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							Operating Systems

     Chapter 11




   Systems Architecture, Fourth Edition   1
                   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


                    Systems Architecture, Fourth Edition   2
Chapter Topics




  Systems Architecture, Fourth Edition   3
   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




       Systems Architecture, Fourth Edition   5
    Operating System Overview
Operating System Layers:
• Command Layer (shell) – the user’s 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
                 Systems Architecture, Fourth Edition   6
Operating System Overview




       Systems Architecture, Fourth Edition   7
Operating System Overview




       Systems Architecture, Fourth Edition   8
         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
                 Systems Architecture, Fourth Edition   9
        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
               Systems Architecture, Fourth Edition   10
          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

                  Systems Architecture, Fourth Edition    11
        Resource Allocation
Real and Virtual Resources:
• Real resource – a computer system’s
  physical devices and associated system
  software
• Virtual resource – the resources that are
  apparent to a process or user



               Systems Architecture, Fourth Edition   12
       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


               Systems Architecture, Fourth Edition   13
       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.
              Systems Architecture, Fourth Edition   14
        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

                 Systems Architecture, Fourth Edition   15
           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
1. Ready State – a thread is waiting for access
   to a CPU
2. Block State – the thread is suspended while
   an interrupt is being processed
3. Running State – the thread has control of
   the CPU.
– The thread or its parent process terminates
   normally
– An interrupt occurs
              Systems Architecture, Fourth Edition   18
            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

                Systems Architecture, Fourth Edition   19
            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
               Systems Architecture, Fourth Edition   20
CPU Allocation




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CPU Allocation




  Systems Architecture, Fourth Edition   22
Technology Focus




   Systems Architecture, Fourth Edition   23
             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.


                 Systems Architecture, Fourth Edition   24
          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

                Systems Architecture, Fourth Edition   27
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.

               Systems Architecture, Fourth Edition   29
Memory Allocation




   Systems Architecture, Fourth Edition   30
          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 process’s
  memory region(s).
• Implement memory allocation and addressing as
  efficiently as possible.


                Systems Architecture, Fourth Edition   31
Memory Allocation




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Memory Allocation




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Memory Allocation




   Systems Architecture, Fourth Edition   34
         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
               Systems Architecture, Fourth Edition   35
Memory Allocation




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Memory Allocation




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Memory Allocation




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Memory Allocation




   Systems Architecture, Fourth Edition   39
Technology Focus




   Systems Architecture, Fourth Edition   40
Technology Focus




   Systems Architecture, Fourth Edition   41
Interprocess Communication




        Systems Architecture, Fourth Edition   42
Interprocess Communication




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Interprocess Communication




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Interprocess Communication




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Interprocess Communication




        Systems Architecture, Fourth Edition   46
                  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


                 Systems Architecture, Fourth Edition   47
                  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

                 Systems Architecture, Fourth Edition   48