DBMS Languages

 DBMS Support a variety of users and provides appropriate languages and interface or each category of users.Here are some languages that are provided by DBMS:
1) Data Definition Language (DDL): This Language is used define data structures and specially database schemas. these statements are used to create, alter, or drop data structures. ALTER ,CREATE ,DROP are some examples of DDL.

2) Storage Definition Language (SDL): This language is used to define internal schema. It defines that what will be the Physical structure of database, How many bites per field will be used, what will be the order of fields, and how records will be accesses etc.

3) View Definition Language (VDL): This language is used to specify user views and their mapping to conceptual schema. It defines the subset of records available to classes of users. It creates virtual tables and the view appears to users like conceptual level.It specifies user interfaces.

4) Data Manipulation Language (DML): It is used at conceptual level and external level and is used to perform operations like Query, Delete Update or Insert.Read Only Queries are also sometimes considered as component of  DML.It modifies the data but not schema or database objects. These languages are further divided into two types:

Hard Disk Drive (HDD)

It is a data storage device and is made up of a thin circular discs, called "Platters", which are coated on both the sides by Magnetic Material. A Disk pack may consist of number of Platters and are mounted on a central shaft that rotates them in same direction with same speed.The Data is written or accessed in a random manner and individual blocks of data can be stored or retrieved in any order rather than just sequentially.It is connected to computer using different interfaces such as IDE, SATA or SCSI.

Information Storage: In HDD information is stored on all surfaces of platters except upper and lower surface of top and bottom platters respectively. thus,a  disk with 5 disk platters will have 8 recording surfaces. Each plotter is consist of no. of invisible concentric circles which are called tracks. A set of such corresponding tracks in all the surfaces is called cylinder. Tracks are further divided into sectors. * bit EBCDIC code is used for the information storage.The Information is stored in form of invisible magnetic spots. The Presence of magnetic spot represents binary 1 and absence represents binary 0.

The Outer tracks has greater circumference then inner tracks but each track consist of same no. of characters.Hence the data density of outer tracks is less as compare to inner tracks.

Computer Generations

First Generation(1942-1955)

This Generation is considered to be form 1942-1955. These Machines were designed with the help of vacuum tubes. The main features of this Generation Computer were:
1. Were designed with vacuum tube’s which was only electronic component available during those days.
2. These computers were fastest of their time and could perform calculations in milliseconds.
3. They were bulk in size due to the use of thousands of vacuum tubs.
4. These devices were non-potable and were more frequent to failure.


Second Generation(1955-1964)

 This Generation is considered from 1955 to 1964. This Generation was based upon transistor Technology. The first transistor invented at AT&T tall Laboratories 1947. The main feature of this Generation Computer were:
1. Used Transistor inspite of vacuum tubes.

Direct Memory Access (DMA)

Data transfer between I/O devices and memory is slow due to the speed mismatch in their speeds. This is avoided by removing the CPU from the path and letting the peripheral device manage the memory buses directly. This   “data transfer between I/O device and memory without the involvement of CPU is called Direct Memory Access or DMA”.  

In this type of data transfer CPU remains free for another processes and has no control of the memory buses.  In DMA CPU make use of two control signals i.e. Bus Request (BR) and Bus Grant (BG). BR signal is used by the DMA controller to request the CPU for the control of buses. When this signal is active, CPU terminate the current execution of instruction and places  data bus, address bus, and read and write lines into high impedance state. This high impedance state works like open circuit and output is disconnected.BG Signal is used by the CPU to inform that all buses are now in high impedance state and the DMA controller can take the control of buses.

When this transfer is over the DMA disables the Bus Request Signal and CPU Disables the Bus Grant Signal and takes back the control of buses and returns back to its normal Operation.

Two Types of DMA Transfer:-When the buses are under the control of DMA data transfer can be in following ways

Functions of Operating System

1) Process Management: - Operating System allocates resources to Processes, enable processes to share and exchange information, protect the resources of each process from other processes and enable synchronization among processes. Deadlock avoidance and recovery is also function of Operating System.

2) Input / Output Management:-Assignment of various input and output devices to one or more programs is done by the operating system.

3) Memory Management: - Allocation of main memory and secondary memory to the system programs as well as user data is done by the Operating System.

4) File Management:-How files will be transferred from one device to another and how they will be saved or accessed is managed by the Operating System

5) Determine and maintain order of Jobs: - It is the Responsibility of the Operating System to decide that jobs will be completed in which order. It decides that first preference should be given to which request and which can be delayed.

Difference Between Structure and Union

Characteristics	Structure	Union
Definition	A structure is a collection of variables under a single name. These variables can be of different types, and each has a name which is used to select it from the structure.	A union is a special data type that enables to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time
Declaration	typedef struct {           char name[64];           char course[128];           int age;           int year;   } student;	union Data { int i; float f; char str[20]; } data;
Memory Allocation	In a structure, all of its data members are stored in contiguous memory locations. The size of an object of a struct is, therefore, the size of the sum of all its data members. Example for Structure: struct tech { char p; int x; float Q; int y; }D; memory allocated for D will be  1+2+4+2= 9 bytes	A union is a class all of whose data members are mapped to the same address within its object. The size of an object of a union is, therefore, the size of its largest data member.

Network Protocols and Their Functions

Protocols

Whenever a person want to talk with another person then their communicating language should be same so that they can easily communicate with each other .If they will use different languages then they can not understand what the other is saying. Similarly in Computer Networks certain rules are defined so that computers can communicate with one another.These Pre defined rules are called Protocols and can be defined as "Protocols are the predefined rules between the communicating devices that govern the communication between them."

Functions Of Protocols

i)  Sequencing of Data:- Protocols split large size messages to equal size pieces called Packets which can be transmitted easily.each packet contains unique identification number and can be easily arranged at receiver end to create original message.

ii) Flow Control of Data:-This is another important function of protocols.if sender is sending continuously data and receiver is not in a condition to receive that data or having less receiving speed then their are the chances of data loss.This data loss is avoided by the use of Protocols.

How Laser Printers Work

Laser printers ware invented at XEROX in 1969 by by researcher Gary Starkweather.Laser Printers are digital printing devices that are used to create high quality text and graphics on plain printer.

Working Principle
1)A laser beam projects an image of the page to be printed onto an electrically charged rotating Photo sensitive drum coated with selenium.
2) Photo conductivity allows charge to leak away from the areas which are exposed to light and the area gets positively charged.
3)Toner particles are then electrostatically picked up by the drum’s charged areas, which have  been exposed to light.
4)The drum then prints the image onto paper by direct contact and heat, which fuses the ink to the paper.