G53OPS Operating Systems

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G53OPSOperating Systems

• Graham Kendall

Operating Systems
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Introduction to Operating Systems

• This section of the course based on (Tanenbaum,
  1992) Chapter 1
• Module Content
• What is an Operating System?
• History of Operating Systems
• Operating System Concepts
• Operating System Structure
• References
• Levy, S. 1994. Hackers.
• Tanenbaum, A., S. 1992. Modern Operating Systems.
  Prentice Hall.

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Introduction to Operating Systems

• Give three functions of an operating system

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What is an Operating System? - 1
5
What is an Operating System? - 2

• Physical Devices
• Integrated circuits, cathode ray tubes, wire etc.

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What is an Operating System? - 2

• Microprogramming
• Firmware
• Provides basic operations that allow
  communication with the physical devices
• Normally in Read Only Memory (and that is why it
  is called firmware)

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What is an Operating System? - 3

• Machine Language
• Defines the instruction set of the computer
• Software but is considered as hardware

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What is an Operating System? - 3

• Operating System
• Layer between the hardware and the software
• Allows us to use the hardware in a user friendly
  way
• Layer of abstraction

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Other Views of an Operating System - 1

• A Resource Manager
• Operating System is seen as a way of providing
  the users of the computer with the resources they
  need at any given time.
• Some resource requests may not be able to be met
  (memory, CPU usage etc.) but the operating system
  is able to deal with scheduling problems such as
  these.
• Other resources have a layer of abstraction
  placed between them and the physical resource
  (e.g. a printer).

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Other Views of an Operating System - 2

• Extended or Virtual Machine
• The operating system is seen as a way of not
  having to deal with the complexity of the
  hardware (e.g. floppy disc controller).

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History of Operating Systems - 1

• Charles Babbage - Analytical Engine.
• Attributed with designing the first digital
  computer
• Never built the computer
• No operating system.

• Charles Babbage - 26th December 1791 to 18th
  October 1871

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History of Operating Systems - 1

• First Generation (1945-1955)
• Developed during the war
• Vacuum Tubes
• Many people were developing automatic calculating
  machines
• Filled entire rooms
• No operating system
• No programming languages
• Had to physically wire the computer to carry out
  the intended instructions

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History of Operating Systems - 2

• Second Generation (1955-1965)
• Transistors
• More reliable
• Became feasible to manufacture computers that
  could be sold to customers
• Job of computer operator invented
• Batch jobs introduced in order to improve
  throughput (via tapes)
• Can be seen as the first operating system

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History of Operating Systems - 3

• Third Generation (1965-1980)
• Integrated Circuits as a replacement for
  transistors
• IBM introduced its System/360 range and ICL
  introduced its 1900 range
• The start of multiprogramming which led to a need
  for a more complex operating system but no
  virtual memory
• Spooling both input and output
• Concept of time sharing introduced which allows
  interactive working
• Operating systems had to become a lot more
  complex in order to deal with all these issues

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History of Operating Systems - 4

• Fourth Generation (1980-present)
• Large Scale Integration (LSI)
• Led directly to the development of the personal
  computer (PC)
• One of the requirements for the original IBM PC
  was an operating system - Bill Gates supplied
  MS-DOS
• On non-Intel processors, the UNIX operating
  system was also being used.

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History of Operating Systems - 7

• Another View
• First, Second and Third generation as before and
  for same reasons
• Fourth generation started in 1971 with the
  introduction of LSI, then VLSI (Very Large Scale
  Integration) and then ULSI (Ultra Large Scale
  Integration)
• Really, just arguing about when the PC revolution
  started. Was it in the early 70s when LSI first
  became available? Or was it in 1980, when the IBM
  PC was launched?

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History of Operating Systems - 5

• Fifth Generation (Sometime in the future)
• What will constitute a fifth generation computer?

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History of Operating Systems - 5

• Fifth Generation (Sometime in the future)
• Computer generations were influenced by new
  hardware (vacuum tubes, transistors, integrated
  circuits and LSI).
• Fifth generation may break with that tradition
• One view is that a fifth generation computer will
  interact with humans in a way that is natural to
  us Computers will be able to reason in a way that
  imitates humans
• Being able to accept (and understand!) the spoken
  word and carry out reasoning is complex,
  requiring advances in software and maybe hardware

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History of Operating Systems - 6

• Fifth Generation (Sometime in the future)
• What advances will have to be made to allow a
  fifth generation computer to be realised?

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History of Operating Systems - 6

• Fifth Generation (Sometime in the future)
• Advances will be needed in AI including NLP
• Maybe need parallel processing
• Maybe a non-silicon computer
• The first time a generation has not been
  motivated by advances in hardware

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Case Study 1 (ICL Mainframe Operating Systems)

• Manual Executive
• Introduced with its 1900 range
• Manual operating system
• Operators had to load and run each program

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Case Study 2 (ICL Mainframe Operating Systems)

• George 1/2/2
• GEneral ORGanisational Environment
• Ran on top of manual exec - so it was not an
  operating system as such
• Allowed you to submit jobs to the machine and G2
  would schedule them
• Batch programs together into a single job
• Parameterised macros (or JCL Job Control
  Language) allowed automation of tasksSubmit many
  jobs at the same time so that G2 would run them
  one after another
• Adjustable scheduling algorithm (via the
  operators console)
• Other scheduling factors (e.g. tape decks or
  memory)
• Under G2 the operators still look after
  individual jobs

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Case Study 3 (ICL Mainframe Operating Systems)

• George 3/4
• Operators no longer looked after individual jobs.
  Instead they looked after the system
• Jobs could be submitted via interactive terminals
  by a scheduling team
• Development staff could issue batch jobs and also
  runs jobs in an interactive environment.
• Any problems were referred to development or
  technical staff

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Case Study 4 (ICL Mainframe Operating Systems)

• George 3/4
• Operators, in some peoples opinion were little
  more than tape monkeys
• G3 was an operating system in its own right
• To use the machine you had to run the job in a
  user
• The Job Control Language (JCL) was much more
  extensive than that of G2.
• It allowed interactive sessions
• It had a concept of filestore

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Case Study 5 (ICL Mainframe Operating Systems)

• VME (Virtual Machine Environment)
• Still the operating system used on ICL mainframes
  today and the foreseeable future
• VME creates virtual machines that jobs run in. If
  you log onto VME, you will create a virtual
  machine.
• VME is written to cater for the many different
  workloads that mainframes have to perform
• Job control language which (SCL) is a lot more
  sophisticated

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Case Study 6 (ICL Mainframe Operating Systems)

• VME (Virtual Machine Environment)
• Still concept of filestore but all disc based
• Amount of filestore available to users or group
  of users is under the control of the operating
  system (and thus the technical support teams).
• Like G3, the operators control the entire system
• Move towards lights out working.

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Operating System Concepts - 1

• System Calls
• Each system call has a procedure associated with
  it so that system calls can be done in a familiar
  way.
• The procedures places the parameters into
  registers and informs the operating system
• OS is informed via a TRAP instruction (sometimes
  known as a kernel call or a supervisor call).

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Operating System Concepts - 2

• CPU switches from user mode to kernel (or
  supervisor) mode
• After carrying out the work the operating system
  will return a result in the same way as a user
  written function written in a high level
  language.
• Example of an operating system call (via a
  procedure) is
• count read(file, buffer, nbytes)
• Use system calls to manipulate objects

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Operating System Concepts - 2

• Give some examples of processes that should only
  be allowed in kernel (supervisor) mode
• Disable all interrupts
• Set the time of day
• Change the memory map

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Operating System Concepts - 3

• Processes
• A running program with all the other information
  that is needed to control its execution
• Needs to be able to start a process from exactly
  the same point as it was left.
• Process information held in a process table.
• A process may create a child process.
• One of the main tasks of an operating system is
  to schedule processes
• Processes may also communicate with other
  processes. The OS must deal with this.

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Operating System Concepts - 4

• Files
• System calls to (for example) create files,
  delete files, move files, rename files, copy
  files, open files, close file, read files, write
  files.
• Files and Directories

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Operating System Concepts - 5

• Example of abstraction
• Text and binary files
• Standard input and standard output
• Pipes.
• DIR SORT
• Redirection
• DIR gt dir.txt

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Operating System Concepts - 6

• The Shell
• Allows the user to interact with the OS
• For example, by means of a CLI or a graphical
  user interface
• Is the shell part of the OS. If not why not?

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Operating System Concepts - 6

• Is the shell part of the OS. If not why not?
• No
• Less flexibility
• Making it a separate program allows different
  users to have different shells
• In general, the more code outside of the OS, the
  better

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Operating System Concepts - 6

• The Shell
• Shell is not part of the operating system but
  worth mentioning as it makes heavy use of
  operating system features
• Example (in UNIX this time)
• cat file1 file2 file3 sort gt /dev/lp

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Operating System Structure - 1

• How is an OS structured internally (up to now
  only an external view)?
• Monolithic Systems
• No structure at all
• OS is simply a collection of procedures
• OS is constructed by compiling all the procedures
  into one huge monolithic system
• But they can naturally fall into a structure

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Operating System Structure - 2
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Operating System Structure - 3

• Layered Systems
• Layer 0 was responsible for the multiprogramming
  aspects of the operating system. It decided which
  process was allocated to the CPU. It dealt with
  interrupts and performed the context switches
  when a process change was required.
• Layer 1 was concerned with allocating memory to
  processes.
• Layer 2 deals with inter-process communication
  and communication between the operating system
  and the console.

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Operating System Structure - 4

• Layered Systems
• Layer 3 managed all I/O between the devices
  attached to the computer. This included buffering
  information from the various devices.
• Layer 4 was where the user programs were stored.
• Layer 5 was the overall control of the system
  (called the system operator)

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Operating System Structure - 5

• Moving through the hierarchy (from 0 to 5) you do
  not worry about the layers you have left behind
• For example, user programs (level 4) do not have
  to worry about where they are stored in memory or
  if they are currently allocated to the processor
  or not, as these are handled in level 0 and level
  1.

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Operating System Structure - 6

• Virtual Machines
• Virtual machine?
• MS-DOS prompt from with Windows 95/98/NT
• ICLs mainframe operating system is called VME
  (Virtual Machine Environment)
• Both of these (Windows 95/98/NT and VME) are
  recent developments
• VM/370 was one of the first OSs to provide a
  virtual machine to each user
• Each user was even able to run different
  operating systems

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Operating System Structure - 7
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Operating System Structure - 8

• Client-Server Model
• Server provides services to any client that
  requests it
• Model is heavily used in distributed systems
• E.g. print server or database
• Operating systems can be designed as a
  client/server

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Operating System Structure - 9

• Benefits include
• Can result in a minimal kernel
• As each server is managing one part of the
  operating system, the procedures can be better
  structured and more easily maintained.
• If a server crashes it is less likely to bring
  the entire machine down as it wont be running in
  kernel mode. Only the service that has crashed
  will be affected.

User Mode
Kernel Mode
Message sent from client to server
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G53OPSOperating Systems

• Graham Kendall

End of Operating Systems