OS Concepts

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OS Concepts

• Computer Software
• 1-system programs (OS)
• 2-application programs
• Os controls all computer resources and provides
  the base upon which the application programs can
  be written
• Os is the software layer that is on top of the
  hardware to manage all parts of the system, and
  present the user with interface or virtual
  machine that is easier to understand and
  program.

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OS Concepts

• A Computer System

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OS Concepts

• The Macroprogram is located in the ROM and
  directly control the hardware
• Some Computers such as RISC does not have
  microprogram and hardware executes the machine
  languages directly.
• OS hides the hardware complexity
• READ BLOCK FROM FILE hides
• all details of moving heads, and so on
• OS runs in the kernel mode or supervisor mode.
  User can change his password but can not write
  his/her own disk interrupt handler.

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OS as a virtual machine

• OS present a user with the extended machine that
  hides the truth about hardware. For example
  reading/writing from a floppy disk requires to
  know
• 16 commands
• 13 parameters packed in 9 bytes
• Address of disk block
• Recording mode, gap spacing, deleted data address
  mark
• Whether the motor is on or off and etc.

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OS as a Resource Manager

• Managing processors, memories , timers , disks,
  mice, network interfaces, laser printers and
  other devices.
• Bringing the order to the chaos for example
  buffering all output of three programs before
  sending them to the printer.
• Managing multiple users when sharing hardware and
  information (files, database and etc.)

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History of Operating Systems

• OS historically have been closely tied to the
  architecture of the computers on which, they run.
  So looking at generation of the computers enables
  us to see what their OS were like.
• The first generation Vacuum tubes and plugboards
  (1945-55)
• The second generation Transistors and batch
  systems (1955-65)

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• An early batch system

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• Structure of a FMS job (Fortran Monitor System)

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• The third generation ICs and multiprogramming
  (1956-1980)

• Spooling and time sharing

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History of OS

• The forth generation Personal Computers (1980-
  present)
• Workstations and network operating systems and
  distributed systems
• MSDOS, UNIX (supports IEEE POSIX standard system
  calls interface), WINDOWS
• MINIX is Unix like operating system written in C
  for educational purposes (1987)
• Linux is the extended version of MINIX

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OS concepts review

• System calls Interface between user and Os.
  Extended instructions provided by OS.
• For example Minix system calls
• Dealing with processes
• Dealing with file system
• Processes are used to get the work done. A
  program in execution. Associated with address
  space, program counters , stack pointers and
  hardware registers.

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Processes

• Process table A link list for each existing
  process that contains all information about the
  process, other than the contents of its own
  address.
• When the process is suspended all of its
  information is saved in process table.
• Command interpreter (Shell) reads the user
  commands from terminal to create a process.

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Processes

• A process can create child processes and
  communicate with them (interprocess
  communication)

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Files

• The main method for storing data. Directory is
  used to keep the files and FFS uses it for
  cylinder grouping technique in Unix ( we will
  study in great details).
• In spite of process hierarchy, file hierarchy can
  be deep and long-lived
• Typically parent process can access the child
  process but files and directories can be read by
  others not just the owner.

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• Root directory and path (INODE in Unix)

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Files

• Files and directories are protecting by assigning
  each a protection code
• 9 bit in Minix/Unix for example rwxr-xx
• means
• owner can read, write or execute
• other group members can read or execute
• everyone else can only execute
• If access to a file is permitted system returns
  an integer (file descriptor) otherwise an error
  code

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Files

• Mounting the files is the ways to deal with the
  files in removable medias (floppy and CDs).
• Floppy in mounted to b. If b had files they
  would not be accessible. So b should be empty.
  

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Pipe

• Pipe is the way to make the communication between
  two processes looks like file read and write.

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Shell

• Shell is a command interpreter (not part of OS)
  such as sh, csh in Unix to process the user
  commands like
• sort lt file1 gt file2
• or cat file1 file2 file3 sort gt /dev/lp
• to concatenate file12 3 and sort and put
  the result at /dev/lp. makes the job to run in
  the background mode

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System Calls

• Can be called from C or other programming
  language to enter the kernel (API in Windows)
• For example when a user program is in the user
  mode and needs a system service such as reading
  form a file by countread(fd,buffer,nbytes)
  switches to kernel mode
• The system calls provide process mng. , file
  mng., directory and file system management and
  other services.

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OS structure

• If we look inside OS, there are four designs
• 1- Monolithic systems
• Collection of the procedures each of which
  calling other ones (The Big Mess)
• No information hiding. Ever procedure is visible
  to other procedures.
• System calls switch to kernel mode (see below)

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• In monolithic model for system calls there are
  service procedures using utility procedures

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OS Structure

• 2- Layered systems
• It is generalization of monolithic approach
• In the user level , they should not worry about
  process, memory, console or I/O managements.
  These jobs are done by lower layers.
• THE was the first layered system developed in
  the Netherlands by Digestra(1968)

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• Structure of THE operating system

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OS Structure

• 3- Virtual machines
• Several virtual machines are simulated in this
  model (providing virtual 8086 on Pentium to be
  able to run MSDOS programs)
• VM provides several virtual OSs such as single
  user, interactive and etc. all run on Bare
  hardware.

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OS Structure

• 4- Client-Server Model
• Moving most of the code up into the higher
  layers made the kernel to be minimal and only
  responsible for communication between clients and
  servers.

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OS Structure

• Advantage of this system is adaptability to use
  in distributed systems.
• Disadvantage is doing OS functions ( e.g. loading
  physical I/O device registers) in the user space
  is difficult.

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OS Structure

• There are two ways to solve this problem
• To have some critical server processes run in the
  kernel with complete access to hardware but still
  communicating with other normal processes.
• Enhance the kernel to provides these tasks but
  server decides how to use it (Splitting policy
  and mechanism)