Monday 21 May 2012

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What is Operating System (OS)?

An operating system, or OS, is a software program that enables the computer hardware to communicate and operate with the computer Application software. Without a computer operating system, a computer would be useless. The purpose of an operating system is to organize and control hardware and software.

What is Application Software’s?

A computer program designed to carry out a specialized task’s for the user, such as database management, word processing, or electronic mail

What is System Software’s?

Systems software refers to the Operating System and all utility programs (like Compiler, Loader, Linker, and Debugger) that manage computer resources at a low level.  Operating systems, such as GNU, Microsoft Windows, Mac OS X or Linux, are prominent examples of system software.

What is Graphical User Interface (GUI)?

A software Designing interface designed to standardize and simplify the use of computer programs, as by using a mouse to manipulate text and images on a display screen featuring icons, windows, and menus.
Generally implemented as a desktop metaphor, with file folders, trash cans, and resource icons.

• Icons represent some item on the system, and respond accordingly when the icon is activated.
• First developed in the early 1970's at Xerox PARC research facility.
• In some systems the GUI is just a front end for activating a traditional command line interpreter running in the background. In others the GUI is a true graphical shell in its own right.
• Mac has traditionally provided ONLY the GUI interface. With the advent of OSX ( based partially on UNIX ), a command line interface has also become available.

What is Kernel?

The kernel of an operating system is the part responsible for all other operations. When a computer boots up, it goes through some initialisation functions, such as checking memory. It then loads the kernel and switches control to it. The kernel then starts up all the processes needed to communiate with the user and the rest of the environment (e.g. the LAN) .
The kernel is always loaded into memory, and kernel functions always run, handling processes, memory, files and devices.

Operating System Structures?

• Simple Structure
• Layered Approach
• Microkernel
• Modules

Simple Structure

When DOS was originally written its developers had no idea how big and important it would eventually become. It was written by a few programmers in a relatively short amount of time, without the benefit of modern software engineering techniques, and then gradually grew over time to exceed its original expectations. It does not break the system into subsystems, and has no distinction between user and kernel modes, allowing all programs direct access to the underlying hardware

Layered Approach

• Another approach is to break the OS into a number of smaller layers, each of which rests on the layer below it, and relies solely on the services provided by the next lower layer.
• This approach allows each layer to be developed and debugged independently, with the assumption that all lower layers have already been debugged and are trusted to deliver proper services.
• The problem is deciding what order in which to place the layers, as no layer can call upon the services of any higher layer, and so many chicken-and-egg situations may arise.
• Layered approaches can also be less efficient, as a request for service from a higher layer has to filter through all lower layers before it reaches the HW, possibly with significant processing at each step.

Microkernels

• The basic idea behind micro kernels is to remove all non-essential services from the kernel, and implement them as system applications instead, thereby making the kernel as small and efficient as possible.
• Most microkernels provide basic process and memory management, and message passing between other services, and not much more.
• Security and protection can be enhanced, as most services are performed in user mode, not kernel mode.
• System expansion can also be easier, because it only involves adding more system applications, not rebuilding a new kernel.
• Mach was the first and most widely known microkernel, and now forms a major component of Mac OSX.
• Windows NT was originally microkernel, but suffered from performance problems relative to Windows 95. NT 4.0 improved performance by moving more services into the kernel, and now XP is back to being more monolithic.

Modules

• Modern OS development is object-oriented, with a relatively small core kernel and a set of modules which can be linked in dynamically.
• Modules are similar to layers in that each subsystem has clearly defined tasks and interfaces, but any module is free to contact any other module, eliminating the problems of going through multiple intermediary layers, as well as the chicken-and-egg problems.
• The kernel is relatively small in this architecture, similar to microkernel, but the kernel does not have to implement message passing since modules are free to contact each other directly.

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