Operating Systems

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Operating Systems

• Amnon Meisels Ehud Gudes
• Office Hrs (314) (303)
• Tuesday 1000-1200 Thursday
• am_at_cs.bgu.ac.il os032/ ehud_at_cs.bg
  u.ac.il

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Operating Systems (OS) - Syllabus

• 1. Introduction - History Views Concepts
  Structure
• 2. Process Management - Processes State
  Resources Threads Unix implementation of
  Processes
• 3. Process Scheduling Paradigms Unix
  Modelling
• 4. Process Synchronization - Synchronization
  primitives and their equivalence Deadlocks
• 5. File Systems - Implementation Directory and
  space management Unix file system Distributed
  file systems (NFS)
• 6. Memory Management - Virtual memory Page
  replacement algorithms Segmentation

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OS - Syllabus (Contd.)

• 7. Security - General policies and mechanisms
  examples of common problems protection models
  authentication
• 8. Distributed Systems a few comments
• Books
• A. Tanenbaum Modern Operating Systems,
  Prentice-Hall, 2nd Edition, 2001
• A. Silbetschatz et. al. Operating System
  Concepts (6th ed.), Addison Wesley,2001.
• G. Nutt Operating Systems (a modern perspective)
  (2nd ed.), Addison Wesley, 1999.
• W. Stallings Operating Systems (3rd ed.),
  Prentice-Hall, 1998.

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Assignments and Grades
Date due Weight Subject Assignment
Week 4 5 A Toy Shell Practical 1
Week 8 15 Processes, Scheduling Practical 2
Week 12 10 Memory Management Practical 3
Week 9 15 up to Files Midterm
? 60 All Final
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Introduction

• Why do we need an operating system ?
• Basis for all software
• Simplifies access (for user processes) to
• Memory
• Permanent Data
• i/o extended machine
• Security and Protection
• of executing user programs
• of resources (devices)

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Layered Hardware-Software Machine Model
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What is an Operating System ?

• The two Views of an Operating System
• Top-down View
• An Extended Machine
• Bottom-up View
• A Resource Manager

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Operating Systems - Extended Machines

• Problems
• Bare machine has a complex structure
• Processors
• Many Devices
• Primitive Instruction Set
• Different for Different Machines
• Solutions provided by the OS
• Simple, easier to use interface
  (machine-independent)
• Hiding of unnecessary details

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Operating Systems as Resource Managers

• Many Resources
• Processors Memory
• Disks Tapes Printers
• Network interfaces Terminals
• Controlled allocation of Resources among
• Users Programs Processors
• Control means sharing/multiplexing, monitoring,
  protection, report/payment

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Multiprogramming Why how its possible?

• CPU much faster than I/O
• Memory large enough (512K!)
• requires protection!
• Scheduler which manages flow of jobs in and out
• and shares CPU between jobs requires
  Timer
• Spooler which takes care of slow I/O (e.g.
  printing).

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Some relevant Hardware aspects
(a)
(b)

• (a) Steps in starting an I/O device and getting
  interrupt
• (b) How the CPU is interrupted

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Memory Protection

• Minimal protection must include the interrupt
  vector and service routines
• General memory protection can use two registers
  -
• base register - smallest legal address
• limit register - size
• memory outside the range is protected
• Hardware performs these checks in all user-mode
  memory references
• The load instructions for the two registers are
  privileged
• In supervisor mode the OS has unrestricted
  access to all memory

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Memory Protection
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Memory protection (Cont.)

• One base-limit pair and two base-limit pairs

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User mode lt--gt Privileged mode (I/o)

• All I/o instructions are privileged instructions
  
• I/o devices and cpu can execute concurrently
• cpu moves data main memory --gt buffers of
  device controllers
• I/o itself is from device to controllers buffer
• Device controllers interrupt upon completion
• Interrupts or Traps enable mode switching..
• control to interrupt service routine (interrupt
  vector)
• Architecture saves address of interrupted
  instruction
• Operating systems are interrupt-driven

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Interrupts part of fetch-and-execute

• While (halt flag not set during execution) /
  every fetch-and-execute/ IR
  memoryPCexecute(IR)PCIf(Interrupt_Reques
  t) memory0 PC PC memory1
• Interrupt handlers loop over all device numbers
  and call the device_handler for which a flag done
  is raised
• The interrupt handling routine uses a
  disable_interrupts instruction to avoid losing
  data while processing an interrupt request
• Hardware may lose queuing interrupts, when the
  Interrupt_Disabled flag is raised and an
  interrupt occurs

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Processes - a central concept

• Programs in Execution
• Timesharing -
• Process Suspension
• Process Table
• Core image
• Process Tree
• Signals
• Uids - Protection...

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Operating System Concepts

• A process tree
• A created two child processes, B and C
• B created three child processes, D, E, and F

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Operating System Concepts

• Two processes connected by a pipe

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Files non volatile data

• File types and operations on files
• Directories - hierarchical structure
• Working directories

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Files non volatile data (cont.)

• Protection and Security
• Unix - user group other (rwx bits)
• File descriptors (handles)
• i/o as a special file
• Block Character special files
• Standard input output error the idea of Late
  binding
• Pipes

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Operating system concepts System calls

• An interface from privileged or system mode to
  user mode (user programs)
• System calls
• create, delete, use various objects
• Examples (Unix)
• fdopen(file_name, mode)
• close(fd) kill(pid,sig) fork()

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A word on System Calls

• Look like standard procedure calls
• put parameters in Registers and TRAP
• operation is performed in privileged mode
• some system calls can be accessed through the
  command interpreter (Shell)
• redirection gt
• pipe
• Do not wait for offspring process

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Steps in Making a System Call

• There are 11 steps in making the system call
• read (fd, buffer, nbytes)

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Some System Calls For Process Management
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Some System Calls For File Management
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Some System Calls For Directory Management
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Some System Calls For Miscellaneous Tasks
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System Calls

• Some Win32 API calls

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The Shell Commands Language

• sort lt file1 gt file2
• cat file1 sort lpr
• The Shell is a process which executes its
  commands as offspring processes
• Processes may call shell commands by using the
  system system call

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Shell structureParent child

• A stripped down shell
• while (TRUE) / repeat forever /
• type_prompt( ) / display prompt /
• read_command (command, parameters) / input
  from terminal /
• if (fork() ! 0) / fork off child process
  /
• / Parent code /
• waitpid( -1, status, 0) / wait for
  child to exit /
• else
• / Child code /
• execve (command, parameters, 0) / execute
  command /

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Initializing a (user) shell

• The init program runs getty on all ports
• Detecting a terminal getty runs login
• Typing in a user name and a password login
  checks the passwd file and if correct runs a
  shell the one specified in the UID entry
• The shell is run with that user ID environment
  parameters
• The user process runs the shell

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Running user commands

• User types grep some_word file_name
• Shell parses the command, inserts the strings
  grep, some_word, file_name into argv and argc
• Next, the shell uses fork() to create a process
  (same user ID)
• Now, it takes the executable name grep and the
  arguments, all from argv, and uses execve() or
  similar system calls to run the grep executable
• Normally the shell uses the wait() system call to
  be rerun after grep exits one user process
  running

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Small-Shell smash

• Your assignment is to build a program smash'
  that will implement the following shell services
  
• Run programs in foreground and background
• List all processes that currently run in the
  background.
• Kill a process running in the background.
• Kill or suspend programs running in foreground
  mode using signals.
• Move programs that run in foreground mode to
  background mode and vice versa.

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OS - Main Components

• Process management
• process creation deletion suspension
• process synchronization communication
• Main-memory management
• Manage used parts and their current users
• Select processes to load
• Allocate memory to running processes
• Secondary storage management
• Free-space management
• Storage allocation

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OS - Main Components (cont.)

• File system management
• File directory creation deletion
• File manipulation primitives
• Mapping files onto secondary storage
• I/o system management
• general device-driver interface
• Drivers for specific hardware devices
• Protection system
• Distinguish between authorized and unauthorized
  usage
• Provide means of enforcement

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OS - Main Components (cont.)

• Networking
• A distributed system is a collection of
  processors that do not share memory or a clock
• The processors are connected through a
  communication network
• Provides user access to various system resources
• Command-interpreter System
• control statements that deal with process
  creation and management I/o handling
  file-system access protection networking
• The program that reads and interprets - shell (in
  Unix)

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Operating system structure

• Monolithic systems have little structure

Service Routines
Utility procedures
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Virtual Machines

• The extreme layered approach
• Provide an interface identical to the underlying
  bare machine
• OS creates multiple processes, each executing on
  its own processor and own (virtual) memory
• Virtual machines provide complete protection of
  system resources - even separate resources
• Difficult to implement, due to the effort
  required to provide an exact duplicate of the
  underlying machine
• Recent use run MS-DOS on top of Windows

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Virtual Machines
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Virtual Machines

• Virtual machine monitors for operating systems

System call
System call
CMS
Trapped
CMS
CMS
VM/370
Trapped
370 Bare Hardware
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Client-Server Model
Memory Server
File Server
Client Process
. . . . . .
Client Process
Kernel
Machine4
Machine3
Machine1
Machine2
Client
File Server
Process Server
. . . .
. . .
Kernel
Kernel
Kernel
Kernel
Network
Distributed System
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Mechanism vs. Policy

• Simple Kernel - modularity minimal privileged
  operation
• Servers for files, memory, etc. - distribution
  user mode operation
• good for distributed systems
• Mechanism in kernel - how to do things..
• Policy outside - decide what to do can be
  changed later..
• Another solution Critical servers in kernel

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• Fig. 9-7. A Distributed System consisting of
  workstations on a LAN.
• Fig 9-1. Advantages of distributed systems over
  centralized ones.

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UNIX

• UserInterface

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UNIX Utility Programs

• A few of the more common UNIX utility programs
  required by POSIX

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UNIX Kernel

• Approximate structure of generic UNIX kernel

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Case Study 2 Windows 2000

• Chapter 11

11.1 History of windows 2000 11.2 Programming
windows 2000 11.3 System structure 11.4
Processes and threads in windows 2000 11.5
Memory management 11.6 Input/output in windows
2000 11.7 The windows 2000 file system 11.8
Security in windows 2000 11.9 Caching in windows
2000
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Windows NT

• Some differences between Windows 98 and Windows NT

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Windows 2000 (1)

• Different versions of Windows 2000

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Windows 2000 (2)

• Comparison of some operating system sizes

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