A computer is basically a tool for data processing- converting the data into information that is useful to people. Any computer – regardless of its type – is controlled by programmed instructions, which give the machine a purpose and tell it what to do. Computers come in many varieties, including the personal computer, tiny computers built into appliances and automobiles, and mainframe machines used by many people simultaneously to run a business. Despite their differences in size and use, all these computers are part of a system and operate on the same fundamental principles. In this lesson we will understand the computer as a system. Understanding the computer as a system is vital to the effective use and management of computers. We should be able to visualize any computer this way, from a microcomputer to a large computer system whose components are interconnected by a telecommunications network and spread throughout a building or geographic area.
The Computer System
A system is a group of integrated parts that have the common purpose of achieving some objective(s). So, the following three characteristics are key to a system:
- a system has more than one element
- all the elements of a system are logically related
- all the elements of a system are controlled in such a way that the system goal is achieved
Since a computer is made up of integrated components (input and output devices, storage, CPU) that work together to perform the basic system function of:
- Inputting: The process of entering data and instructions into the computer system.
- Storing: Saving data and instructions so that they are available for initial or for additional processing as and when required.
- Processing: Performing arithmetic and logical operations on data in order to convert them in to useful information.
- Outputting: The process of producing useful information or results-such printed reports, or visual display- for the user.
- Controlling: Managing the manner and sequence in which all of the above functions are performed.
All the components of a computer system (to be discussed shortly) are integrated and interacting. The Input or Output units cannot function until they receive signals from the CPU. Similarly, the Storage unit or the CPU alone is of no use. So the usefulness of each unit depends on other units and can be realized only when all units are put together (integrated) to form a system.
Components Of A Computer System
A complete computer system consists of four parts: hardware, software, people, and data (see Figure 2-1).
The mechanical devices that make up the computer are called hardware. In other words, hardware is any part of the computer we can touch. Hardware consists of interconnected electronic devices that we can use to control the computer’s operation, input and output.
Computer hardware consists of:
- Input Devices: The input devices of a computer system include keyboards, touch screens, trackballs, joysticks, digital cameras, microphones, pens, electronic mice, optical scanners, and so on. They convert data into electronic machine-readable form for direct entry or through telecommunication links into a computer system.
- Output Devices: The output devices of a computer system include video display units, printers, audio response units, and so on. They convert electronic information produced by the computer system into human-intelligible form for presentation to end-users.
- Storage Devices: The storage function of a computer system takes place in the storage circuits of the computer’s primary storage unit, or memory, and in secondary storage devices such as magnetic disk and tape units. These devices store data and program instructions needed for processing. Various secondary storage devices include tape drives, optical drives, removable hard drives, DVDs floppy disks, and different kinds of compact disks such as CD-ROM, CD-R, CDRW.
- Central Processing Unit (CPU): The central processing unit (CPU) is the main processing component of a computer system. (In microcomputers, it is the main microprocessor) The electronic circuits of the arithmetic-logic unit, one of the CPU’s major components, perform the arithmetic and logic functions required in computer processing.
Software is a set of electronic instructions consisting of complex codes (also known as programs) that make the computer perform tasks. In other words, software is the nontangible component of the computer system that tells the computer what to do. They are generally in the form of electric or magnetic impulses.
Computer software consists of
- System Software: These programs exist primarily for the computer’s use and help the computer perform and manage its own tasks. System software has its orientation more towards the operation of the hardware components of the computer system. DOS, UNIX, Windows 2000 etc. are some examples of system software.
- Application Software: These programs exist primarily for the user and enable the computer to perform tasks, such as creating documents or drawing pictures. Thus, application software has its orientation more towards performing user tasks. Payroll System, Airline Reservation System etc. are some examples of application software.
People are the personnel involved in using and maintaining the computer system. People are
- Users: who actually uses the computer system
- Trained Professionals: who design, build, program, repair and maintain the computer system.
Data consists of raw facts, which the computer stores and reads in the form of numbers. The computer manipulates data according to the instructions contained in the software and then forwards it for use by people or another computer. Data can consist of letters, numbers, sounds, or images. No matter what kind of data is entered into a computer, however, the computer converts it to numbers. Consequently, computerized data is digital, meaning that it has been reduced to digits, or numbers. Within the computer, data is organized into files. A computer file is simply a set of data or program instructions that has been given a name. A file that the user can open and use is often called a document. Although many people think of documents simply as text, a computer document can include many kinds of data. For example, a computer document can be a text file (such as a letter), a group of numbers (such as a budget), a video clip (which includes images and sounds), or any combination of these items. Programs are organized into files as well, but because programs are not considered data, they are not document files.
Basic Computer Organisation
The capacity, size, cost and internal architectural design of computers differ from one model to another. However, the basic organisation remains the same for all computer systems. A block diagram is shown in Figure 2-2, which displays the five basic building blocks or functional units, of a digital computer system. These five units correspond to the five basic operations performed by all computer systems. The function of each of these units is described below.
1. INPUT UNIT
Data and instructions must enter the computer system before any computation can be performed on the supplied data. The input unit that links the external environment with the computer system performs this task. Data and instructions enter input units in forms that depend upon the particular device used. For example, data is entered from a keyboard in a manner similar to typing, and this differs from the way in which data is entered through a card reader which is another type of input device. However, regardless of the form in which they receive their inputs, all input devices must provide a computer with data that are transformed into the binary codes that the primary memory of a computer is designed to accept. This transformation is accomplished by units called input interfaces. Input interfaces are designed to match the unique physical or electrical characteristics of input devices to the requirements of the computer system. In short, an input unit performs the following functions:
- It accepts (or reads) the list of instructions and data from the outside world.
- It converts these instructions and data in the computer acceptable form.
- It supplies the converted instructions and data to the complete system for further processing.
2. OUTPUT UNIT
The job of an output unit is just the reverse of that of an input unit. It supplies information and results of computation to the outside world. Thus, it links the computer with the external environment. As computers work with binary code, the results produced are also in the binary form. Hence, before supplying the results to the outside world, it must be converted to human acceptable (readable) form. This task is accomplished by units call output interfaces. Output interfaces are designed to match the unique physical or electrical characteristics of output devices (terminals, printers, etc.) to the requirements of the external environmental. In short, an output unit performs the following functions:
- It accepts the results produced by the computer, which are in coded form and hence cannot be easily understood by us.
- It converts these coded results to human acceptable (readable) form.
- It supplies the converted results to the outside world
3. STORAGE UNIT
The data and instructions that are entered into the computer system through input units have to be stored inside the computer before the actual processing starts. Similarly, the results produced by the computer after processing must also be kept somewhere inside the computer system before being passed on to the output units. Moreover, the intermediate results produced by the computer must also be preserved for ongoing processing.
The storage unit at the primary/main storage of a computer system is designed to cater to all these needs. It provides space for storing data and instructions; space for intermediate results; and also space for the final results.
In short the specific functions of the storage unit are to hold (store):
- All the data to be processed and the instructions required for processing (received from input devices).
- Intermediate results of processing.
- Final results of processing before these results are released to an output device.
Two Kinds of Memory
The main memory, housed inside the computer unit, is built from two different kinds of memory chip: the first kind, called ROM (read only memory), has permanently built into information and instructions the computer needs to know in order to operate properly; the second kind of memory, called RAM (random access memory), holds the program and other information typed in at the keyboard.
The RAM is a ‘read and write’ memory. This means we can store, or ‘write’, information into this memory and later recall it, or ‘read’ it out again. The ROM, on the other hand, can only be read; we cannot write information into it. This ensures that we do not destroy the vital information held in ROM by over-writing it.
An important difference between the two types of memory is that RAM is ‘volatile’, i.e. it loses all the information stored when the power is switched off. ROM, on the other hand, is ‘non-volatile’; its information is not lost when the power is switched off. The secondary storage medium stores data, instructions and output for archival purpose so that whenever any data or instructions is required in the future it can be retrieved for reference or for further processing.
4. CENTRAL PROCESSING UNIT
The Arithmetic Logic Unit and the Control Unit of a computer system are jointly known as the Central Processing Unit (CPU). The CPU is the brain of any computer system. In a human body, the brain takes all major decisions and the other parts of the body function as directed by the brain. Similarly in a computer system, all major calculations and comparisons are made inside the CPU and the CPU is also responsible for activating and controlling the operations of other units of a computer system.
5. ARITHMETIC LOGIC UNIT
The Arithmetic Logic Unit (ALU) of a computer system is the place where the actual execution of the instructions takes place during the processing operation. To be more precise all calculations are performed and all comparisons (decisions) are made in the ALU. The data and instructions stored in the primary storage prior to processing, are transferred as and when needed to the ALU where processing takes place. No processing is done in the primary storage unit. Intermediate results generated in the ALU are temporarily transferred back to the primary storage until needed at a later time. Data may, thus, move from primary storage to ALU and back again to storage many times before the processing is over. After the completion of processing the final results, which are stored in the storage unit, are released to an output device.
The type and number of arithmetic and logic operations that a computer can perform is determined by the engineering design of the ALU. However almost all ALU’s are designed to perform the four basic arithmetic operations (add, subtract, multiply, divide) and logic operations or comparisons such as less than, equal to, or greater than.
6. CONTROL UNIT
How does the input device know that it is time for it to feed data into the storage unit? How does the ALU know what should be done with the data once they are received? And how is it that only the final results are sent to the output device and not the intermediate result? All this is possible because of the Control Unit of the computer system. By selecting, interpreting, and seeing to the execution of the program instructions, the Control Unit is able to maintain order and direct the operation of the entire system. Although, it does not perform any actual processing on the data, the Control Unit acts as a central nervous system for the other components of the computer. It manages and coordinates the entire computer system. It obtains instructions from the program stored in main memory, interprets the instructions, and issues signals that cause other units of the system to execute them.
How Does A Computer Work?
The working of a computer can best be illustrated by an example. Suppose we have a routine clerical task to carry out and are given a willing helper who can follow instructions but who is new to the job. How would we proceed? Firstly, we would sit our helper at a desk and provide him with an ‘in’-tray for material coming in (the input) and an ‘out’-tray for material that had been processed (the output). If the work involved calculations, we would also provide a desk calculator to help with the arithmetic, and a pad of paper for working things out. Next, we would put in our helper’s ‘in’-tray all the figures and information, such as prices and orders placed, needed as the ‘raw material’ to work on. This is the data.
Our willing helper is now ready to start work – processing the data – but first needs to be told what to do. He knows nothing about the job and so the instructions we give him will have to be very detailed and precise. Such a set of instructions is called a program. He would have to remember our more general instructions, and so memory would play an important part in the whole process. He would probably note down the program of detailed instructions on his pad so that he can quickly refer to it should he forget what he is expected to do. The note-pad, then, is a type of memory, used not only for calculations but also for storing the program.
We would also give him some reference books, such as a catalogue of prices or a telephone directory, which might be needed from time to time. These provide a second kind of memory: they store information, like the note-pad, but the information is permanent and cannot be changed. The books are ‘read only‘ memories (they can only be read), while the note-pad is a ‘read and write‘ memory, since information can be written on the pad and changed or deleted when necessary.
Finally, he might also need as a ‘backing‘ memory – a secondary storage– a filing cabinet for those records, such as customer accounts, to which he may have to refer occasionally. It is now up to our helper to co-ordinate and control the various operations required by the program to complete the task.
A computer works in much the same way as this and, incorporates the same basic features
|The original information||Data|
|is fed into the computer,||Input|
|together with the instructions.||Program|
|These are stored||Memory|
|and the computer carries out any calculations required,||ALU|
|and supervises and checks the whole process||Control|
|The completed answer is read out of the computer||Output|
There is in every computer system a section to carry out each of these operations, and the operations are combined to solve problems in the same way as they were combined to complete the clerical task.