Wednesday 29 February 2012

Define protocol?


                        A protocol is a set of rules that govern data communication. In networks, communication occurs between the entities in different systems. Two entities cannot just send bit streams to each other and expect to be understood. For communication, the entities must agree on a protocol.

Define DNS?



The Domain Name System (DNS) is a hierarchical naming system built on a distributed database for computers, services, or any resource connected to the Internet or a private network. DNS is an Internet service that translates domain names into IP addresses.

Define TCP/IP?



TCP (Transmission Control Protocol) is a reliable, point-to-point, connection-oriented, full-duplex protocol.

Define UDP?



UDP (User Datagram Protocol) is a connectionless unreliable transport protocol. It extends IP’s host-to-host delivery service to process-to-process communication service.

Multitasking vs Multithreading:



·        Process is nothing but the program that is executing.
·        Process based multitasking (or) multitasking is the feature that allows the system to run two or more programs concurrently.
·        Example for multitasking: This multitasking enables us to use the java compiler at the same time when we are using text editor.
·        Threads are separate tasks running within a program.
·        In Thread-based multitasking (or) Multithreading, a single program can execute two or more tasks simultaneously.
·        Example for multithreading: a text editor can format the text at the same time that it is printing.
·        In multitasking, a program is the smallest unit of code that can be dispatched by the scheduler.
·        In multithreading, the thread is the smallest unit of code that can be dispatched by the thread scheduler.
·        The processes in multitasking (process-based multitasking), are referred to as heavyweight tasks. Since,
o   Each process requires its own separate address space.
o   Context switching from one process to another is CPU-intensive task needing more time.
o   Inter-process communication between the processes is again expensive (as the communication mechanism has to span separate
·        Multitasking threads cost less in terms of processor overhead (or) simply Multitasking threads requires less overhead than multitasking processes because of the following reasons:
o   Multiple threads in a program share the same address space, and cooperatively share the same heavyweight process.
o   Context switching from one thread to another is less CPU intensive.
o   Inter-thread communication, is less expensive (as threads in the program communicate in the same address space).
·        Multithreading are used to write very efficient programs that make maximum use of CPU, because idle time of the CPU can be kept down to a minimum.
·        This is really important for the interactive and networked internet environments.
Eg:-
    There may be an instruction in a program that is reading a file from a different host on the network. Knowing that reading from a local file system itself is slow compared to the speed of the CPU, reading from a file on a different host on the network is an even slower process. In single threaded environment, the rest of the program waits till the I/O instruction returns. Once the CPU dispatches the I/O instruction it is free. This idle time of the CPU should be put to good use by executing some other part of the system.

Multitasking:




·        The process of executing several programs simultaneously is known as multitasking.
·        Multitasking is further classified as:
o   Process-based multitasking
o   Thread –based multitasking
·        Process-based multitasking usually referred to as multitasking whereas thread-based multitasking is referred as multithreading.

Multithreaded Programming


·        A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread.
·        Threads are separate tasks running within a program.

Need for Multithreading:
·        Have you faced the following situations?
o   The browser cannot skip to the next web page because it is downloading a file?
o   You cannot enter text into your current document until your word processor completes the task of saving the document to disk.
·        If the program consists of a single thread, it can handle only one activity at a time, which results in the above situations.
·        To handle such situations effectively, multithreaded programming has been developed.

Exception Handling in JAVA:



·        In Java, exception handling is managed via five keywords:
o    Try
o    Catch
o    Throw
o    Throws
o    finally
·        Program statements that should be monitored for exceptions are contained within a try block.
·        If an exception occurs within the try block, it is thrown. The exception which is thrown should be caught by the code (using catch) and that should also be handled.
·        System-generated exceptions are automatically thrown by the Java run-time system. To manually throw an exception, use the keyword throw.
·        Any exception that is thrown out of a method must be specified as such by a throws clause.
·         Any code that absolutely must be executed before a method returns is put in a finally block.

This is the general form of an exception-handling block:

try
{
// block of code to monitor for errors
}
catch (ExceptionType1 exOb)
{
// exception handler for ExceptionType1
}
catch (ExceptionType2 exOb)
{
// exception handler for ExceptionType2
}
// ...
finally
{
// block of code to be executed before try block ends
}

·        Exception Type is the type of exception that has occurred.

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