Operating Systems

1
Operating Systems

• Shyh-Kang Jeng
• Department of Electrical Engineering/
• Graduate Institute of Communication Engineering
• National Taiwan University

2
Operating Systems

• Interface between a user and the computer
  hardware
• Provide an environment in which a user can
  execute programs
• Goals
• Make the computer system convenient to use
• Use the computer hardware (resources) in an
  efficient manner

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Batch Processing
Especially suitable for payroll processing
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Interactive Processing
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Real-Time Processing and Time-Sharing Systems

• Real-time processing
• Processing whose response time is restricted
• Time-sharing systems
• Shuffle jobs by dividing time into intervals
• Multitasking for a one-user system
• Efficient than the sequential way especially for
  jobs which must wait for peripheral devices

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Multiprocessor Systems

• Sharing information and resources among different
  machines
• Networks
• Coupled computer systems
• Software controlling a network is as a
  network-worldwide operating system
• Load balancing
• Dynamically allocate tasks to various processes
  so that all processors are used efficiently
• Scaling
• Break tasks into a number of subtasks compatible
  with the number of processors available

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Scalability

• Capability of a system to adapt to increased
  service load
• A scalable system reacts more gracefully to
  increased load than does a non-scalable one
• A scalable system has the potential to grow
  without problems like generating additional
  indirect load on other resources or incurring
  design modification

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Application and System Software

• Application software
• Performs tasks particular to the machines
  utilization
• System software
• Performs tasks common to computer system in
  general
• OS utility software
• Utility software
• Collection of software units extending the
  capabilities of OS

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Software Classification
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Shells
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Shells

• Portion of an OS to define the interface between
  the OS and its users
• Types of shells
• Textual interface
• e.g., Borne shell, C shell, Korn shell in Unix
  MS-DOS for Microsoft Windows
• Graphical user interface
• e.g., WinXP, Mac
• Window manager

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Kernels

• Performs the very basic functions required by the
  computer installation
• Main components
• File manager
• Device drivers
• Memory manager
• Scheduler and dispatcher

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File Manager
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File Systems

• Directory or folder
• Path C\animals\prehistoric\dinosaurs
• Any access to a file by other software units is
  obtained at the discretion of the file manager
• File descriptor
• Information needed to find and manipulate the file

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Device Drivers
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Device Drivers

• Software units that communicate with the
  controllers to carry out operations on the
  peripheral devices attached to the machine
• Each device driver is uniquely designed for its
  particular type of device
• Example Device driver for a printer contains
  software to read and decode status word as well
  as other handshaking details. Other software
  does not have to deal with those technicalities
  in order to print a file
• A generic OS can be customized for particular
  peripheral devices by merely installing
  appropriate device drivers

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Memory Manager

• Charged with the tasks of coordinating the
  machines use of main memory
• Such duties are extensive in multi-user or
  multitasking environments
• Many programs and blocks of data reside in main
  memory concurrently
• Must find places to fulfill memory requirements
  and keep track of memory areas no longer occupied

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Virtual Memory

• Illusion of additional memory space by rotating
  programs and data back and forth between main
  memory and mass storage
• Useful when total main memory space required
  exceeds the space actually available in the
  machine
• Divides the required space into units called
  pages and stores these pages in mass storage
• Memory manager exchange pages required in memory
  for pages that are no longer required
• Other software units can execute as though there
  were large main memory in the machine

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PC Memory Manager

• 4 GB addressable
• space for 32-bit
• addressing
• Virtual memory
• Page
• Page fault

Mass Storage
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Scheduler and Dispatcher

• Scheduler
• Determines which activities are to be considered
  for execution in a time-sharing system
• Dispatcher
• Controls the allocation of time slices to the
  scheduled activities

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Boot Strapping (Booting)

• Procedure performed by the machine each time it
  is turned on
• Read-only memory (ROM) vs. Random access memory
  (RAM)
• A small part of the main memory is constructed
  from ROM
• Bootstrap program is stored in ROM and executed
  automatically when the machine is turned on

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Bootstrap Program

• Directs the CPU to transfer material from a
  predetermined location in mass storage into the
  volatile area of main memory
• In most cases this material is the operating
  system
• Once the operating system has been placed in main
  memory, the bootstrap program directs the CPU to
  execute a jump to that area of memory
• At this point, the operating system takes over
  and begins controlling the machines behavior

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Booting Process
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Bootstrap in Personal Computers

• Try to extract the operating system from a floppy
  disk first
• If no such disks are found, the bootstrap will
  automatically try to extract the operating system
  from the machines hard disk
• If floppy disk is inserted, and that disk does
  not contain a copy of the operating system, the
  bootstrap will pause with an error message to the
  machines operator

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Concept of Process

• Program
• A static set of directions (passive entity)
• Process
• Program (application and utilities) in execution
  (active entity)
• Needs resources to accomplish its task
• Resources are given to the process when created
• Can be executed in parallel
• A single program can be associated with more than
  one process, each with its own data and rate of
  progress

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Process Administration

• Handled by scheduler and dispatcher within kernel
• Scheduler
• Maintains a process table
• Dispatcher
• Ensures the scheduled processes are actually
  executed
• Time slice (or quantum) typically no more than
  50 milliseconds
• Process switch

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Process Table

• Record of the process present in the computer
  system
• Contains information of
• Memory area assigned to the process
• Priority of the process
• Status of the process
• Ready can continue
• Waiting currently delayed until some external
  event occurs

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Program and Processes
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Processes in Windows
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Time-Sharing Between Processes
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Process Switch

• The procedure of changing from one process to
  another
• Occurs when an interrupt signal is generated at
  the end of the quantum
• Process state of an executing process is saved
  and the process becomes idle
• Interrupt is handled by interrupt handler
• Process state of the process to be executed next
  is loaded
• The new process starts running

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Process State

• A snapshot of the machine at that time
• Defined by its current activity
• Includes program counter, values in CPU registers
  and associated memory cells, etc.
• Changes as a process executes

new
ready
running
halted
waiting
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Interrupt Handler

• Part of the dispatcher
• Preempt the current process and transfer control
  back to the dispatcher
• At this point, the dispatcher allows the
  scheduler to update the process table (e.g.,
  changing priorities)
• The dispatcher then selects the process from the
  process table that has the highest priority among
  ready process
• Restarts the timer circuit, and allow the
  selected process to begin its time slice

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Terminating a Time Slice Early

• For example, if a process executes an I/O
  request, the time slice of that process will be
  terminated
• The scheduler will update the process table to
  reflect the processs waiting status
• The dispatcher will assign a new quantum to a
  process that is ready
• When the controller indicates that I/O request
  has been completed, the scheduler will reclassify
  the process as ready, and the process will again
  compete for a time slice

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Mini Review

• If each quantum in a time-sharing system is 50
  milliseconds and each context switch requires at
  most a microsecond, how many processes can the
  machine service in a single second?
• If each process uses its complete quantum in the
  machine in last question, what fraction of the
  machines time is spent actually performing
  process? What would this fraction be if each
  process executes an I/O request after only a
  microsecond of its quantum?

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Answer

• If each process consumed its entire time slice,
  the machine could provide a complete quantum to
  almost 20 processes in one second. If processes
  did not consume their entire time slices, this
  value could be much higher, but then the time
  required to perform a context switch may become
  more significant

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Answer (continue)

• A total of 5000/5001 of the machines time would
  be spent actually performing processes. However,
  when a process requires an I/O activity, its time
  slice is terminated while the controller performs
  the request. Thus if each process made such a
  request after only one microsecond of its
  quantum, the efficiency of the machine would drop
  to ½.

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Threads
Thread
Thread
Thread
Thread
CPU
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
J. Richter ??, ?????, Microsoft Windows
??????????, ??, 2001
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Interprocess Communication

• To coordinate activities of processes
• For examples,
• Scheduling a new process the scheduler must
  obtain memory space from the memory manager
• Accessing a file in mass storage the process
  must first obtain information from the file
  manager

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Client/Server Model forInterprocess Communication
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Client/Server Model

• Client makes requests
• Server satisfies the requests made by clients
• Simplifies inter-process communications
• To gain uniformity among all types of
  communication taking place in the system, no
  matter they are at the same machine or different
  machines

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Client-Server Communications
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Communications between Software
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Common Object Request Broker Architecture (CORBA)
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Allocation of Resources

• An important task of an operating system is the
  allocation of resources to the processes in the
  system
• Resources machines peripheral devices as well
  as features within the machine
• Access to files and disk space (file manager)
• Memory space (memory manager)
• Space in process table (scheduler)
• Time slices (dispatcher)

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Competition among Processes

• Two alternatives
• Disable interrupt in test and set
• Test and set as a single instruction

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Competition for Resources

• Assume a process needs to print
• Request the OS to give it access to the printers
  device driver
• OS decide whether to grant the request, depending
  upon whether the printer is already being used by
  another process
• If the printer is used by another process, the OS
  should deny the request and classify the process
  as waiting until the printer is ready
• If two processes were given simultaneous access
  to printer, the results would be worthless to both

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Using Flag to Control Access

• OS must keep track of whether the printer has
  been allocated
• Use a flag (a bit in memory) as set or clear to
  indicate that the printer is currently allocated
  or not
• Testing and possibly setting the flag requires
  several machine instructions

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A Possible Problem in the Flag Approach
Process 2
OS
Printer Device Driver
Process 1
Check flag
Request for printer
Grant access to printer
clear
Interrupt process
Check flag
Request for printer
clear
Grant access to printer
Set flag
Access printer
Resume process
Access printer
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Two Solutions

• Use the interrupt disable and interrupt enable
  instructions provided in most machine language
• When OS starts the flag-testing routine, it
  disables the interrupt, and ends it by enabling
  the interrupt
• Use the test-and-set instruction available in
  many machine language
• Direct the CPU to retrieve the flag, note the
  value received, and set the flag, all within a
  machine instruction

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Semaphores

• Properly implemented flag

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Critical Region and Mutual Exclusion
???, Visual C 6 ????????, ????, 1999.
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Critical Region and Mutual Exclusion

• Critical region A sequence of instructions that
  executed by only one process at a time
• Mutual exclusion Requirement that only one
  process at a time to execute a critical region

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Examples of Deadlock

• One process has access to printer but is waiting
  for tape drive, while another process has access
  to tape drive but is waiting for printer
• Scheduler has no space left in the process table
  and each process in the system must create an
  additional process before it can complete its task

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Deadlock

• A situation that two or more processes are
  blocked from processing because each is waiting
  for access the resources allocated to another
• Degrade systems performance
• Deadlock handling
• Prevention
• Avoidance
• Detection and correction

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Deadlock
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Prerequisites of Deadlock

• Competition for non-sharable resources
• Resources are requested on a partial basis
• Once a resource has been allocated, it can not be
  forcibly retrieved

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Removing Deadlocks

• Deadlock detection and correction schemes
• Requiring each process to request all the
  resources at one time
• Converting non-shareable resources into shareable
  ones

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Examples of Removing Deadlock

• If deadlock occur due to a full process table,
  the administrator merely uses his powers to
  remove (kill) some of the process
• Spooling holding data for output at a later but
  more convenient time
• Each time a process requires the printer, the OS
  grants the request. However, OS connects the
  process to a device that stores the information
  to be print on a disk
• When the printer is available, OS transfers the
  data from the disk to printer

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Mini Review

• Suppose the following solutions have been
  proposed for removing the deadlock that occurs on
  a single-lane bridge when two cars meet.
  Identify which condition for deadlock is removed
  by each solution
• Do not let a car onto the bridge until the bridge
  is empty
• If cars met, make one of them back up
• Add a second lane to the bridge

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Answer

• This guarantees that the non-shareable resource
  is not required and allocated on a partial basis
• This means that the non-shareable resource can be
  forcibly retrieved
• This makes the non-shareable resource shareable,
  which removes the competition

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Another Competition Problem

• A file manager could grant several processes
  access to the same file if the processes merely
  read data
• Conflicts occur if more than one process tries to
  alter a file at the same time
• File manager may allow only one process have
  write access at any given time
• Other systems divide the file into pieces so that
  different processes can alter different parts of
  the file concurrently

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Security from outside attacks

• Most common protection require user name and
  password
• Problem password stealing
• Problem automated password guessers
• Countermeasures
• Always tell user when he/she last logged in
• Report repeated bad guesses
• Log the guesser into a captive account to spy on
  the guesser

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Security from attacks from within

• Operating system prevents illegal access to
  resources
• Different memory for different processes
• Privileged instructions only allowed in kernel
• All file access passes through the kernel
• Other devices can only be accessed through the
  kernel

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Homework 3

• 4, 7, 10, 11, 14, 18, 25, 29, 31, 37, 41