Download unit1

1
STUDY MATERIAL
DEPARTMENT
: SOFTWARE SYSTEMS
CLASS
: II M.Sc
SUBJECT
: OPERATING SYSTEMS
UNIT
:I
Syllabus: Objectives of an operating system. Operating system as a resource manager, operating system
as a user computer interface, ease of evolution. The evolution of the operating system.
Definition for Operating System:
Program that controls the execution of application programs and acts as an interface
between the user of computer and the hardware.
Objectives :
 Convenience : Make computer more ease to work.
 Efficiency : Resources in efficient manner.
 Ability to evolve : Permit effective development , testing , and introduction of
without interfering the service.
new system function
Three Aspects of Operating System:
 Operating System as a user / computer interface ,
 Operating System as Resource Manager ,
 Ease Of Evolution
Operating System as a user / computer interface :
 User of these Application – End User
 He can view the system in terms of Applications
 Application is a Program Developed by a Programmer
 To help the application programmer, a set of system programs are provided called as Utilities.
 This is used for calling frequently used functions , management of files ,and for Control of I/O devices.
 Most important system program is Operating System.
 Operating System hides the details of hardware from the programmer .
End User
Application Program
Utilities
Operating System
Hardware
Services Of Operating System:
2
i)Program Creation:
Editors and Debuggers are the utility Programs accessible
through the OPERATING SYSTEM.
ii)Program Execution :
Instruction and data must be loaded into main memory , I/O devices and file must be initialized
and other resources must be prepared which
are governed by OPERATING SYSTEM.
iii)Access To I/O devices :
Each I/O device , peculiar set of instructions , control
SYSTEM .
signals are governed by OPERATING
iv)Controlled Access to files:
In the file Cases , control with addition to the storage format on I/O devices are governed by
OPERATING SYSTEM . It also provide security in case of Multi User mode.
v) System Access:
In an shared or public mode , OPERATING SYSTEM provide
Protection Mechanism to the resources and Resolve Conflict in use of A resource.
vi)Error Detection and Response:
During running , Varity of errors can occurred so the Operating System to make the response that
clears the error with least impact on running Application.
The response may range from ending a program that cause error , or to retrying the operation , to simply
reporting the error in the application.
vii) Accounting:
Operating System has the statistics about available resources , which is useful for future Up
gradation.
Other Services are Communication, Resource Allocation etc.,
Operating System As a Resource Manager:

Operating System is also a Program, and the key difference is that it directs the processes in the use of
other system resources.
 This Program is Executed by the Processor.
 Processor releases its control to the process and regain its control after the process has finished the job.
 This allocation is managed by the Kernel or nucleus where most frequently used functions of
operating system are present which are kept in main memory.
In allocating Resources it has to follow the below steps ;
1.Status (Free or Allocated) ,
2.Priority among the Process,
3.Allocate the Processor to the Process(Relinquish the Control),
4.Deallocate the Process (Regain its Control).
Ease Of Evolution:
 Hardware Upgrades and New Type of Hardware
Evolution of Paging Mechanism ,
Evolution of Graphic or Window mode Terminals.

New Services:

Fixes
Due to the user request , new kind of services are evolved by the operating System.
:
3
Unexpected errors are found and some fixes are made.
Evolution of Operating System:




Serial Processing
Simple Batch System
Multi Programming Environment
Time Sharing Environment
Serial Processing:
 No concept of Operating System.
 Programs are in Machine Code.
 If error occurs program comes to halt state.
 There are switches and lights , input (Program in Machine Code)is given to the switches And
output is sent to printer through the switches. If any error occurs light will glow.
 Programmer correct the error by checking Registers ,Memory etc., and then proceed the above
step to get the output .
Scheduling:
Reserving the Machine time for Processing ----- Scheduling.
If Process time for a process < Reserved time –Processor put as idle.
If Process time for a process > Reserved time –Process is stopped without completion.
Setup Time:
For a Process to be completed, then some basic steps to be done ;
 Loading Complier(Program) + Source Program to then Memory.
 Compiling the Source Program so that source Program is converted to Object
Program , which has to be saved in memory.
 Linking has to be done so that Object Program is linked with the Built in
Functions.
So for the above process some amount of time will be consumed. This amount of time is
consumed is called as setup time.
If any error occurs while running, then we have to startup from the first which make to consume
large amount of time.
Due to this two cases , we are moving to the Batch Processing Environment.
Simple Batch Systems:
 First of this kind is developed for General Motors by IBM in mid of 1950’s .
 Monitor the heart of this kind.
 User submit his job to the Operator.
 Operator batches all the jobs and load it as input to the monitor.
 Monitor send its control to the first job .
 After Processing of first job , control is branch back to the monitor.
Monitor:
 Resident monitor , part of the monitor resides in main memory.
 Rest of monitor consists of utilities and common functions that are loaded as
subroutines to the user program at the beginning of any job that requires them.
 Monitor send its control to the first job which is in main memory.
 After Processing of first job , control is branch back to the monitor.
 Results are sent to the Printer.
Processor:
 Processor first executes set of instructions , so that monitor fetch a job.
 After that Processor executes the set of instruction in the user program until it ends or
error occurs.
 Control is passed to a job , that is it fetches an instruction from the user program and
executes it.
 Control is passed to the Monitor , that is it fetches an instruction from the monitor
Program and executes it.
Job Setup:
4
 Job Control Language (JCL) , special type of language which provide instructions to
the monitor.
 For Example , if user submits a program(Fortran) with data cards and job control
instructions denoted by $ at the beginning.
 If following tasks are done for specific card has been read
 $FTN card load Fortran complier from its tape.
 Compiler transfer Source Code to the Object Code and store in memory.
 If it is stored in memory , then it is termed as compile , load and go
 If it is stored in tape ,$LOAD card is required.
 $LOAD card is read by monitor and invokes loader , which stores the object program into
main memory that is in the place of compiler.
 Input instruction cause the monitor to fetch the data from data cards until next JCL card
has been read.
Hardware features:
 Memory Protection : Protection is given to the memory where Monitor resides.
 Timer: Timer is to prevent a single job to monopolize the system.
 Privileged Instruction: To secure the resources such as I/O devices Some set of
instructions can be executed only by the Monitor. If any user try to execute these set of
instruction , then error occurs and stops current execution. These set of instruction are
called as Privileged Instruction.
 Interrupts: Flexible for to release and to regain its control to the or from the process or
job.

Multi Programming Batch System:
In Simple Batch System , if any process is waiting to get control over some I/O devices ,
then processor is to wait for some time until the process has received the control .(Refer to the example
)
To avoid that we are moving to the new concept that is, if a process is waiting for some
event had to occur, processor can send its control to the next job which is waiting for the processor
control. This concept is called as Multiprogramming or Multitasking .
Run
Run
Wait
Wait
Time
UniProgramming
Program A
Run
Wait
Run
Run
Program B
Combined
Wait
Run
Wait
Run
Run
Wait
Wait
Run
Run
Wait
Time
Multi Programming with Two Programming
5
In the above figure , if Program A is waiting for a control of I/O device , then Program
B is receive the Processor control .
Comparison between Simple Batch and Multi Batch Systems:
Time taken by each job to complete its task are;
Job 1 = 5 min
Job 2 = 15 min
Job 3 = 10 min
In simple Batch System , Job2 has to wait for 5min until job1 has been finished. Job 3 has been
wait for 20 min so that job1 & job 2 has been over ( 5 + 15).So that total time to process all the three
jobs are 30 min.
But in Multiprogramming batch system when job1 is over , 1/3 of job2 and ½ of job3 has been
completed . so that total time taken by the processor to complete all the task is 15 min.
So total time taken by the processor in multi programming mode is less than the time taken by
the processor in simple batch system.
Time Sharing System:
Processor’s time is shared among all the process .
For example , if a machine with 32K of main memory , with the 5K of resident monitor. User
program and data are loaded into 27K of main memory. A system clock generates interrupt at the rate of
0.2 seconds. When interrupt occurs , control is regained by the processor and sent its control to the next
process.
Assume that there are four jobs ,
Job1 = 15K
Job 2 = 20K
Job 3 = 5 K
Job 4 = 10 K

Monitor loads job1 and transfer its control to it.

Monitor if wants to load job 2 which needs more memory , so that job 1 is written out
into the disk first , then job 2 is loaded.

Monitor if wants to load job 3 which has need less memory , so that part of job 2 is
written out into the disk first to avoid disk write time , then job 3 is loaded.

Monitor if wants to load job 1, so that part of job 2 , and job 3 are written out into the
disk first , then job 1 is loaded.

Monitor if wants to load job 4 which has need less memory , so that part of job 2 and
part of job 1 is written out into the disk first to avoid disk write time , then job 4 is
loaded.

Monitor if wants to load job 2, so that job 1 is written out into the disk first and missing
portion of job 2 is loaded into the memory ,
32K
32K
25K
Job 2
Job1
5K
5K
Monitor
Monitor
6
32K
32K
25K
25K
Job 2
(Job 2)
20K
10K
Job 1
Job 3
5K
5K
Monitor
0
Monitor
0
32K
32K
25K
25Kk
(Job 2)
20K
(Job 1)
Job 2
15K
Job 4
5K
0
5K
Monitor
Monitor
Monitor
0
Questions:
Services Provided By the Operating System.
Evolution Of Operating System.
Comparison of Serial , simple batch ,Multi Programming and
Time Sharing System.
Hardware facilities available in Simple Batch System.
How Operating System work as a Resource Manager.