OS provide a user-friendly environment and manage resources of the computer system.

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• OS provide a user-friendly environment and manage
  resources of the computer system.
• Operating systems manage
• Processes
• Memory
• Storage
• I/O subsystem

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Process Management

• A process is a program in execution. It is a unit
  of work within the system. Program is a passive
  entity, process is an active entity.
• Operating system controls execution of user and
  system processes.
• Process needs resources to accomplish its task
• CPU, memory, I/O, files
• Initialization data
• Process termination requires reclaim of any
  reusable resources

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Process Management Activities

• The operating system is responsible for the
  following activities in connection with process
  management
• Creating and deleting both user and system
  processes
• Suspending and resuming processes (context
  switching)
• Providing mechanisms for process synchronization
• Providing mechanisms for process communication

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Memory Management

• Memory management activities
• Keeping track of which parts of memory are
  currently being used and by whom
• Deciding which processes (or parts thereof) and
  data to move into and out of memory
• Allocating and deallocating memory space as
  needed

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Storage Management

• OS provides uniform, logical view of information
  storage
• Abstracts physical properties to logical storage
  unit - file

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Storage Management

• File-System management
• Files usually organized into directories
• Access control on most systems to determine who
  can access what
• OS activities include
• Creating and deleting files and directories
• Primitives to manipulate files and dirs
• Mapping files onto secondary storage
• Backup files onto stable (non-volatile) storage
  media

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Disk Management

• OS activities
• Free-space management
• Disk-space allocation
• Disk scheduling
• Some storage need not be fast
• Tertiary storage includes optical storage,
  magnetic tape (often used for back-ups).

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I/O Management

• One purpose of OS is to hide peculiarities of
  hardware devices from the user
• I/O subsystem responsible for
• Memory management of I/O including buffering
  (storing data temporarily while it is being
  transferred), caching (storing parts of data in
  faster storage for performance), spooling
  (intercepting concurrent requests for device such
  as printer and ensuring sequential order, i.e.,
  no interleaving of files).
• General device-driver interface
• Drivers for specific hardware devices

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OS as Execution Environment

• Can also view the operating system as providing
  an environment for the execution of programs.
• Provides services for user as well as user (and
  system) applications.

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Services for User

• User interface - Almost all operating systems
  have a user interface (UI)
• Varies between Command-Line Interface (CLI),
  Graphics User Interface (GUI), Batch
• Program execution - The system must be able to
  load a program into memory and to run that
  program, end execution, either normally or
  abnormally (indicating error)

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Operating System Services

• I/O operations - A running program may require
  I/O, which may involve a file or an I/O device.
• File-system manipulation - The file system is of
  particular interest. Obviously, programs need to
  read and write files and directories, create and
  delete them, search them, list file Information,
  permission management.
• Communications Processes may exchange
  information, on the same computer or between
  computers over a network
• Communications may be via shared memory or
  through message passing (packets moved by the OS)

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Operating System Services

• Error detection OS needs to be constantly aware
  of possible errors
• May occur in the CPU and memory hardware, in I/O
  devices, in user program
• For each type of error, OS should take the
  appropriate action to ensure correct and
  consistent computing
• Debugging facilities can greatly enhance the
  users and programmers abilities to efficiently
  use the system

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Other Services

• Another set of OS functions exists for ensuring
  the efficient operation of the system itself via
  resource sharing
• Resource allocation - When multiple users or
  multiple jobs running concurrently, resources
  must be allocated to each of them
• Many types of resources - CPU cycles, main
  memory, file storage, and I/O devices.
• Accounting - To keep track of which users use how
  much and what kinds of computer resources

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Operating System Services (Cont.)

• Protection and security - The owners of
  information stored in a multi-user or networked
  computer system may want to control use of that
  information, concurrent processes should not
  interfere with each other
• Protection involves ensuring that all access to
  system resources is controlled
• E.g., base-limit registers for memory protection.
• Access lists on files.
• Security of the system from outsiders.
• E.g., requiring user name and password.

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User Operating System Interface - CLI

• CLI allows direct command entry
• Sometimes implemented in kernel, sometimes by
  systems program
• Sometimes multiple flavors implemented shells
• Primarily fetches a command from user and
  executes it
• Commands may be implemented in shell
• Implemented through system calls.
• Looks for program of that name. If found,
  executes it. If not, returns an error message.

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User Operating System Interface - GUI

• User-friendly desktop metaphor interface
• Usually mouse, keyboard, and monitor
• Icons represent files, programs, actions, etc
• Various mouse buttons over objects in the
  interface cause various actions (provide
  information, options, execute function, open
  directory (known as a folder)
• Invented at Xerox PARC

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System Calls

• Interface between executing program and OS
  defined by set of system calls OS provides.
• System call causes a TRAP to switch from user to
  kernel mode and starts execution at interrupt
  vector location for TRAP instruction.
• Operating system looks at requested operation and
  any parameters passed by the application.
• Dispatches the correct system call handler
  through a table of pointers to system call
  handlers.
• Handler completes and (may) return to user code
  at the next instruction. OS may schedule another
  process to execute.

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Transition from User to Kernel Mode
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System Calls

• Mostly accessed by programs via a high-level
  Application Program Interface (API) rather than
  direct system call use
• Three most common APIs are Win32 API for Windows,
  POSIX API for POSIX-based systems (including
  virtually all versions of UNIX, Linux, and Mac OS
  X), and Java API for the Java virtual machine
  (JVM)
• Why use APIs rather than system calls?

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System Calls

• Why use APIs rather than system calls?
• Portability Code should run on any system that
  supports the same API.

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System Calls

• Why use APIs rather than system calls?
• Portability Code should run on any system that
  supports the same API.
• Ease of use.
• Some system calls are quite complex involving,
  for example, assembly code.

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System Call Implementation

• High-level languages provide system-call
  interface.
• Run-time libraries added by the compiler.
• Program makes an API call.
• Trapped by the run-time library.
• RTL places number of requested system call in
  correct register.
• Places parameters in appropriate locations.
• Makes system call.

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System Call Implementation

• The standard I/O library in C (C) is another
  high-level API.
• Examples fopen, printf, scanf, cin, cout
• These are functions available to the program, but
  they are not system calls.
• Rather, they are replaced (at compile time) with
  calls to user-level libraries.
• In C, these libraries are loaded into the
  applications address space via the include
  directive.
• Actual system call made in the library.

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Standard C Library Example

• C program invoking printf() library call, which
  calls write() system call.
• Library handles details of making system call
  (e.g., where to put parameters, system call id,
  etc. )