Why do we need an operating system

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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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What is an Operating System

• A program that acts as an intermediary between a
  user of a computer and the computer hardware.
• Operating system goals
• Execute user programs and make solving user
  problems easier.
• Make the computer system convenient to use.
• Use the computer hardware in an efficient manner.

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Operating System Definitions

• Resource allocator manages and allocates
  resources.
• Control program controls the execution of user
  programs and operations of I/O devices .
• Kernel the one program running at all times
  (all else being application programs).

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What is an Operating System

• It is an extended machine
• Hides the messy details which must be performed
• Presents user with a virtual machine, easier to
  use
• It is a resource manager
• Each program gets time with the resource
• Each program gets space on the resource

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Layered Hardware-Software Machine Model
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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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Computer System Components

• 1. Hardware provides basic computing resources
  (CPU, memory, I/O devices).
• 2. Operating system controls and coordinates
  the use of the hardware among the various
  application programs for the various users.
• 3. Applications programs define the ways in
  which the system resources are used to solve the
  computing problems of the users (compilers,
  database systems, video games, business
  programs).
• 4. Users (people, machines, other computers).

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Abstract View of System Components
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History of Computing Systems

• First generation 1945 - 1955
• vacuum tubes, plug boards
• Second generation 1955 - 1965
• transistors, batch systems
• Third generation 1965 1980
• ICs and multiprogramming
• Fourth generation 1980 present
• personal computers

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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 -- 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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Simple Batch Systems

• Hire an operator
• User ? operator
• Add a card reader
• Reduce setup time by batching similar jobs
• Automatic job sequencing automatically
  transfers control from one job to another. First
  rudimentary operating system.
• Resident monitor
• initial control in monitor
• control transfers to job
• when job completes control transfers back to
  monitor

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History of Computing Systems

• Early batch system
• bring cards to 1401
• read cards to tape
• put tape on 7094 which does computing
• put tape on 1401 which prints output

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Memory Layout for a Simple Batch System
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History of Computing Systems

• Structure of a typical FMS job 2nd generation

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Control Cards

• Problems
• 1. How does the monitor know about the nature of
  the job (e.g., Fortran versus Assembly) or which
  program to execute?
• 2. How does the monitor distinguish (a) job from
  job?(b) data from program?
• Solution
• Introduce control cards

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Control Cards (Cont.)

• Special cards that tell the resident monitor
  which programs to runJOBFTNRUNDATAEND
• Special characters distinguish control cards from
  data or program cards in column 1// in column
  1 and 2709 in column1

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Control Cards (Cont.)

• Parts of resident monitor
• Control card interpreter responsible for
  reading and carrying out instructions on the
  cards.
• Loader loads systems programs and applications
  programs into memory.
• Device drivers know special characteristics and
  properties for each of the systems I/O devices.
• Problem Slow Performance I/O and CPU could
  not overlap card reader very slow.
• Solution Off-line operation speed up
  computation by loading jobs into memory from
  tapes and card reading and line printing done
  off-line.

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Spooling

• Overlap I/O of one job with computation of
  another job. While executing one job, the OS.
• Reads next job from card reader into a storage
  area on the disk (job queue).
• Outputs printout of previous job from disk to
  printer.
• Job pool data structure that allows the OS to
  select which job to run next in order to increase
  CPU utilization.

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Multiprogramming - Integrated Circuits
(1965-1980)

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

Job_1
Memory Partitions
Job_2

Job_9
Operating System
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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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OS Features Needed for Multiprogramming

• I/O routine supplied by the system.
• Memory management the system must allocate the
  memory to several jobs.
• CPU scheduling the system must choose among
  several jobs ready to run.
• Allocation of devices.

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User mode Privileged mode

• All I/o instructions are privileged instructions
  
• I/o devices and cpu can execute concurrently
• cpu moves data main memory -- 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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Multiprogramming - Issues

• Protect user programs from each other and OS
  from User programs
• Schedule CPU between jobs
• Schedule I/O between jobs
• Avoid concurrent access to exclusive devices (e.g
  Printer)
• Control job flow into and out the system

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Multiprogramming (Cont.)

• 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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Time-Sharing SystemsInteractive Computing

• The CPU is multiplexed among several jobs that
  are kept in memory and on disk (the CPU is
  allocated to a job only if the job is in memory).
• A job is swapped in and out of memory to the
  disk.
• On-line communication between the user and the
  system is provided when the operating system
  finishes the execution of one command, it seeks
  the next control statement not from a card
  reader, but rather from the users keyboard.
• On-line system must be available for users to
  access data and code.

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Multics(beg. Of 70s) - Innovations

• Programmed in high-level language PL/I
• Time-sharing via terminals
• Virtual memory including Segmentation paging
• Files as segments
• Strong emphasis on protection including
  protection rings and segment descriptors

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Unix (middle of 70s) - Innovations

• First OS on a mini-computer (PDP-11 by Dec)
• High-level language (C )
• Hierarchical file system
• Powerful and convenient shell
• Standard interface to OS via C system calls
• Unification of many concepts via files (e.g.
  devices, pipes, etc.)
• Protection? forget it! Convenience was more
  important! (fixed over the years).

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Personal-Computer Systems

• Personal computers computer system dedicated to
  a single user.
• I/O devices keyboards, mice, display screens,
  small printers.
• User convenience and responsiveness.
• Can adopt technology developed for larger
  operating system often individuals have sole use
  of computer and do not need advanced CPU
  utilization of protection features.

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VLSI and personal computers (1980-

• 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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The Operating System Zoo

• Mainframe operating systems
• Server operating systems
• Multiprocessor operating systems
• Personal computer operating systems
• Real-time operating systems
• Embedded operating systems
• Smart card operating systems

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Comparison of some operating system sizes
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Migration of Operating-System Concepts and
Features
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Parallel Systems

• Multiprocessor systems with more than one CPU in
  close communication.
• Tightly coupled system processors share memory
  and a clock communication usually takes place
  through the shared memory.
• Advantages of parallel system
• Increased throughput
• Economical
• Increased reliability
• graceful degradation
• fail-soft systems

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Parallel Systems (Cont.)

• Symmetric multiprocessing (SMP)
• Each processor runs an identical copy of the
  operating system.
• Many processes can run at once without
  performance deterioration.
• Most modern operating systems support SMP
• Asymmetric multiprocessing
• Each processor is assigned a specific task
  master processor schedules and allocates work to
  slave processors.
• More common in extremely large systems

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Symmetric Multiprocessing Architecture
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Real-Time Systems

• Often used as a control device in a dedicated
  application such as controlling scientific
  experiments, medical imaging systems, industrial
  control systems, and some display systems.
• Well-defined fixed-time constraints.
• Hard real-time system.
• Secondary storage limited or absent, data stored
  in short-term memory, or read-only memory (ROM)
• Conflicts with time-sharing systems, not
  supported by general-purpose operating systems.
• Soft real-time system
• Limited utility in industrial control or robotics
• Useful in applications (multimedia, virtual
  reality) requiring advanced operating-system
  features.

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Distributed Systems

• Distribute the computation among several physical
  processors.
• Loosely coupled system each processor has its
  own local memory processors communicate with one
  another through various communications lines,
  such as high-speed buses or telephone lines.
• Advantages of distributed systems.
• Resources Sharing
• Computation speed up load sharing
• Reliability
• Communications

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Distributed Systems (Cont.)

• Network Operating System
• provides file sharing
• provides communication scheme
• runs independently from other computers on the
  network
• Distributed Operating System
• less autonomy between computers
• gives the impression there is a single operating
  system controlling the network.

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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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Operating System Structure

• The client-server model

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Advanced Architectures

• Multi-processors
• Parallel Computers
• Networks and Distributed systems
• Each of these architectures requires a
  corresponding Operating System

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• Fig. 9-5. A bus-based multiprocessor
• Fig 9-8. (a) Grid. (b) Hypercube.

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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.