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December 31, 2013

Operating System Structures


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Friends, in our last post we have discussed about the introduction of Operating Systems. Today we shall discuss about various structures of Operating System. Mainly two different operating system structures are there, those are,
  1. Monolithic Systems
  2. Layered Systems
1. Monolithic System : In monolithic systems, the operating system is a collection of procedures, each of which can call any other procedure whenever it wants. Each procedure in the system has a well-defined interface in terms of parameters and results., and each one is free to call any other one, if the later provides some useful computation that the former needs.

To construct the actual program of operating system, programmer compiles all the individual procedures, all files containing procedures and binds them together into single object program with the help of linkers. Here there is no information hiding as every procedure  an call every other procedure.

The services provided by operating system are requested by putting the parameters in well-defined places, such as in registers or in the stack and then executing a special trap instruction known as kernel call or supervisor call.  But with registers, there is a problem because the number of parameters may exceed number of registers. Hence most of the operating systems put parameters in memory only. When the system is executed, the system switches from user mode to kernel mode and transfers, the control to operating system. The operating system then examines the parameters of one system call to determine which system call is to be carried out. After system call is finished, the service of the operating system is provided to the program that executed system call. now the control returns from kernel mode to user mode. Now the user program continues execution from where it left off.

Layered System :
In the layered approach, the operating system has six layers as shown below, 
0 - Processor Allocation and Multiprogramming
1 - Memory management
2 - Operator - Process Communication
3 - Input / Output Management
4 - User Programs
5 - The Operator 
Layer 0 deals with allocation of CPU, switching between processes, when interrupt occurs or timers expires. i.e., Layer 0 deals with multiprogramming of the CPU. In a multiprogramming environment different processes request CPU simultaneously but CPU is given to only one process based on First Come First Serve (FCFS), Shortest Job First (SJF), Round Robin etc., and the other processes will be waiting in the ready queue. For the CPU if it is a time sharing system CPU will be given to each process for definitive time interval. Once the time quantum is completed, the process execution will be suspended and it is put at the end of ready queue. Now the CPU is given to the next waiting process in the queue.

Layer 1 deals with memory management. It allocates space for processes in main memory. In a multiprogramming environment the CPU will be shared by many processes simultaneously. Since may programs must be present in main memory; each program is given small amount of memory space called partition. If the size of the program exceeds the length of the partition, then it is not possible to store the entire program in same partition. Hence the program is divided into pages and the required pages will be present in the partition. Bringing the essential pages into core is known as Demand paging.

Layer 2 deals with operator's process communication. When the process is being executed it requires some I/O which is supplied by operator.

Layer 3 takes care of managing I/O i.e., information is transferred into or transferred out of buffer.

Layer 4 is the place where user's programs are found. they need not worry about processor, memory etc., since they are taken care at lower layers.

The system operator process is at layer 5.  

That's all for now friends. In our next post we shall discuss about Multiprogramming in detail. Happy Reading :)

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  1. Hi anybody got interview call letters for KVB Asst. Mgr.................... :-)

    1. Hi Suresh, we are also waiting for which state u belongs...........

  2. is there any need of CV in ibps po 3 interview, as every details is there in the application form and it is not written to bring CV ????????? Mam plz reply........Thank u in advance

    1. hi friend no need of any CV...... application forms enough..........

    2. cv is mandatory guys

    3. thank you Suresh......

    4. mam plz give the reply is needed or not???????

  3. haii.....mam please clarify my doubt .....regarding last year cutoff one should get above 140 for selected to interview in IBPS SO... how one can get 140 marks in that paper...... while the paper will be in PO standard.... i am able to attempt only 15questions both in reasoning and data interpretion in PO paper......but in SO paper we should attempt atleast 40 questions in each section to score 140 marks having negative marks............. please clarify my doubt

    1. what is the twss cuttoff of ibps so it officer 2 exam.if it 140 then it i impossibe to score 140 marks paper like po standard

    2. hi friends, cut off depends on the paper difficulty.... last year paper was moderate, so it was 128 ( from 140 they reduced to 128 later).........This year PO paper was very tough, so they given cut off 60 only na........... so dont concentrate on cutoffs ........prepare well

  4. Kindly give me the link for last post on 'Operating System'


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