Operating Systems

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Operating Systems
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What is an operating system

• an extended machine, easier to program for the
  user
• a resource manager, with more multiple users
  using the system efficient and correct use of the
  available resources is conducted by the operating
  systems.

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Typical Services Provided by an O.S.

• CPU scheduling how to serve the program
  execution (batch, multiprogramming, timesharing,
  real time)
• Memory management how to allocate the memory
• Swapping how to move data between the main
  memory and secondary storage
• I/O device drivers how to operate I/O devices

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• File system organize mass storage (disk) into
  files and directories
• Utilities date/time, accounting, file copy, etc.
• Command interpreter allow users to enter
  commands interactively
• System calls allow user programs access to OS
  services
• Protection keep processes (and users) from
  interfering with each other and system
• Communication Resource sharing allow users/
  processes to communicate (within a computer or
  over networks) and share resources (e.g. laser
  printers, disks, etc.)
• Security protect machines from intruders and
  unauthorized people.

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Evolution of the Operating Systems

• It is better tie up the evolution of the
  operating systems to the hardware, to give a true
  picture, as done by Tanenbaum.

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First generation (1945-1955) characterized by
vacuum tubes and plug-boards

• First Computers
• All the programming was in machine language,
  often by wiring up plug-boards to control
  machine's basic functions. All the problems
  programmed were numeric calculations such as
  tables of sines and cosines. Punch-cards were
  introduced in 1950 which replaced plug-boards

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Second generation Characterized by Transistors
(1955-1965)

• Batch systems,
• Commercial use of computers has become possible.
  They were small enough and reliable enough to be
  manufactured and sold. They were kept in special
  dust-free air conditioned rooms and attended by
  operators only.
• Use of high level languages such as FORTRAN has
  become possible, other than the assembly
  language.
• Batch systems were order of the day as they
  allowed the best utilization of the equipment
  under the present condition (see Figure 1.2 and
  1.3)
• Examples IBM 1401 and 7094.

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Simple Batch Systems in more details

• Are the first operating systems
• The user submit a job (written on card or tape)
  to a computer operator
• The computer operator place a batch of several
  jobs on a input device
• A special program, the monitor, manages the
  execution of each program in the batch
• Resident monitor is in main memory and available
  for execution
• Monitor utilities are loaded when needed

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The Monitor

• Monitor reads jobs one at a time from the input
  device
• Monitor places a job in the user program area
• A monitor instruction branches to the start of
  the user program
• Execution of user pgm continues until
• end-of-pgm occurs or error occurs
• Causes the CPU to fetch its next instruction from
  Monitor

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Job Control Language (JCL)

• Is the language to provide instructions to the
  monitor
• what compiler to use and what data to use
• Each read instruction (in user pgm)
• causes one line of input to be read
• causes (OS) input routine to be invoked, to
• check for not reading a JCL line
• skip to the next JCL line at completion of user
  program

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Batch OS

• Alternates execution between user program and the
  monitor program
• Relies on available hardware to effectively
  alternate execution from various parts of memory

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Desirable Hardware Features

• Memory protection
• do not allow the memory area containing the
  monitor to be altered by user programs
• Timer
• prevents a job from monopolizing the system
• an interrupt occurs when time expires
• Privileged instructions
• can be executed only by the monitor
• an interrupt occurs if a program tries these
  instructions
• Interrupts
• provides flexibility for relinquishing control to
  and regaining control from user programs

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Third generation computers, Characterized by Ics
(1965-1980) Multiprogramming

• The most representative machine of this
  generation is IBM 360, which has combined the
  numeric and non-numeric applications in one
  machine.
• The operating systems were written in assembly
  language and have soon become very bulky as they
  have to work both on newer as well as older
  systems.
• One important break from the second generation
  was multiprogramming which was very important
  for the utilization of the machine. The CPU could
  be multiplexed between the programs residing in
  the memory. When one tied up with IO, the other
  would be tied up with execution.
• Another important characteristic of the third
  generation is SPOOLING, which comes from
  simultaneous peripheral operation on line. The
  job flow from card readers to disk, IO from/to
  disk, print-out from disk to printers could take
  place concurrent with CPU operation.
• The desire for quick response time speeded up the
  birth of timesharing systems. This has made
  interactive on-line of systems order of the day.
• Examples IBM 360, 370, 43xx, and 30xx series.

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Multiprogrammed Batch Systems

• I/O operations are exceedingly slow (compared to
  instruction execution)
• A program containing even a very small number of
  I/O ops, will spend most of its time waiting for
  them
• Hence poor CPU usage when only one program is
  present in memory
• If memory can hold several programs, then CPU can
  switch to another one whenever a program is
  awaiting for an I/O to complete
• This is multitasking (multiprogramming)

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Requirements for Multiprogramming

• Hardware support
• I/O interrupts and (possibly) DMA
• in order to execute instructions while I/O device
  is busy
• Memory management several ready-to-run jobs must
  be kept in memory
• Memory protection (data and programs)
• Software support from the OS
• Scheduling (which program is to be run next)
• To manage resource contention

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MULTICS

• Multics project was initiated to integrate all
  the novelty of multiprogramming, timesharing, and
  user requirements into one operating system.
  Unfortunately the project was never fully
  complete, initiated by ATT
• Minicomputers have allowed computers to enter
  small business and scientific institutions PDP,
  Interdata, VAX, Prime, etc.
• People who have taken part in MULTICS were able
  to come up MULTICS like but far simpler (far less
  ambitious) operating system- UNIX for
  minicomputers, with no written objectives.

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Time Sharing Systems (TSS) in summary

• Batch multiprogramming does not support
  interaction with users
• TSS extends multiprogramming to handle multiple
  interactive jobs
• Processors time is shared among multiple users
• Multiple users simultaneously access the system
  through terminals
• Because of slow human reaction time, a typical
  user needs 2 sec of processing time per minute
• Then (about) 30 users should be able to share the
  same system without noticeable delay in the
  computer reaction time
• The file system must be protected (multiple
  users)

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Fourth generation (1980- ) Personal Computers,
Workstations, and Distributed Systems

• Chip technology (development in LSI) allowed mass
  production, thus the present state of cheap but
  powerful computers finding their way to homes as
  well as offices.
• The dominant operating systems UNIX, NT,
  WindowsXX, etc.
• Networking (TCP/IP) allowed operating systems
  like UNIX to evolve into network operating
  systems, in network environment.
• Distributed operating systems Multiple
  processors will appear one single system to the
  user.

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Fifth generation (1990- ) Intelligent systems

• (still to come!), distributed computing, parallel
  computing, very high speed communication (order
  of many giga bits per second), www, mobile
  systems