Operating System Structure

An operating system contains a kernel, command processor or shell and graphical user interface.

Kernel

It is the control module of an operating system.
It is that part of the operating system, that loads first and remains in the main memory.
It is a bridge between applications and the actual data processing done at the hardware level.
It is responsible for memory management, process and task management, and disk management.
Thus Kernel’s primary function is to manage the computer’s resources and allow other programs to run and use these resources.
Kernel provides the lowest level abstraction layer for various resources like processors and I/O devices that application software must control to perform it function.
When a user gives command for performing any operation, then the request goes to the shell.
The shell translates these human programs into machine language and then transfers the request to the kernel.
The kernel receives the request from shell, processes it and displays the result on the Screen.
All these functions are performed by kernel in a transparent manner.

To run an application, it sets up an address space for the application, loads the file containing the application code into memory, sets up a stack for the program and branches to a given location inside the program, thus starting its execution.
It controls the state of the processes i.e. checks whether the process is running, waiting or blocked.
Kernels also provide methods for process synchronization and process communication (IPC).
Kernel performs memory management and provides the memory for processes that are running on the system.
The kernel maintains a time table for all the processes that are running. Thus, in a multitasking system, kernel decides the schedule for various processes for how long each process will get CPU. It also maintains waiting and suspended processes in different memory areas.
When a kernel determines that the logical memory does not fit to store the program. It then uses the concept of the physical memory which will store the program into temporary manner.
It also maintains all the files and decide the manner in which they are stored into the computer system. It also provides the facility to password protect the various files.
The kernel manages and controls the various I/O devices and their drivers installed on the system and allocates these devices in efficient manner to various processes.

A shell is a software that provides an interface for users of an operating system which provides access to the services of a Kernel.
The name shell originated from shells being an outer layer of interface between the user and the internals of the operating system (the kernel).
Operating system shells are divided into two categories command line and graphical.
Command line shell is used in MS-DOS and graphical shells are used in Windows that provides GUI facilities.

It is that part of an operating system which receives and executes operating system commands.
Whenever a command prompt is displayed the command processor waits for a command.
After the user enters a command, the command processor makes sure that command is valid and then either executes it or issues an error message.
For an operating system with graphical user interface, the command processor interprets mouse operations and executes the appropriate command. It is also known as command line interpreter.

It is provided so that the user need not remember tedious syntax of the command language, instead pointing to an option by means of a mouse can do his/her job.