A database is a collection of related information stored so that it is available to many users for different purpose. The content of a database is obtained by combining data from the entire different source in an organization. So that data are available to all users and redundant data can be eliminated or at least minimized. A Computer database gives us some electronic filing can be eliminated large number of ways of cross referring and this allows the user many different ways in which to reorganize and retrieve data.

The Three Level Architecture of Database management system

• External level or subschema
• Conceptual level or conceptual schema
• Internal level or physical schema
• Mapping between different levels.

Database management system facilities

Data definition language;

Database management system provide a facility known as the data definition language (DDL), which can be used to define the conceptual schema and also give some details about how important this schema in the physical devices used to store data.

Data Manipulation Language;

DML is a language that enables users to access or manipulate as organized by the appropriate data model. Data manipulation involves retrieval of data from the database, insertion of new data into the database and deletion or modification of exiting data.

Elements Of a database management system;

1. DML Precompiler
2. DDL compiler
3. File manager
4. Database manager
5. query processor
6. Database Administrator
7. Data Dictionary

COMPUTER VIRUS

The evolution of virus;

The concept of virus dates back to 1949, when John von meumann submitted a paper putting forward the concept of a “Self Replicating” program; the idea seemed impossible and was dropped. Subsequently, the first virus like program appeared in the form of a recreational game called “CODE-WARS” at the bell labs of American telephone and Telegraphy Company. In “CODE-WARS” two players were to code a set of program that would destroy the other players programs. Concurrently at Massachusetts institute of technology, students were carrying out experiments with computer, which no body had ever tried.

Classification of Virus;

Viruses are classified on the basis of their mode of existence and there are three categories of viruses;

1. BOOT infectors.

1. SYSTEM infectors.

1. GENERAL EXECUTABLE PROGRAME infectors.

BOOT infectors;

As the name suggest, they are characterized by the fact that they physically reside in the boot sector [0(Zero)] sector of the disk. A system infected by such a virus will have the virus residing in a particular area of the disk rather than in a program file. These viruses get loaded soon after the power on self test and control the system and remains in control at all times. They sometimes have the capability to trap soft booting (i.e. CTRL ALT DEL) and remain in control even if the system is booted from a non-infected floppy, thereby contemning the clean floppy.

SYSTEM infectors;

This second category of viruses deals with the components of the system itself. All machines without exception require an operating system in order to create an environment in which the operator works. In MS-Dos, Command.com contains all not loaded onto the memory when the machine is booted. The system infectors attach themselves to a file such as COMMAND.Com or other memory resident files and manipulate these files.

GENERAL .COM or .EXE Infectors;

From the infection point of view, these viruses are most dangerous and devastating of the three classes of viruses. They attach themselves to program files and can spread to almost any executable program in any system. These viruses change the original program instructions into a Jump to its own code and follow that code with a return to the original program. As a result, whenever the program is executed, the viruses get loaded and executed first and then allow the original program to proceed. It remains memory resident and infects each and every program that is loaded for execution.