01/04/2014

Computers Knowledge Files and Systems PDF


Files and Systems notes


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.

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.

File Organisation

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

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

Direct Access Organisation: 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.

A number of desirable features of a comprehensive operating system are:
  • Job control language
  • Failure and recovery
  • File security
  • Logging
  • Scheduling
  • Monitoring system status
  • Multi-access control

Translation

Compiler: A compiler translates a program written in a high-level language to executable machine instructions. The compiler treats source-program instructions as data. Each instruction is accessed in turn and translated into one or more lines of object code in machine language.

Interpreter: Some high-level programming languages often use an interpreter instead of a compiler to translate instructions into machine code. 

Application Programs: An application program is designed to handle a particular task required by the end-user. It handles all aspects of a routine application, including error situation, the display of menus to aid the user, thus making it possible for a user having very little computer expertise to process the application.

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