Operating Systems

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Operating Systems

• Amnon Meisels
• Office Hrs (314) Monday 1000-1200
• am_at_cs.bgu.ac.il os002/

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Operating Systems (OS) - Syllabus

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

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OS - Syllabus (Contd.)

• 6. Security - General policies and mechanisms
  examples of common problems protection models
  authentication
• 7. Input/Output - Interrupts Drivers Disks
• 8. Distributed Systems - Inter Process
  Communication (RPCs) Synchronization
• A. Tanenbaum Modern Operating Systems,
  Prentice-Hall, 1992.
• A. Silbetschatz et. al. Operating System
  Concepts (5th ed.), Addison Wesley, 1997.
• 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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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

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Operating Systems History

• Computer hardware evolved in 4 steps
• Vacuum tubes Plug boards (1945-1955)
• One user per machine --gt User as Operator
• Early Software - Assemblers, compilers,
  Libraries of common subroutines, Device drivers
• Secure
• Inefficient use -
• low CPU utilization
• Significant set-up time
• No concept of an Operating System

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B. Batch systems - Transistors (1955-1965)

• Cards into card readers
• Batch similar jobs (to save on set-up time)
• Transferring control automatically, from one
  batch job to another --gt a rudimentary OS
• Job Control Language (JCL)
• Program LOAD RUN Data EOF.
• Resident monitor (user ? operator)
• Initial control in monitor
• transfers control to job
• upon completion, control transfers back to
  monitor

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C. Multiprogramming - Integrated Circuits
(1965-1980)

• Several programs reside on disk (and in memory)
  at the same time

Memory Partitions
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C. Multiprogramming (Contnd.)

• I/o routines supplied by the system - device
  allocation
• Memory management - allocate memory among jobs
• Processes contend for CPU time - cpu scheduling
• Spooling (Simultaneous Peripheral Operations On
  Line)
• Timesharing i.e. Terminals - jobs kept in
  memory and on disk control statements issued
  by user (keyboard)
• On-line file system is needed, for users access
  to data
• Examples CTSS Multics Unix.

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D. VLSI and personal computers (1980-

• Machine per user
• I/o devices - keyboards, displays, mice,
  printers
• Friendly operating software
• Examples Unix (here too ?) Windows-NT
• Parallel (multiprocessor) systems shared
  memory symmetric processing - same OS (no
  master)
• Networks (distribution)
• Distributed file systems - shared among many
  systems
• Network operating systems - protection for
  simple OSs
• Distributed operating systems - distribute
  computation among several machines/processors

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Operating system concepts - Protection

• 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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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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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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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 instructiones 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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CPU Protection

• Timer - interrupts computer after specified
  period, to ensure OS maintains control
• timer is decremented every clock tick
• when timer reaches zero, an interrupt occurs
• timer is set whenever control is transferred to
  user processes (or any process..)
• timer is mainly used to implement time-sharing or
  multiprocessing
• load-timer is a privileged instruction

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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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and in the 1st Assignment ..

• Processes are simulated by threads
• No process tree, but, the special thread - the
  Shell - creates all other threads
• The process table is very simplistic, the
  Scheduler keeps track on whos ready
• Inter process communication is enabled for all
  threads - performed by the Mailer
• Threads have Ids, essential for the routing

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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 (contnd.)

• 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
• Pipes

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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 (contnd.)

• 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 (contnd.)

• 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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Layered Systems

• Layers of detail hiding, protection
• passing parameters through layers - inefficient..

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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

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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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and in the 1st Assignment ..

• The Mailer is a Server
• All threads are Clients
• There are only two modules in the System
• the Scheduler
• the Mailer
• The Mailer requests services from the Scheduler
  thread_status --gt suspended

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and in the 1st Assignment ..

• System calls are simulated by
• send(), receive()
• But, there are all the real system calls
• suspend(), block(), release()
• So, the best policy is to let these
  extra-functions be called from the system
  module
• Remember, the (global) function called is part
  of the thread of control...

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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