Operating Systems

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OperatingSystems
CSCI-4210-01Spring 2009

• David Goldschmidt, Ph.D.
• Computer Science
• The College of Saint Rose

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ReviewInterrupt Mechanism

• Interrupts are handled much like calling a
  function or method in a programming language

I/O devicestypically use abuffering strategy
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What is an Operating System?

• An Operating System is the software interface
  between hardware and users
• Operating systems
• Execute user and system programs
• Manage and coordinate computer hardware
• Use computer hardware efficiently
• Serve as resource allocators
• Are typically interrupt-driven

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Design Goals of an Operating System

• From a users perspective, an operating system
  should be
• easy to use
• easy to learn
• reliable
• safe
• fast

• System goals ofan operating system typically
  include
• reliability
• flexibility
• extensibility
• speed
• efficiency
• maintainability

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Functional Components

• Functional components of anoperating system
  include
• Process manager
• Memory manager
• Virtual memory manager
• File manager
• Device manager

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History of Operating Systems

• In the beginning
• the 1940s

ENIAC
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History of Operating Systems

• Automation in the 1950s using punch cards

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Batch Jobs

• A job is a unit of work submitted by a user
  tothe operating system, typically consisting of
• a program either in a sourcelanguage or in
  binary form
• a set of input data used bythe program when it
  executes

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History of Operating Systems

• IBM 360 introduced
• in 1964

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Multiprogramming
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Multiprogramming

• In multiprogramming, several jobs reside in
  memory simultaneously
• CPU use is sharedand managed by theoperating
  system

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Multiprogramming

• Multiprogramming provides efficient use of the
  computer (CPU) and its resources (e.g. I/O)
• One user cannot keep theCPU and I/O devices
  busyat all times
• Each running program iscalled a job
• Multiprogramming organizesjobs such that the CPU
  alwayshas exactly one to execute

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Multiprogramming

• Computer is often idle why?
• CPU and hardware muchfaster than I/O
• When a user or process isblocked waiting for
  I/O,the operating system switchesto another job
• A subset of jobs is storedin memory, awaiting
  CPUor other resources

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Timesharing (Multitasking)

• To ensure fairness, consider a timesharing scheme
  in which the CPU cycles through all jobs
• Each job (process) is givena fixed amount of CPU
  time
• Switching from one runningprocess to another is
  calleda context switch
• A process may relinquish itstime if blocked on
  an I/O request

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Timesharing (Multitasking)

• Unless blocked on I/O, processes line
  up,awaiting their turn with the CPU
• CPU scheduling algorithmis necessary to
  efficientlydecide the order in whichprocesses
  execute
• If processes do not fit intomemory, the
  operating systemwill swap them in and out
  ofmemory from virtual memory

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History of Operating Systems

• Personal computer revolution
• of the late 1970s and early 1980s

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History of Operating Systems
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History of Operating Systems

• World Wide Web and Internet revolution
• of the 1990s and early 2000s

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External View of a Computer
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Operating System Structure

• Using a layered approach, the operating system is
  divided into N levels or layers
• Layer 0 is the hardware
• Layer N is the top-leveluser interface
• Each layer uses functionsand services of lower
  layers
• Layer 1 often is the kernel

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Operating System Structure
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Operating System Kernel

• The core program running in anoperating system
  is the kernel
• When a computer is switched on,a bootstrap
  program executes from ROM
• Bootstrap program initializes all aspects of the
  system, then loads the operating system kernel
  and starts its execution

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User and Kernel Modes

• Dual-mode operation provides a user mode anda
  kernel mode of operation
• Allows the operating system to protect itself and
  other system components

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User and Kernel Modes

• Kernel relinquishes control to a user
  process,but may set a timer to ensure a process
  does not run beyond its allotted time
• Avoid infinite loops, memory leaks, memory hogs,
  etc.
• Not entirely effective in practice?
• How many times have you fought to stop a runaway
  process?

• Kernel also provides securityvia privileged
  instructions

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Operating System Services

• An operating system provides services to its
  users
• Program execution
• Load programs into memory, run/suspend/halt
  programs, handle/display errors
• I/O operations
• Seamlessly interact with I/O devices,
  includingdisks, networks connection, etc.
• Filesystem manipulation
• Read/write/traverse filesystem directories,read/w
  rite files, enforce permissions, search for
  files, etc.

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Operating System Services

• Other services an operating system provides
• Inter-Process Communications (IPC)
• Processes exchange information via shared
  memory,message passing, sockets, pipes, files,
  etc.
• Might span multiple computer systems
• Error detection and recovery
• Errors arise in CPU, memory, I/O
  devices,processes, etc.
• Operating system should detect and recover from
  errors, ensuring correct and consistent operations

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System Calls via APIs

• Operating system services available to usersvia
  GUI and CLI, which are just software programs
• System calls are made via programming
  interfacecalled an Application Program Interface
  (API)
• Common operating system APIs
• Win32 API for Windows
• POSIX API for POSIX-based systems,including
  UNIX, Linux, Mac OS X
• Java API for Java Virtual Machine
• C Standard Library

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System Calls via APIs

• Types of system calls include
• Process control (e.g. start/suspend/stop a
  process)
• Debugging information, too
• File management
• Device management
• Information retrieval and maintenance
• Current date/time, number of current users, OS
  version, amount of free memory, process
  information, etc.
• Communications (e.g. IPC, network)

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System Calls via APIs

• System call sequence for a file copy program

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System Calls via APIs

• The API successfully hides the implementation
  detailsof the operating system
• Programmer just needs to obeythe API
  specifications
• Change the operating system,but maintain the API
  such thatit is backwards compatible
• Deprecate old behavior over long period of time

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System Calls via APIs
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System Calls via APIs

• Example using theprintf() functionfrom C

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System Calls via APIs

• Pass parameters to the operating systemvia
  registers or a block of data in memory

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Reading Assignments

• Read this week
• Chapters 1 and 2of the Garrido book