Unit 1 introduction to operating system

Introduction
To
Operating System
Prepared By
Prof. Bhushan Pawar
www.bhushanpawar.com

Basics
Prof.Bhushan Pawar

Operating System
• It is a collection of software that manages
computer hardware resources and provides
common services for computer programs.
• The operating system is a vital component of
the system software in a computer system
• Definition- “An operating system is a program
that acts as an interface between the user and
the computer hardware and it controls the
execution of all kinds of programs”
Prof.Bhushan Pawar

Operating System
Prof.Bhushan Pawar

History of UNIX
• In mid-1960s, Massachusetts Institute of
Technology, AT&T Bell Labs and General
Electric were (Ken Thompson, Dennis Ritchie,
Doug McIlroy, and Joe Ossanna) developing an
experimental time-sharing & hierarchical file
system operating system called Multics.
• In 1970, Peter Neumann coined the project name
UNICS (UNiplexed Information and Computing
Service) as a pun on Multics (Multiplexed
Information and Computer Services)
Prof.Bhushan Pawar

History of UNIX (Continued…)
• In 1972, Unix was rewritten by Dennis Ritchie in
the C programming language .
• In 1980, Bell developed multiple versions of Unix
for internal use, such as CB UNIX, PWB/UNIX
(Programmer's Workbench)
• Microcomputer Unix became commercially
available in 1980, when Onyx Systems released its
Zilog Z8000-based C8002 and Microsoft announced
its first Unix for 16-bit microcomputers called
Xenix, which the Santa Cruz Operation (SCO)
ported to the 8086 processor in 1983
Prof.Bhushan Pawar

History of UNIX (Continued…)
• Can find out more at:
http://en.wikipedia.org/wiki/History_of_Unix
Prof.Bhushan Pawar

Unique Features of Unix
• Multitasking
• Multiuser
• Portability
• Application Libraries and Tools (e.g
shell, gedit etc.)
Prof.Bhushan Pawar

Benefits of Unix
• High level language “C” which makes it easier to
read understand and update.
• Complex programs can be built on simple programs.
• Hierarchal file system which allows easy
maintenance and efficient implementation.
• Provides simple and consistent interface to the
peripheral devices.
• It completely hides the machine architecture from the
user which makes it easier for the programmers to
write programs independent of hardware.
Prof.Bhushan Pawar

System Structure
Prof.Bhushan Pawar www.bhushanpawar.com

System Structure
Prof.Bhushan Pawar

System Structure
• It can be viewed as a set of layers
• Lowermost layer is Hardware Layer which is not
the part of the UNIX operating system
• Middle layer is called the System Kernel or the
kernel which is a actual operating system code and
functionality. It is totally isolated from user’s
program. The programs interact with the kernel by
using the system calls
• User programs can be built on top of the lower
level programs using these lower level programs
and system calls
Prof.Bhushan Pawar

User Perspective
• User perspective is basically based
on 3 scenarios
–The File System
–Processing Environment
–Building Block Primitives
Prof.Bhushan Pawar

File System
• Characteristics of UNIX file system
– A hierarchal structure.
– Consistent treatment of data
– Ability to create and delete files
– Dynamic growth of files
– Peripheral devices are also treated as files
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File System- Diagram 1
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File System- Diagram 2
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File System
• A full path name starts with the root directory i.e.
a slash character ( “/” ) and specifies the file that
can be found by travestying the tree.
• The path that starts from the root directory is
called the Absolute Path.
• Alternatively we can give path of any file relative
to any other directory. This path will be called
Relative Path.
• The files are just stream of bytes it is up-to the
program to interpret these bytes.
Prof.Bhushan Pawar

File System
• Permission to any file is governed by the file
access permissions.
• Access permissions are set independently for
read, write and execute. These permissions are
set independently for the file user, file group
and others.
– E.g. rwx rwx rwx
–
u g o
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Prof.BhushanPawar

Processing Environment
• Program is an executable file and Process is an
instance of program in execution.
• Many processes can execute simultaneously on
UNIX systems (sometimes this feature is
known as Multiprogramming or Multitasking)
• Various system calls allow processes to create
new processes, terminate processes,
synchronize stages of process execution, and
control reaction to various events
Prof.Bhushan Pawar

Processing Environment
Prof.Bhushan Pawar

Building Block Primitives
• Unix allows user to write small, modular
programs, which are later on use as a building
block for complex programs.
• It has 3 standard files
– Standard Input File
– Standard Output File
– Standard Error File
Prof.Bhushan Pawar

Building Block Primitives
• e.g
– One primitive building block available to the shell
user is the redirect I/O
• ls – this command gives the list of all files in current
directory
– The second building block primitive is the PIPE
• ls | more
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Operating System Services
• Memory Management
• Processor Management
• Device Management
• File Management
• Security
• Control over system performance
• Job accounting
• Error detecting aids
• Coordination between other software and users
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• Process Controlling
– Controlling the Creating, Termination and Suspension
of processes.
• Scheduling Processes
– Since many programs can execute simultaneously in
UNIX the process scheduling is also done by the
kernel.
• Main Memory Management
– allocating main memory to the user programs and
protecting the memory region where kernel is running.
Also, protecting the memory region of one process
from another process.
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• Secondary Memory Management
– Managing the secondary storage for the efficient and
timely retrieval and storage of data.
• Virtual Memory
– Managing the swap device and handling the swapping
system. Controlling the pages in the paging
system(memory allocation)
• Controlling Peripheral Devices
– Kernel controls the peripheral devices such as
terminals, disk drives and network devices.
Prof.Bhushan Pawar

Assumptions about the Hardware
• When a process executes on UNIX it executes
on two levels or we can say it executes in two
modes
– User level
– Kernel level
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User Level Vs. Kernel Level
• Processes in user mode can access their own
instructions and data but not kernel
instructions and data (or those of other
processes).
• Processes in kernel mode, can access kernel
data and instructions as well as user data and
instructions.
Prof.Bhushan Pawar

Assumption 1
Interrupts and exceptions
• UNIX supports asynchronous interrupt coming
facility.
• After receiving interrupt kernel saves the context and
services the interrupt, After servicing the interrupt it
reloads its context and resumes whatever it was
doing.
• There might be a possibility that kernel is servicing
one interrupt and another interrupt may occur. So
whether or not to service that interrupt and stop
whatever kernel was doing is decided by the interrupt
priority(or interrupt levels).
Prof.Bhushan Pawar

• If a high priority interrupt occurs the kernel stops the
previous one and jumps onto second. If a lower priority
interrupt occurs the kernel will not stop what it was
doing and that interrupt will have to wait. In other words
the lower priority interrupt is blocked if kernel is
servicing some high priority interrupt.
• An exception occurs when a process does something
unexpected. Exceptions are different from interrupts
they occur as events. If an interrupt occurs in the middle
of instruction, that instruction will be restarted after
handling the exception. If the exception is not caused by
the instruction but because of some other reasons and
between two instructions then the next instruction is
processed after handling the exception.
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Prof.Bhushan Pawar

Assumption 2
Processor Execution Levels
• Computers typically have a set of privileged
instructions that set the processor execution level in
the processor status word. Setting the processor
execution level to certain values masks off interrupts
from that level and lower levels, allowing only
higher-level interrupts
Prof.Bhushan Pawar

Assumption 3
Memory management
• It is functionality of an operating system which
handles or manages primary memory.
• It keeps track of each and every memory location
either it is allocated to some process or it is free.
• It checks how much memory is to be allocated to
processes, decides which process will get memory at
what time.
• It tracks whenever some memory gets freed or
unallocated and correspondingly it updates the status.
Prof.Bhushan Pawar

Assumption 3
Memory management
• It consist of 7 operations
– Dynamic Loading
– Dynamic Linking
– Logical versus Physical Address Space
– Swapping
– Memory Allocation
– Fragmentation
– Paging
– Segmentation
Prof.Bhushan Pawar

Any Question???
• If you having any doubt then you can freely
ask me question on
bhushan.pawar@mescoepune.org
Or
contact me on (+91)-7588318728
Prof.Bhushan Pawar
36

Unit 1 introduction to operating system

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Unit 1 introduction to operating system