File Organization

For information to be useful it should not only be recorded, but it should also be easy to access and retrieve the information. File organization may be:
1.  Serial
2. Direct access
3. Indexed sequential access

Serial Organization: In serial file organization, records are held and accessed in a predetermined sequence of keys. Records can be organized in numerical, alphabetical or chronological order.

Direct Access Organization: Direct access files are stored on magnetic disks or other devices where each record is assigned a physical address.

Indexed Sequential Access: The computer records of an indexed sequential file are stored in the main storage portion of the file, which is divided into sections called segments. Usually, all segments are the same physical size e.g. one cylinder.



Files and Systems



Computer Files: It is convenient to store similar information together and this is the idea behind both manual and computer files.

Data Field: The smallest unit of data is the data field. The data field consists of a group of related characters treated as a single entity.

Record: A collection of related data items treated as a single unit is called a record.

File: Records are grouped to form files. A file is a number of related records that are treated as unit representing a particular transaction.

Master File: Master files are perpetual files, i.e. apart from the time of their creation they are never empty. Further, they maintain information that remains constant over a relatively long period of time. When the information changes the master file may be updated. The normal methods of updating are by adding, deleting or editing records in a file.

Transaction File: Transaction files are files in which data prior to the stage of processing is recorded.

The data in transaction records may be collected automatically or may be initially recorded on source documents and later converted to machine-readable format.


Main Input Devices of Computers


The following are the main input devices:
a) Keyboards:
The key board resembles a typewriter. But there are additional keys that handle control functions. The computer keyboard has three categories of keys.
=> Alphanumeric keys 
=> Special key
=> Function keys    
The Alphanumeric Keys comprises of alphabets (A-Z or a-z), numbers (0-9) and other characters, like space, ./ × ' ; : — ~ ! @ # $ % ^ & * ( ) - + = | \ { } [ ].

The Special Keys perform specific tasks. Some of the special keys are, Enter or the Return key, Backspace key, Del key, the Ins key, Shift keys, e Caps lock key, Num Lock key, Ctrl key and the Alt key.

The Function Keys are used to perform a set of operations by a single keystroke. The function keys can be used for various functions. They can be used as short hand for a sequence of keystrokes, configured accordingly.

Mouse: One of the most popular types of specialized input devices for terminals or micro computers is a mouse. Which is used as a pointing device, the mouse is a small box, about the size of a tape cassette with a round ball on the bottom and one or more buttons on the top. The mouse is attached to a terminal or micro computer by a cable. A mouse enables the user to manipulate a pointer or an arrow on a terminal or micro computer screen.
Joystick: Joysticks are designed in the shape of handles that swivel in 360 degree arcs, enabling their users to control screen figures.
Its utilization for commercial data processing applications is limited.  

Barcode Reader: Data can be coded in the form of light and dark bars with coded spacing’s and thickness. These are called barcodes which are commonly used to identify items. Each item is labeled with a Universal Product Code (UPC). The code is read by a wand or pen which transmits a laser beam and receives the reflection from the label. These pulses are compared with standard codes stored in the computer. A barcode is commonly seen on the back of any book published in recent times.
 

OMR: The Optical Mark Reader (OMR) is a device which can detect the presence or absence of a mark on a paper. Light is incident onto the paper and the reflected light is detected. The presence of a mark is detected due to intensity of light being reflected from the mark. OMR is used in reading answer sheets, questionnaires.
OCR: Optical Character Reader (OCR) is an improvement over OMR. This can not. Only detect a mark but can also recognize its shape and identify characters directly from source documents. The amount of light reflected differs depending on the shape of the character and the OCR can detect and interpret these minor differences.  
MICR: Magnetic Ink Character Recognition (MICR) systems use special ink which can be magnetized, to print characters that can then be read and decoded by special magnetic devices.
This system is widely used by banks for processing cheques. The cheque number, the bank and branch code and the account number are printed with ink containing magnet sable particles of iron oxide.


Functional Components of Computer





Functional Components:
The hardware of a computer system can be classified into the Central Processing Unit (CPU) and peripherals.
The heart of the computer is called CPU or Processor.

The CPU Performs:
I) Arithmetic Operations
II) Logical Operations
III) Input-Output Operations
IV) Internal data movements (moving data between various parts of storage)
V) Data manipulation

To perform these operations the CPU has various components:
    
a. Arithmetic-Logic Unit (ALU): It performs the actual calculations (addition, subtraction, division, multiplication) and comparison (equal to, greater, lesser, positive, negative, zero etc).
    
b. Control Unit (CU): It coordinates the operation of the hardware, the flow and execution of data and instructions that are fed into the memory or main storage via the CPU.

c. Memory Unit (Main Store): It holds data instructions (that are being interpreted and executed), intermediate results and final results ready for output. The data and instructions are passed from the main store into ALU, or to and from the storage devices under the control of the CU.
          
Within the CPU there are a number of high speeds, special purpose memory units called registers. These registers carry out critical functions in the execution of programmes.

A computer has four basic types of registers:
1) Accumulator register (ACC)
2) Store operand register (SOR)
3) Control register (CR)
4) Instruction address register (IAR).

Buses: The control unit and internal storage are linked together by sets of parallel electrical conducting lines called buses. The buses that carry data are called data buses. The retrieval of data from memory is carried by the memory address bus.

Peripheral Devices:
The peripheral devices can be classified as input devices, output devices and backing storage devices.

Input Devices allow the user to get data into the machine. A large range of input, devices are available, e.g. keyboard, mouse.

Output Devices are peripherals used to output results to the user. They include printers, plotters and VDUs (Visual Display Units).

Backing Storage Devices or the secondary storage devices are mainly hard disk, CDS, disk packs, floppy disks, and tapes used to store data and programmes permanently.

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