Operating Systems

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Lecture 12

• Operating Systems
• (SG, 6.4)

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Read SG ch. 7,7.1 7.8(High-Level
Languages)for next week
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Operating Systems

• Wait for user requests
• Launch programs (e.g., assemblers, loaders) to
  service those requests
• Provide many other services to the user

(slide lt C. Hundhausen)
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User Interface

• Operating systems most important task Wait for
  user requests and process them
• Receptionist
• Dispatcher
• The user interface performs this task

(slide adapted lt C. Hundhausen)
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Text-OrientedUser Interface

• Traditional user interfacecommand line
• e.g., DOS, Linux, UNIX
• gt cd mydocs/history
• gt ls
• gt ls al gt out.txt
• gt grep "http" .html more

(slide adapt. lt C. Hundhausen)
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GraphicalUser Interface(GUI)

• Modern alternative Graphical User Interface
• Pioneered by Apple Macintosh (1984)
• Windows, Icons, Menus, Pointer (WIMP)
• Directories represented by folder icons
• Files represented by specialized icons

(slide adapt. lt C. Hundhausen)
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Security

• In the 50s and 60s, physical security restricted
  access to computers and the files and programs
  they stored
• In modern times, that responsibility has shifted
  to the operating system
• Operating system controls access to computer by
  requiring users to log in with a username and
  password

(slide lt C. Hundhausen)
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Encrypted Passwords

• OS encrypts passwords so that they cant be
  stolen
• Encryption an active research area in computer
  science
• Need both encrypted text and algorithm for
  decoding it
• Modern public key cryptography based on
  extremely large prime numbers

(slide lt C. Hundhausen)
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File Access Permissions

• OS restricts users from accessing the files of
  others
• Files can be set so that they are readable,
  writable, and executable by only a limited set of
  people, e.g.
• Only the owner
• Only users in the owners group
• Anyone
• Authorization lists maintain this information
  they are encrypted so that they cannot be
  modified by unauthorized users
• OS prevents you from harming yourself

(slide adapt. lt C. Hundhausen)
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Hierarchy of File Access Permissions

• Delete
• Change
• Append
• Read

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Process Scheduling

• Operating system ensures that the processor is
  being utilized efficiently
• While waiting for I/O operations to complete, the
  computer can do useful work
• This is done by maintaining a queue of processes
  (programs) to be run
• Programs have three status levels
• Running
• Ready
• Waiting

(slide lt C. Hundhausen)
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Process Scheduling (2)
waiting ready running event
A, B, C D
D B, C
A
A, D C
B
B, D C
A
Etc.
(slide adapt. lt UT Austin)
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Deadlock

• Operating system aims to prevent deadlock a
  state in which no useful work can be done
• How deadlock happens (example)
• Programs A and B both want to print a file
  located on the CD-ROM
• A requests CD-ROM, then printer
• B requests printer, then CD-ROM
• A obtains CD-ROM, but is told that printer is not
  available
• B obtains printer, but is told that CD-ROM is not
  available
• Deadlock!
• Both are waiting for a resource allocated to the
  other
• the only way to end the wait is for one of them
  to free the resource that the other needs

(slide lt C. Hundhausen)
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Deadlock Prevention

• How to prevent deadlock
• If all resources needed by a program are
  available
• allocate the resources to the program
• If not all resources needed by a program are
  available
• do not allocate any resources to the program
• require the program to make a new request for
  resources
• This algorithm would prevent deadlock in the
  preceding example
• A and B both want to print a file located on
  CD-ROM
• A requests CD-ROM and printer
• A obtains CD-ROM and printer
• B requests printer and CD-ROM. The request is
  denied
• A finishes and releases CD-ROM and printer
• B re-requests printer and CD-ROM
• B obtains printer and CD-ROM
• Deadlock is prevented!

(slide adapt. lt C. Hundhausen)
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First Generation (194555)

• No operating systems
• Assemblers and loaders only
• Programmers themselves managed allocation of
  resources and execution of programs
• They signed up for blocks of time
• They brought in their punched card programs
  during their allocated block of time
• They manually loaded punched cards into computers
• They pressed buttons on console to initiate
  assembler translation process
• They manually loaded computer program into memory
  and began its execution
• This was a very tedious process!

(slide adapt. lt C. Hundhausen)
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Second Generation (195565)

• 1st generation systems often sat idle, because
  programmers spent a lot of their allocated time
  thinking and troubleshooting
• Led to development of batch operating systems
• Computer programmers hand in programs to computer
  operator, who runs collections of programs in
  batches

(slide adapt. lt C. Hundhausen)
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Second Generation (2)

• Command languages introduced
• Enable programmers to specify to operating system
  what they want done
• Usually a mix of programs, data, and commands
• Role of operating systems as receptionist and
  dispatcher was born

(slide adapt. lt C. Hundhausen)
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Third Generation (1965-1985)

• As computational speeds improved, it became
  unacceptable for computers to sit and wait for
  I/O operations
• The goal Minimize CPU idle time
• Multiprogramming operating systems were born
• Many user programs loaded simultaneously into
  memory
• If program has to pause for I/O, another program
  begins executing
• Substantial improvements in CPU utilization were
  realized

(slide adapt. lt C. Hundhausen)
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Third Generation (2)

• Issue With multiple programs in memory, programs
  run risk of corrupting other programs by writing
  in their memory space
• Solution
• Keep track of beginning and ending address of
  each programs memory space
• If a program attempts to reference/write to
  memory outside of its boundaries
• report an error message
• shut down the program
• resume execution of another program

(slide adapt. lt C. Hundhausen)
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Third Generation (3)

• Time sharing systems emerged
• Took advantage of emerging network technologies
• Like multi-programming OS
• But programs do not have to be loaded into memory
  in advance
• Rather, programs can be loaded dynamically by
  users sitting at remote terminals illusion of
  single-user computer
• Need for computer security emerged, because
  computer could now be accessed remotely
• Since many users accessed the computer
  simultaneously, I/O events could no longer be the
  only event that triggered a switch to a new
  program
• A given program is run for a designated time
  slice
• When time slice is up or an I/O operation occurs,
  the CPU begins running another program
• This proceeds in round-robin fashion

(slide lt C. Hundhausen)
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Fourth Generation (1985-present)

• Personal computers became cheap and powerful
  enough that they could get work done more
  efficiently than a timeshare system
• At the same time, computer peripherals remained
  expensive
• This suggested that local computation was
  desirable, with remote access to more expensive
  peripherals
• Return to single-user computer but with much
  better OS

(slide lt C. Hundhausen)
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Fourth Generation (2)

• Network operating systems emerged
• Manages resources of local computer (client)
• Manages shared resources (servers) of a local
  area network (LAN)
• Typical servers include file, mail, and print

(slide adapt. lt C. Hundhausen)
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Fourth Generation (3)

• Real-time operating systems
• Manage resources of embedded systems that are
  placed inside equipment (e.g., automobiles,
  airplanes, ovens, watches)
• Prioritizes requests, so that most critical ones
  are serviced first
• E.g., request for collision avoidance in an
  airplane would be serviced before request to turn
  up the heat in the cabin

(slide lt C. Hundhausen)
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Future Generations

• Multimedia, multimodal, and tangible user
  interfaces (speech, gesture, virtual reality,
  etc.)
• Issue commands via speech, gesture, interaction
  with tangible objects
• Parallel processing
• Distributed operating systems
• No boundaries between local area networks and
  global networks
• Users no longer need to be aware of where a
  resource is coming from
• Users can utilize global resources seamlessly
  as though they are local resources
• But, physical location is not irrelevant

(slide lt C. Hundhausen)
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Ad Hoc Networks

• Wireless mobile communication
• No fixed network structure (pattern of
  interconnectivity)
• Each node discovers keeps track of which other
  nodes it can communicate with
• Messages are routed in accordance with current
  configuration of nodes
• Self-organize adapt like social networks

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Smart Dust

• Currently available motes
• Bottle-cap size
• 100200 each (1 in 5 years)
• Sense temperature, light, motion, energy use,
  GPS, gas, pressure,
• Set up ad hoc network
• Battery lasts for years
• 8K program memory, 512K RAM
• coded in C, runs TinyOS
• See Dust Inc. lthttp//www.dust-inc.comgt
• Privacy issues?

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(fig. lt IEEE Computer)