Operating Systems

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Operating Systems

• Chapter 11

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Objectives

• Describe the functions and layers of an operating
  system
• List the resources allocated by the operating
  system and describe the complexities of the
  allocation process
• Describe how an operating system manages
  processes and threads
• Compare and contrast alternative CPU scheduling
  methods
• Explain how the operating system manages memory
• Describe signals and pipes and how they
  coordinate threads and processes

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Chapter Topics
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Operating System Overview

• Operating System Management Functions
• Oversees all hardware resources and allocates
  them to user and applications as needed
• Performs many low-level tasks on behalf of users
  and application programs

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Operating System Overview
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Operating System Overview

• Operating System Layers
• Command Layer (shell) the users interface to
  the operating system
• Service Layer contains a set of functions
  called by application programs and the command
  layer
• Kernel manages resources and directly interacts
  with computer hardware

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Operating System Overview
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Operating System Overview
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Resource Allocation

• Resource Allocation Process
• Keeps detailed records of available resources
• Knows which resources are used to satisfy which
  requests
• Schedules resources based on specific allocation
  policies to meet present and anticipated demands
• Updates records to reflect resources commitment
  and release by processes and users

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Resource Allocation

• Single Process Resource Allocation (application
  program and operating system)
• When an application program begins executing, the
  operating system grants it control of all unused
  resources
• Most service calls are actually indirect requests
  for system resources

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Resource Allocation

• Multiple Process Resource Allocation
• Multitasking operating systems are the norm for
  general-purpose computers
• Multitasking operating systems must be able to
  handle multiple programs and users
• A multitasking operating system manages hardware
  resources to achieve the following
• Meet the resource needs of processes
• Prevent processes from interfering with one
  another
• Efficiently use hardware and other resources

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Resource Allocation

• Real and Virtual Resources
• Real resource a computer systems physical
  devices and associated system software
• Virtual resource the resources that are
  apparent to a process or user

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Process Management

• Process Control Data Structures
• A process is a unit of executing software that is
  managed independently by the operating system
• A process can request and receive hardware
  resources and operating system services

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Process Management

• Process Control Data Structures
• Process Control Block (PCB) keeps track of each
  process by creating and updating a data structure
• The PCB is created when the process is created,
  updated when the process changes and deleted when
  the process terminates.

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Process Management

• Data included in a PCB
• A unique process identification number
• The current state of the process
• Events for which the process is waiting
• Resources allocated exclusively to the process
• Machine resources consumed
• Process ownership and/or access privileges
• Scheduling priority

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CPU Allocation

• Concurrent Execution (Interleaved Execution)
• A thread is a portion of a process that can be
  scheduled and executed independently.
• It controls the CPU for no more than a few
  milliseconds before it relinquishes control and
  the operating system gives another thread a turn

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CPU Allocation
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CPU Allocation

• Ready State a thread is waiting for access to a
  CPU
• Block State the thread is suspended while an
  interrupt is being processed
• Running State the thread has control of the
  CPU.
• The thread or its parent process terminates
  normally
• An interrupt occurs

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CPU Allocation

• Scheduling
• Preemptive Scheduling
• a thread can be removed involuntarily from the
  running state
• a running process controls the CPU by controlling
  the content of the instruction pointer
• CPU control is lost whenever an interrupt is
  received

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CPU Allocation

• Interrupt Processing
• A blocked thread is waiting for an event to occur
• If the event is the correction of an error and it
  can be corrected, the thread remains in the
  blocked state until the error condition is
  resolved otherwise, the thread is halted

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CPU Allocation
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CPU Allocation
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Technology Focus
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CPU Allocation

• Priority-Based
• First-come first served
• The scheduler always dispatches the ready thread
  that has been waiting the longest.
• Explicit priority
• Uses a set of priority levels and assigns a level
  to each process or thread.

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CPU Allocation

• Shortest Remaining Time
• Chooses the next process to be dispatched based
  on the expected amount of CPU time needed to
  complete the process.

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Technology Focus
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Memory Allocation

• Single-Tasking Memory Allocation
• Multitasking Memory Allocation
• Memory Fragmentation
• Noncontiguous Memory Allocation
• Virtual Memory Management
• Memory Protection
• Memory Management Hardware

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Memory Allocation
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Memory Allocation

• Single-Tasking Memory Allocation
• Contiguous Memory Allocation
• The bulk of the operating system occupies lower
  memory addresses.
• The application program is loaded above the
  operating system.
• The remaining space is available for user
  programs and data.

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Memory Allocation
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Memory Allocation

• Multitasking Memory Allocation
• Allow as many active processes as possible.
• Respond quickly to changing memory demands of
  processes.
• Prevent unauthorized changes to a processs
  memory region(s).
• Implement memory allocation and addressing as
  efficiently as possible.

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Memory Allocation
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Memory Allocation
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Memory Allocation
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Memory Allocation

• Memory Fragmentation
• As processes are created, executed, and
  terminated, memory allocation changes accordingly
• Memory partition allocation and deallocation
  leads to an increasing number of small free
  partitions separated by allocated partitions

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Memory Allocation
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Memory Allocation
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Memory Allocation
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Memory Allocation
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Technology Focus
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Technology Focus
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Interprocess Communication
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Interprocess Communication
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Interprocess Communication
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Interprocess Communication
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Interprocess Communication
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Summary

• The operating system is complex software that
  allocates hardware resources to individual user
  processes on demand
• The operating system stores information about
  each process in a PCB
• Application software is simper to develop if
  programs are unaware of resource allocation
  functions

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Summary

• An active thread is always in one of three states
  ready, running, or blocked
• Memory is divided into fixed-size partitions and
  processes are allocated one or more memory
  partitions to store instructions and data
• Modern operating systems implement a form of
  memory allocation and management called virtual
  memory management
• Processes and threads often need to synchronize
  their actions or transfer data among themselves
  using signals and pipes