Operating systems11 9-07

Asst. Prof. Rajni
Dept. of CS
Pt. Mohan Lal SD College for Women Gurdaspur
Subject: Operating Systems

Contents
 Today's Topic: Introduction to Operating Systems
 We will learn
1. What is Operating System?
2. What OS does?
3. Structure of OS
4. Evolution of OS
 Batch Processing, Multiprogramming, Time
sharing systems
5. Operating System Functions
6. Main Funtions of OS
7. Types of OS
 Single User, Multi User systems

What is OS?
 Operating System is a software, which makes a
computer to actually work.
 It is the software the enables all the programs we use.
 The OS organizes and controls the hardware.
 OS acts as an interface between the application
programs and the machine hardware.
 Examples: Windows, Linux, Unix and Mac OS, etc.,

What OS does?
An operating system performs basic tasks such as,
 controlling and allocating memory,
 prioritizing system requests,
 controlling input and output devices,
 facilitating networking and
 managing file systems.

Structure of Operating System:
Application Programs
System Programs
Software (Operating System)
HARDWARE
(Contd…)

Structure of Operating System
(Contd…):
 The structure of OS consists of 4 layers:
1. Hardware
Hardware consists of CPU, Main memory, I/O
Devices, etc,
2. Software (Operating System)
Software includes process management
routines, memory management routines, I/O
control routines, file management routines.
(Contd…)

Structure of Operating System
(Contd…):
3. System programs
This layer consists of compilers, Assemblers,
linker etc.
4. Application programs
This is dependent on users need. Ex. Railway
reservation system, Bank database
management etc.,

Evolution of OS:
 The evolution of operating systems went through seven
major phases.
 Six of them significantly changed the ways in which
users accessed computers through the open shop, batch
processing, multiprogramming, timesharing, personal
computing, and distributed systems.
 In the seventh phase the foundations of concurrent
programming were developed and demonstrated in
model operating systems.
(Contd…)

Evolution of OS (contd..):
Major
Phases
Technical
Innovations
Operating
Systems
Open Shop The idea of OS IBM 701 open shop
(1954)
Batch
Processing
Tape batching,
First-in, first-out
scheduling.
BKS system (1961)
Multi-
programming
Processor multiplexing,
Indivisible operations,
Demand paging,
Input/output spooling,
Priority scheduling,
Remote job entry
Atlas supervisor
(1961),
Exec II system (1966)
(Contd…)

Evolution of OS (contd..):
Timesharing Simultaneous user
interaction,
On-line file systems
Multics file system
(1965),
Unix (1974)
Concurrent
Programming
Hierarchical systems,
Extensible kernels,
Parallel programming
concepts, Secure parallel
languages
RC 4000 system (1969),
13 Venus system
(1972),
14 Boss 2 system
(1975).
Personal
Computing
Graphic user interfaces OS 6 (1972)
Pilot system (1980)
Distributed
Systems
Remote servers WFS file server (1979)
Unix United RPC (1982)
24 Amoeba system
(1990)

Batch Processing:
 In Batch processing same type of jobs batch (BATCH- a
set of jobs with similar needs) together and execute at a
time.
 The OS was simple, its major task was to transfer control
from one job to the next.
 The job was submitted to the computer operator in form
of punch cards. At some later time the output appeared.
 The OS was always resident in memory. (Ref. Fig. next
slide)
 Common Input devices were card readers and tape
drives.

Batch Processing (Contd…):
 Common output devices were line printers, tape drives,
and card punches.
 Users did not interact directly with the computer
systems, but he prepared a job (comprising of the
program, the data, & some control information).
OS
User
program
area

Multiprogramming:
 Multiprogramming is a technique to execute number of
programs simultaneously by a single processor.
 In Multiprogramming, number of processes reside in
main memory at a time.
 The OS picks and begins to executes one of the jobs in
the main memory.
 If any I/O wait happened in a process, then CPU
switches from that job to another job.
 Hence CPU in not idle at any time.

Multiprogramming (Contd…):
OS
Job 1
Job 2
Job 3
Job 4
Job 5
• Figure dipicts the layout of
multiprogramming system.
• The main memory consists of 5
jobs at a time, the CPU executes
one by one.
Advantages:
•Efficient memory utilization
•Throughput increases
•CPU is never idle, so
performance increases.

Time Sharing Systems:
 Time sharing, or multitasking, is a logical extension of
multiprogramming.
 Multiple jobs are executed by switching the CPU
between them.
 In this, the CPU time is shared by different processes, so
it is called as “Time sharing Systems”.
 Time slice is defined by the OS, for sharing CPU time
between processes.
 Examples: Multics, Unix, etc.,

Operating Systems functions:
 The main functions of operating systems are:
1. Program creation
2. Program execution
3. Input/Output operations
4. Error detection
5. Resource allocation
6. Accounting
7. protection

Types of OS:
Operating System can also be classified as,-
 Single User Systems
 Multi User Systems

Single User Systems:
 Provides a platform for only one user at a
time.
 They are popularly associated with Desk Top
operating system which run on standalone
systems where no user accounts are
required.
 Example: DOS

Multi-User Systems:
 Provides regulated access for a number of users by
maintaining a database of known users.
 Refers to computer systems that support two or more
simultaneous users.
 Another term for multi-user is time sharing.
 Ex: All mainframes and are multi-user systems.
 Example: Unix

Operating systems11 9-07

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