Operating Systems

Operating Systems Integrated set of programs which manage overall operation of a computer.  It makes Computer System user friendly.  Responsible for Smooth & Efficient operation of entire computer system.  Go by different names such as, Monitor, Executive, Supervisor, Controller & Master Control Programs. 

Functions 

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Processor Management Memory management Input/Output management File Management Job Priority System Automatic Transition from job to job.

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Interpretation of Command Coordination among compilers, assemblers & other utility software. Data Security & Integrity. Facilitates Easy Communication.

Batch Processing One of Oldest method of running programs.  Based on the idea of automatic job-to-job transition facility.  Also known as serial, sequential, off line, or stacked job processing.  Input data are introduced into the computer & processed automatically, one by one without user intervention. 

Batch Processing(Contd.) Method reduces Idle time of a computer system.  Appropriate for Processing Payroll or preparation of customer statements.  Disadvantages: 1. Reduces Timeliness. 2. Increases Turnaround Time. 3. Difficult to provide Priority. 

Spooling Simultaneous Peripheral Output On Line.  Used for reducing speed mismatch & in turn the idle time of the CPU.  Process of placing all data that comes from input devices on either a magnetic tape or disk.  Primary Reason for Spooling is to keep the program & data readily available to CPU. 

Card Reader

Input

Disk Or Tape

Output

Main Memory

Spooled Input

Spool Disk or Tape Output

Process Of Spooling

Printer

Multiprogramming 

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In order to overcome the problem of underutilization of main memory & CPU, the concept of Multiprogramming was introduced. Multiprogramming is the name given to the interleaved execution of two or more different & independent programs by the same computer. CPU switches from one program to another instantaneously.

Operation Of Multiprogramming Main Memory Supervisor Writing Output Data

Secondary Disk Storage

Prog. A Prog. B Program C (waiting for CPU)

CPU

Three Different States of a Program

Running

Program must wait for I/O completion

Program Executed is Resumed

Ready

Blocked I/O Completed

Requirement of Multiprogramming System 

Large Memory

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Memory protection

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Program Status Preservation

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Proper Job Mix

Multiprocessing  

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The systems that uses more than one CPU are known as Multiprocessing Systems. Multiprocessing means interconnected computer configurations with two or more independent CPU’s that have the ability to simultaneously execute several programs. For Ex: In case of a multiprocessing systems with two CPU’s , one may be used to process only on-line jobs while another one may be meant for processing only batch applications.

CPU 1

Main Memory

CPU 2

I/O Processors

I/O Processors

I/O Units

I/O Units

Basic Organization Of Multiprocessing System

Advantages Multiprocessing Improves Performance of a computer systems by allowing parallel processing of segment of programs.  It also facilitates more efficient utilization of all the other devices of the computer system.  It provides a built-in backup.If one of the CPU’s breaks down, the other one takes over. 

Limitation Multiprocessing 

Design of such OS is time taking job & requires highly skilled computer professionals.

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Large main memory is required.

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Very Expensive

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Maintenance is Costly.

Timesharing 

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It is used to describe a processing system with a number of independent, relatively low speed, online, simultaneously usable stations. It refers to the allocation of computer resources in a time dependent fashion to several programs simultaneously. Idea behind timesharing systems is to allow all user programs to have a brief share of the CPU time in turn.

Timesharing(Contd.) The short period of time during which a user gets the attention of the CPU is known as a time slice & of the order of 10 to 20 milliseconds.  The processing speed of the system & the use of multiprogramming in conjunction with timesharing allows the CPU to switch from one user station to another to do a job allocated. 

Concept of Timesharing System

Main Memory Terminal 1 Terminal 2

Time Sharing Supervisor

User 1 User 2

User 1 User 2

… … .

Terminal 3

Terminal N

User N CPU

User N Online Storage

Timesharing(Contd.) 

Users who are using a timesharing system will fall in one of the following three status groups: a) Active: user’s program currently has control of the CPU. One user will be active at a time. b) Ready: user’s program is ready to continue but is waiting for its turn to get the attention of CPU. c) Wait: user has made no request for execution of his job or the user’s program is waiting for some I/O operation.

Switching between 3 status

Active Process must wait for I/O completion

Process is allocated CPU time Allotted time slice over

Ready I/O completed

Wait

Advantages Timesharing 

Reduces CPU idle time.

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Offers computing facility to small users.

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Provides advantages of quick response.

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Reduces the output of paper.

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Avoids duplication of software.

Limitation Timesharing 

Question of security.

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Problem of reliability.

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Problem of data communication.

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Question of overhead involved.

ON-LINE Processing Also called Direct-Access or RandomAccess processing.  It permits transaction data to be fed under CPU control directly into secondary on-line storage devices from the point where data originates without being sorted.  Access & Retrieval of data is quick & direct.  Timesharing system is an example. 

On-LINE Processing

On-Line Station Transaction Input Enquiry Input/ System Response

CPU

Direct Access On-Line Storage Device

Real-Time Processing 

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A system in which a transaction accesses & updates a file quickly enough to affect the original decision making is called Real Systems. Real time means immediate response from the computer. It’s a on-line system with severe time limitations. One of the early system was American Airlines SABRE reservation system. They are required to be highly reliable.