Model of operating system

operating system structure:

An operating system might have many structure. According to the structure of the operating system; operating systems can be classified into many categories.
Some of the main structures used in operating systems are:

1. Monolithic architecture of operating system

monolithic sturucture of operating system

It is the oldest architecture used for developing operating system. Operating system resides on kernel for anyone to execute. System call is involved i.e.  Switching from user mode to kernel mode and transfer control to operating system shown as event 1. Many CPU has two modes, kernel mode, for the operating system in which all instruction are allowed and user mode for user program in which I/O devices and certain other instruction are not allowed.  Two operating system then examines the parameter of the call to determine which system call is to be carried out shown in event 2. Next, the operating system index’s into a table that contains procedure that carries out system call. This operation is shown in events. Finally, it is called when the work has been completed and the system call is finished, control is given back to the user mode as shown in event 4.

2. Layered Architecture of operating system
The layered Architecture of operating system was developed in 60’s in this approach; the operating system is broken up into number of layers. The bottom layer (layer 0) is the hardware layer and the highest layer (layer n) is the user interface layer as shown in the figure.

layered architecture

The layered are selected such that each user functions and services of only lower level layer. The first layer can be debugged wit out any concern for the rest of the system. It user basic hardware to implement this function once the first layer is debugged., it’s correct functioning can be assumed  while the second layer is debugged & soon . If an error is found during the debugged of particular layer, the layer must be on that layer, because the layer below it already debugged. Because of this design of the system is simplified when operating system is broken up into layer.
Os/2 operating system is example of layered architecture of operating system another example is earlier version of Windows NT.
The main disadvantage of this architecture is that it requires an appropriate definition of the various layers & a careful planning of the proper placement of the layer.

3. Virtual memory architecture of operating system

virtual memory architecture

Virtual machine is an illusion of a real machine. It is created by a real machine operating system, which make a single real machine appears to be several real machine. The architecture of virtual machine is shown above.
The best example of virtual machine architecture is IBM 370 computer. In this system each user can choose a different operating system. Actually, virtual machine can run several operating systems at once, each of them on its virtual machine.
Its multiprogramming shares the resource of a single machine in different manner.
The concepts of virtual machine are:-
a. Control program (cp):- cp creates the environment in which virtual machine can execute. It gives to each user facilities of real machine such as processor, storage I/0 devices.
B. conversation monitor system (cons):- cons is a system application having features of developing program. It contains editor, language translator, and various application packages.
c.    Remote spooling communication system (RSCS):- provide virtual machine with the ability to transmit and receive file in distributed system.
d. IPCS (interactive problem control system):- it is used to fix the virtual machine software problems.

4. client/server architecture of operating system
A trend in modern operating system is to move maximum code into the higher level and remove as much as possible from operating system, minimising the work of the kernel. The basic approach is to implement most of the operating system functions in user processes to request a service, such as request to read a particular file, user send a request to the server process, server checks the parameter and finds whether it is valid or not, after that server does the work and send back the answer  to client server model works on request- response technique i.e. Client  always send request to the side in order to perform the task, and on the other side, server gates complementing that request send back response. The figure below shows client server architecture.

client server model

In this model, the main task of the kernel is to handle all the communication between the client and the server by splitting the operating system into number of ports, each of which only handle some specific task. I.e. file server, process server, terminal server and memory service.
Another advantage of the client-server model is it’s adaptability to user in distributed system. If the client communicates with the server by sending it the message, the client need not know whether it was send a ……. Is the network to a server on a remote machine? As in case of client, same thing happen and occurs in client side that is a request was send and a reply come back.