Survey Paper

1
Survey Paper 1

• Modern Operating Systems
• Unix
• by
• Sajida Begum
• Samina F Choudhry

2
Outline

• Brief History of UNIX
• Goals of UNIX
• Processes and Processes Management
• Memory Management
• File System Architecture and Management
• Interprocess Communication

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Brief History of UNIX

• 1960s MULTICS ( Multiplexed Operating and
  Computing System)
• by General Electric, Massachusetts Institute
  for Technology
• and Bell Laboratories
• 1969 UNICS (Uniplexed Operating and Computing
  System )by
• Bell Labs .Though the OS has
  changed, the name stuck and
• was shortened to Unix
• 1973 rewritten in C language.
• 1974 5th addition of UNIX
• 1978 7th addition a dividing point for
  SVR4(System V) and BSD
• BSD Berkley version of UNIX
• Linux Borrows from both SVR4 and BSD

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Features of UNIX

• UNIX is an operating system made by
  programmers to programmers.
•       Multitasking capability.
•       Multi-user capability.
•       Portability.
•       Easy to understand.
•       Written in High-level language rather that
  assembly language.
•   Open system.

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Processes Process Management

• Process
• The process is OS abstraction for execution.
• - A process is a program (file).
• - It is a unit of execution.
• - It is a dynamic execution context of a
  program.
• A process is sometimes called a job or a task or
  a sequential process.
• A sequential process is a program in execution
• -It defines the sequential,
  instruction-at-a-time execution of a program.

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Process Components

• A process contains all of the state for a program
  in execution.
• An address space.
• The code for the executing program.
• The data for the executing program.
• An executing stack encapsulating the state of
  procedure calls.
• The program counter (PC) indicating the next
  instruction.
• A set of general-purpose registers with current
  value.
• Operating system resources like open file, n/w
  connections etc.

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Process state

• The process has an execution state that indicates
  what it is doing currently ?
• Running Executing instructions on the CPU
• - It is the process that has control of the
  CPU.
• Ready Waiting to be assigned to the CPU
• - Ready to execute but another program is
  executing on
• the CPU.
• Waiting Waiting for an event, eg. I/O
  completion
• - It cannot make progress until event is
  signaled.
• Stopped Not allowed to execute.
• As a process executes it moves from state to
  state

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Process state Graph
Create Process
New
Ready
I/O Done
Waiting
Unscheduled Process
Schedule Process
I/O Page fault etc.
Terminated
Running
Process Exit
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Process Creation in UNIX

• A process is created by another process.
• - Parent is creator and child is created -
  Process user id is inherited
• - After creating a child the parent may either
  wait for it to finish its task or continue in
  parallel or both.
• The processes are created using fork() int fork()
• Fork()
• Creates and initializes a new PCB.(Process
  Control Block)
• Creates a new address space.
• Initializes the address space with a copy of the
  entire contents of the address space of the
  parent.
• Initializes the kernel resources to point to the
  resources used by parent (eg., open file).
• Places the PCB in the ready queue.

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Fork

• Int main( int argc, char argv)
• char name argv0
• int child_pid fork()
• if(child_pid 0)
• printf(Child of s is d\n, name,getpid())
• return 0
• else
• printf(My child is d\n, child_pid)
• return 0
• // What does this program prints ?

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Fork

• Output
• My child is 486
• Duplicating Address
• // Child_pid 486 //
  Child_pid 0
• Child_pid fork() Child_pid
  fork()
• if(child_pid 0) if(child_pid
  0)
• printf(Child )
  printf(Child)
• else
  else
• printf(Parent)
  printf(Parent)
• // Parent
  // Child

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Parents Child processes

• The processes which call fork( ) is the parent.
• The process which result from the fork( ) is the
  child.
• Fork( ) returns 0 if the process is the child
  process
• Fork ( ) returns the PID of the child if the
  process is the parent process.
• In UNIX all the processes ultimately come from
  the init process (PID 1)

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Long-Term Processes Daemons

• Daemon Daemon processes run in the background
  and spend most of their time waiting for an event
• to occur, which they provide them service
• these are started automatically when the system
  is booted.
• a typical daemon cron daemon
• daemons are needed to
• -schedule activities
• -startup periodic activities

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Process Management Description
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Memory Management in UNIX

• The memory management subsystem is one of the
  
• most important parts of the operating
  system.
• It allows
• Large Address Spaces
• Protection.
• Memory Mapping
• Fair Physical Memory Allocation
• Shared Virtual Memory

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Contd

• Physical vs. Virtual Memory
• What is a Page Memory?
• Cache Memory
• How the Kernel Organize Memory
• - Dividing the RAM
• - System and User Areas
• Paging vs. Swapping

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File System in Unix

• Introduction
• What is File System?
• The abstraction used by kernel to represent and
  organize the storage resources.
• UNIX File System in general
• File system is organized in tree structure.
• File tree can be arbitrarily deep.
• File name must NOT LONGER than 256 chars.
• Single path name must NOT LONGER than 1023 chars.

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Creating File System

• Mounting File System
• File tree is composed of File System
• Use mount command to map a directory within the
  existing file tree (mount point) to the root of
  the new file system.
• mount /dev/hda2 /usr
• Use umount command to detach the file system.
• Detaching will fail if the file system is busy.

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Types of files

• Every thing in Unix is a file.
• There are 3 types of files
• Regular files , Directory files and Device files.
• Regular files
• Most common type.
• Hold programs and data.
• Example
• - Source code of a program
• - Email received from a friend
• - Executable programs and applications such
  as (ls, cat etc.,)

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Types of files cont..

• 2. Directory files
• A directory is like a file folder, it can contain
  files, and other directories and hold programs
  and data.
• Like a cabinet in real life, the files that
  contains other files.
• Saves information of files such as location, size
  etc.,
• A directory in the file system tree may have many
  children, but it can only have one parent.

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Types of files (cont.)

• 3. Device Files
• 1. Allows programs to communicate with the
  hardware.
• 2. Kernel modules handles device management.
• - Character Device
• Accepts a stream of characters, without regard to
  any block structure.
• It is not addressable, therefore no seek
  operation
• - Block Device
• Information stored in fixed-sized block.
• It is addressable, therefore seek operation is
  possible.

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Types of files cont..

• Device files (cont..)
• Devices are also represented by special files.
• Each hardware is treated as a file like
  Terminals, keyboards, printers etc.
• Example
• - read the user input from the keyboard device
• - print a text file copy a file to the
  printer device

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Organizing of The File System
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File System Structure
/
lib
user
bin
etc
usr
staff
stud
admin
jon
stev
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File Structure cont..

• Absolute path The path to a location can be
  defined as an absolute path from the root anchor
  point.
• Relative path The path starting from the current
  location.
• Current working directory The place in the
  filesystem tree where you are located is called
  the current working directory.

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INODES

• INODES
• Kernel maintains file information in a structure
  called inode.
• Creation, modification time stamps
• Ownership, file size etc.
• Commonly used INODE information can be found by
  using ls command
• Group information and be modified by using chgrp
  command.

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File Management

• File File is a collection of permanent data with
  a global name.
• - The data are permanent in the sense that
  they
• remain in existence until they are
  destroyed
• explicitly by a process or a user (through
  some
• process).
• - The name is global in the sense that it
  does not
• belong to the address space.
• they have a name
• the name refers to some data
• the data may be examined or modified
• access to the name and the data is controlled

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File and Directory permissions

• Three class of users and three types of
  permissions.
• Classes of Users
• Owner I created the file.
• Group The owner and I are in the same working
  group.
• World I just want to use the file.
• Types of Permissions
• Read permission Read.
• Write permission Alter, add or delete.
• Execute permission Run.

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Access Methods

• Definition An access method defines the way
  processes read and write files.
• Sequential AccessThe file is associated with a
  read/write mark, which is advanced on each
  access. If several processes are reading or
  writing from the same file, then the system may
  define one read/write mark or several.
• Direct Access This access allows a user to
  position the read/write mark before reading or
  writing. This feature is useful for applications
  such as editors that need to randomly access the
  contents of the file.

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Access Methods cont..

• 3. Mapped AccessWhen a process opens a file, it
  is mapped to a segment. The open call returns the
  number of this segment. The process can thus
  access the file as part of its virtual store.
• 4. Structured Files. Database applications
  often wish to treat them as records that may be
  accessed by some key. To accommodate these
  applications, some systems support typed or
  structured files that are considered streams of
  records

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Interprocess Communication using Unix Domain

• Socket A socket is an endpoint of communication
  between processes.
• It is a software entity that provides the basic
  building block for interprocess communications.
• Sockets function either as clients or servers.
• A server typically advertises some service at a
  well-known address and waits for clients to
  request connections.
• Clients requiring a particular service attempt to
  connect to a server at the server's advertised
  transport address.

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Unix Domain Sockets

• Types of Sockets
• Unix Domain Socket UNIX domain sockets
  communicate between processes on the same system
  only.
• Internet domain sockets which can be used to
  communicate with processes on other systems
• Two types of Unix Domain Sockets
• Stream sockets Most commonly used socket, It
  provides bi-directional, sequenced, and
  unduplicated flow of data without boundaries.
• - This service is reliable and
  connection-oriented.
• - Data is transmitted on a full-duplex,
  flow-controlled byte stream.

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Unix Domain sockets cont..

• Cont.. - Message boundaries are not preserved on
  a stream socket.
• - it is supported by the Internet
  transport protocol TCP.
• 2. Datagram socketsA datagram service is
  connectionless and unreliable.
• - used in applications where reliability of
  an individual packet is not essential.
• - message boundaries are maintained. This
  means that individual datagrams (i.e., messages
  sent with separate calls) will be kept separate
  when they are received at the other end.
• - it is supported by the Internet transport
  protocol UDP.

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How to create a socket ?

• s socket(domain, type, protocol)
• domain The domain is specified as a manifest
  constant.
• Type, Types are SOCK_STREAM and SOCK_DGRAM
• Protocol is a set of communication conventions
  for regulating the exchange of information
  between two parties eg. TCP, UDP.