Operating Systems

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Operating Systems

• Introduction

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OS is a Control Program

• OS Functions are
• 1) OS is a resource allocator. Resources are
• CPU time
• I/O devices
• Files
• Memory, internal (SIMM, DIMM)
• Mass Storage (HD, MTU)
• 2) To make Computer System user friendly
• GUI, Sound, Use of human (like) languages

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CS Components
User 1
User 2
User 4
User 3
User 5
Application Programs
Operating System
Computer Hardware
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Early Computers (Mainframes)

• First Computers had no OS
• programmer had to control hardware (011101110)
• Single User machine
• User had to book time to use CS
• CS was often not available, or was idle
• Computers were set up to run a (ONE) job
• load, run and unload compiler, (removable disk of
  5MByte)
• load assembler, (JOB, FOR,), another disk

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Batch System

• Job Cards were used to specify steps in the job
  process
• load compiler, one instruction, one card
• run compiler,
• unload compiler,
• load assembler,
• Changes in the program were done by punching new
  cards
• Human operators had to batch jobs together
• jobs requiring the same compiler run in the batch

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Resident Monitor First OS

• Monitor Program was doing job scheduling
• User is no longer in control of the CS
• Resident Monitor was always in memory
• RM was first OS, with components
• Loader
• Job Sequencer
• Control Card Interpreter (Command Interpreter)
• The computer efficiency was increased

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Monitor Program

• While the user application is running, it must
  not be allowed to alter the monitor
• CPU hardware is designed to protect the monitor
  in memory
• an error message is generated if protected memory
  area is to be accessed
• Monitor must take control of CS if a application
  enters an infinite loop
• Timer is a hardware to support that
• when timer goes off, monitor program gain control
  back

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Monitor

• Monitor prevents possible hardware damage, caused
  by application software, by using
• Privileged instructions
• CPU allows only the monitor to run instructions
  which use the I/O devices, and which will report
  back with an error in case of any failure
• How does CS hardware put Monitor Program back in
  control of CS?
• Traps
• Interrupts

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Next Step - Spooled Batch Systems

• I/O activities are too slow comparing to CPU
  activities, order of ms, sec vs. ns
• Card reader is slow, and following that, CPU is
  mostly idle as it waits for data to come in
• Writing output / printing is also slow, and CPU
  also mostly idle
• Computer CPU time was always expensive,
• SPOOL-ing (Simultaneous Peripheral Operation On
  Line) was THE solution

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Spooled Batch Systems

• Cheaper machines do the Input and record details
  on a tape or disk
• Tape or Disk is then used by the main computer
• Output is written on the disk, or tape, which
  another cheaper machine reads to print out
• CPU Idle time is down from few min to few sec, ms.

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Multi-programmed Batch Systems1965

• Spooling still means that the CPU is mostly idle
  while accessing data from
• A disk ( few 10s ms) or
• A tape (few min)
• Next step Multi-programmed Batch Systems
• Introduce more than one user application into
  memory. While I/O is occurring, CPU run another
  job
• Operating System, or Control Program, now have to
  do Job Management
• Priority, FCFS, RR,

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Multi-programming

• Memory allocation for multi-programmed system

Operating System
Job 1, (User)
Job 2
Job 3
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Concurrent Jobs

• Operating System is now doing resource allocation
  for concurrent jobs in memory
• By the time an input device is available, few
  jobs are most likely waiting ( in a i/o queue for
  that device)
• OS must ensure that the data goes to the correct
  job i.e. user application
• OS must protect devices from competing jobs
• Fairness of resource allocation is to be in place
• Efficiency of the system is always an issue
• Deadlock prevention, and/or recovery should be
  enforced

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New OS functions to Support Multitasking

• I/O routines to handle concurrent requests
• More sophisticated memory management
• multiple jobs must be placed in memory and
• be protected from each other
• CPU scheduling,
• to choose which ready-to-run job gets the CPU
  first
• I/O devices, and Mass storage devices,
  allocation algorithms are developed to handle
  concurrent requests
• Disk Management FCFS, SSTF, SCAN, CSCAN, LOOK,
  CLOOK
• Printer queue FCFS

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Batch Jobs, still in place

• Modern Operating Systems like Windows (DOS),
  UNIX,, can perform batch jobs, as well.
• UNIX batch shell script
• mkdir new_dir
• cp .txt new_dir
• startx
• logout

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Multi-user Interactive Systems

• Batch Systems There was no interaction between
  programmer and application during execution
• User have to provide all possible inputs in
  advance,
• Question was, how to handle errors, in the
  process?
• Multi-user Interactive Systems, or Time sharing
  was introduced in 1970s
• User application and OS can
• take inputs, interactively, and
• Produce outputs, in a reasonable time
• CS serve many users (Honeywell, IBM 128, 256
  users)

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Multi-user Interactive Systems

• CPU schedules a small time-slice to each user
  application
• For n users,
• give each user (job, task) approximately 1/n of
  the CPU time, over a short interval (Round Robin
  Algorithm)
• Such OS requires File System with easy interface
  for an interactive user.

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Finally, PC Systems, late 1970,

• Hardware is becoming cheap,
• A PC is a single user machine,
• As being a single user CS, used to have just a
  few, hardware and resources, protection
  mechanisms
• PC is quite sophisticated now,
• Well known PC Operating Systems
• CPM, DOS, WINDOWS,

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Other CS and OS

• Parallel Systems, Multiprocessing
• More than one CPU sharing the same clock, bus,
  and sometimes the same memory
• Parallel processing, load sharing
• Two Types
• Symmetric systems
• each CPU runs a copy of the OS. The separate
  operating systems can communicate with each other
  when necessary in order to balance workloads and
  coordinate resources
• Asymmetric systems
• only one OS. Different OS processes can be
  running on different CPUs

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Networked Systems

• User is able to access files/peripherals across a
  network and can log in to remote machines
• CPUs time is a network resource as well
• Network types
• LAN, MAN (broadcasting mostly, but switching as
  well)
• WAN (switching mostly)
• Internet Computing

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Distributed Systems

• User sees the system as it is just his local
  system,
• in reality, files and running programs are on
  different machines.
• On some systems, running applications are able to
  migrate from system to system
• Internet Computing
• On other systems, a running application has to
  stay on the computer that started it.

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Real-time OS

• Systems defined according response time.
• Real Time OS guarantees that a request to the OS
  will be done within a certain time-limit (short
  time usually).
• Mainly used for embedded systems that need to
  control processes, or equipment
• Automatic pilot,
• medical equipment,
• some military systems,
• power station,

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Real-time OS

• Hard limit real-time
• Absolutely guarantees that any request will be
  done within a time limit
• system usually limits types of activities the
  user application can perform to ensure a request
  can be met
• Soft limit real-time
• to do the request on time

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References

• Silberscharz A. and Galvin P., Operating System
  Concepts, John Wiley Sons, 1999
• Tanenbaum A., Modern Operating Systems, 2nd
  edition, Prentice Hall, New Jersey, 2001