Typical Process State Diagram

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Typical Process State Diagram
New
Admit
Suspend / High Priority Job entries
Admit
Dispatch
No allocated Memory
Activate
Release
Ready Suspend
Ready
Running
Exit
Time Out
Event Occurs
Swap out Disk
Event Wait (I/O wait)
I/O Complete
Wait / Blocked
Activate
Event Occurs
Blocked Suspended
Swap out to Disk
No allocated Memory
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Process State Diagram of UNIX
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Thread State Diagram of WINDOWS
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LINUX Thread State Diagram
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Significance of States Transitions - 1

• NEW gtCreate Process Id PCB.
• NEW ? Ready or ? Ready / Suspend
• gt Process is ready to execute waiting for
  CPU allocation OR Process is waiting only for
  main memory.
• Ready / Suspend ? Ready gt Process is ready
  to run waiting to be scheduled/ dispatched.
• Blocked gt Waiting for an event to Complete.

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Significance of States Transitions - 2

• Blocked ? Blocked/ Suspend gt used to make
  available more memory to the Ready processes
  and/ or to the Running Process.
• Blocked/Suspend ? Ready Suspend gt Process
  waiting for event completion is over but no main
  memory is currently available.
• Ready ? Ready/ Suspend adopted if that is the
  only way to make available more memory to a high
  priority / currently running process.

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Significance of States Transitions - 3

• Blocked /Suspend ? Blocked used to bring a high
  priority waiting process into memory.
• Running ? Ready /Suspend implies completion
  of time quantum but no memory is currently
  available since a high priority process has
  either moved into the blocked state or has become
  unblocked.

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Reasons for Process Creation

• New batch job presented to O.S. for execution.
• A new user logs on in an interactive system.
• Spawned /Created by any existing process for
  purpose of modularity or to exploit parallelism.
• e.g. User process calling an application by a
  command.
• User process executing a
  Process Creation
• Call (an SVC).

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Process Creation Steps 1

• State New -gt Create a new process. This
  involves the following
• Assign an Unique Process Identifier (PID).
• Allocate Space for the entire Process Image in
  the Virtual Memory. Space requirement information
  is obtained from
• 1) Default based on the process type.
• 2) Specified by user via Job
  Specification.
• 3) Supplied by the Parent Process.

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Process Creation Steps 2

• Initialize the Process Control Block (PCB) in
  the following manner
• 1) The processor state information
• a) Instruction Pointer set to the
  code entry point.
• b) System Stack pointers set to
  stack
• boundaries.
• c) All other registers Flags
  are initialised to
• all zero .

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Process Creation Steps 3

• 2) Process Control Information Section is
  initialised
• based on default values like
• a) Process State New ? Ready (If Memory
  Available)
• OR New ? Ready / Suspend
  otherwise.
• b) Lowest priority / Specified at the
  time of creation /
• Inherited from the parent.
• c) No resources hold except for Disk Swap
  Space /
• Explicit resource request/ Inherited
  from the parent.

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Process Creation Steps 4

• Set appropriate linkages / Put in the relevant
  Queue (Ready OR Ready/ Suspend).
• Create or Expand other data structures like
  accounting / system log.

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Reasons for Process Blocking/Waiting

• Waiting for I/O to complete.
• Waiting for User response.
• Waiting for the required resource other than CPU
  memory.
• Waiting for the spawned child process(s) to
  complete.

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Reasons for process suspension

• Swapped out to release main memory for high
  priority Process / Currently running process.
• Suspended by O.S. (background process, utility).
• Periodic execution after certain interval like a
  Screen Saver.
• Parent process request.
• User request.

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Reasons for Process Termination - 1

• Normal Completion.
• Max. time limit exceeded.
• Memory unavailable.
• Bounds of memory violation (illegal unauthorized
  memory access).
• Protection exception e.g. Bypass the allowable
  access mode of a file.
• Arithmetic error e.g. division by zero.
• Wait overdue. Waiting for too long.
• I/O failure.

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Reasons for Process Termination - 2

• Executing invalid instruction / non existing
  instructions.
• Trying to execute privileged instructions.
• Accessing improper data / invalid type / not
  initialized.
• Operator intervention e.g. too large a print
  out, infinite looping.
• O.S. intervention to prevent deadlock from
  occurring.
• Terminated by the parent process.
• Termination requested by the parent process.

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Possible Reasons for Context Switching of the
Processor (CPU)

• 1) Timer Interrupt (Time Quantum over for
  the
• currently running process).
• 2) A process having higher priority has
  become
• ready .
• 3) The currently running process has
  become blocked or
• has Terminated which leaves the CPU
  idle (triggered
• by a Mode Switch) thereby
  necessitating
• a new process to be scheduled.
• 4) Main Memory has become FULL /
  AVAILABLE.

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Context Switching of the Processor (Salient
Features)

• Brings in the following Operating system
  processes into stand alone execution
• a) The Context Switcher to accomplish
  saving old
• Execution status Thread loading
  new Status .
• b) The Dispatcher / Scheduler for picking
  out
• the next process to be allotted the
  CPU (usually
• from the Ready Queue) based on a
  predefined
• short term scheduling policy (to be
  illustrated later).
• c) The Enqueuer / Dequeuer for updating
  various
• process queues at different states.

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Context Switching / Change of a Process State (
The Steps Involved )

• Save the current Processor Context Currently
  running Process context The Current Thread in
  relevant (User ) Stack
• Update the PCBs of all those processes that are
  changing state.
• Move the requisite PCBs in the relevant Queue.
• Update Memory Management data structures to
  facilitate address translation.
• Restore / Reload Processor Context Some Prev.
  /A New Thread

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Types Modes of Execution

• Two types of processes
• User Processes.
• Supervisor/ Monitor/ Kernel Process.
• In most contemporary Operating Systems like
  UNIX / Windows 2000 , in the context of the same
  process the computation changes mode from user to
  kernel mode , then execution continues at a
  higher priority and privilege level subsequently
  it comes back to user mode.

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The Kernel the User Relationships

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Mode Switching Scenario - 1

• User Mode to Kernel Mode caused by one of the
  following
• 1) User process asks for an O.S. Service
  via a Supervisor Call /
• Software Interrupt.
• 2) A trap / Exception occurs in an
  instruction while executing in
• the user mode.
• 3) A device interrupt occurs .
• In ALL cases , however, the state of
  the currently running
• process almost always changes (from
  Running -gt Ready
• OR ? Ready /Suspended OR ? Terminated OR
  alternately from
• Running --gt Blocked ) followed by a
  Context Switch to invoke
• the Dispatcher / Scheduler.

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Mode Switching of any Process ( from User to
Kernel ) Steps performed in Kernel Mode

• Save the context of the currently running
  program (the current thread) in the relevant
  stack (usually in the System Stack).
• Set up appropriate flag(s) properly in the
  Processor status Register to indicate mode
  switching.
• Transfers control to the start position of the
  relevant System Set Up Routine.
• Initiate necessary State Transitions in the
  System (if needed).

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Mode Switching Scenario - 2

• Kernel Mode to User Mode normally happens
  either
• 1) By executing some privileged instruction.
• 2) By an ordinary RETURN instruction after
• completion of service.

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Process State Diagram of UNIX
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Thread State Diagram of WINDOWS
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LINUX Thread State Diagram