up of records. A record contains information relating to one person or one company and is made up of individual pieces of information called fields. For example, you may create your records having the following fields: company name, address, telephone number, credit limit, amount owed, payment due etc.
When all the information has been entered the database may then be used by searching for particular information, called crossreferencing (e.g. to find out at the end of the month who the company owes money to and how much, in order to settle their accounts). New information can be fed in and the database is changed (updated) to include the new data, so that it is always up to date. Large databases are best used with static information (remains the same), with new records being added to the end. An example of this type is a Library with records on all the books in stock.
The computer operated by the DVLA in Swansea (Driving Vehicle Licensing Authority) might be an example of a large database. Information relating to every motor vehicle in the U.K. is stored on computer and can be sorted or searched to find out, for example, who owns a particular car.
Word Processing
In word processing a computer behaves like a much enhanced form of typewriter. Facilities are provided for entering, manipulating, storing and retrieving blocks of text. This means that standard letters and lists of names and addresses can be generated separately and then letters to everyone on the list can be printed without retyping the document. The processor retrieves the letter and the first name and address, prints the letter, retrieves the next name and address, changes the information in the letter, prints the next letter and so on.
RealTime Systems
An example of a realtime system is Airline Booking. This is a system which updates the data immediately it changes. All the terminals in all the booking offices are connected via a telephone line to a large computer in the head office of the company. This is so that information can be accessed immediately and be changed on confirmation of a booking immediately. For example, if you wanted to book a flight, you would feed details into the terminal about the flight, and back would come a message saying perhaps that only two seats remain on that flight. If you then take these seats by sending that information back, confirmation would take place and then noone anywhere in the world would be able to book a seat on that particular flight (even if they tried to book only ten seconds after you).
Spreadsheet
A spreadsheet is a special application package, which allows the user to set up tables of information and add up rows and columns which make up the table. They are used mainly for displaying cash forecasts of companies, in the form of a table where all the figures are interconnected. Each intersection of a row and a column is called a 'cell'. The cells are filled with figures and then changes can be made and the spreadsheet calculates all the related cells, alters and reprints them. Figures can be changed and the overall effect on the complete table can be seen immediately.
