Typical Process State Diagram

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Typical Process State Diagram
New
Memory Shortage Pre-Empted Swapped Out
Admit / Load
Admit
Scheduled THEN Dispatched
No allocated Memory
Load
Ready Suspend
Ready
Running
Terminated
Time Out Pre-Empted
Event Occurs
Memory Shortage Swapped out
Event/ Peripheral Wait / Peripheral Operation
Peripheral Finished
Waiting / Blocked
Memory Available/Activate
Event Occurs / Peripheral Finished
Waiting /Blocked Suspended
Memory Shortage Swap out to Disk
No allocated Memory
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Key O.S. Functions for Process Handling

• Process Creation.
• Process Admission in Memory Ready Queues
  along with Pre Assigned Associated Priority for
  each Process.
• Process Scheduling Select to Dispatch Next.
• Process Dispatching Allocate CPU.
• Process Pre-Emption Context Switching between
  processes very important for Multi Programmed
  OR Multi Tasked System supported by various
  types of Interrupts Timer Interrupt, Device
  Interrupt, Supervisor Call SVC / Software
  Interrupt.
• Synchronization of Concurrent Processes in a
  Multi Tasked Environment which involves enforcing
  Memory Resource Sharing Protocols.
• Process Termination ( Normal / Abnormal).
• Handling Process request for resources other
  than CPU ( Peripheral , Memory) normally done via
  Supervisor Call SVC / Software Interrupt.
• Process Swapping between Main Memory Virtual
  Memory ( Disk Swap Space) on Demand.

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Process Creation

• State New -gt Any Process is created by the
  O.S. in response to user command or by itself .
  All processes are first created in the Virtual
  Space ( Secondary Memory ) . At any point of
  time several user processes may exist so typical
  of a Multi- Programmed / Multi Tasked Scenario.

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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.
• Created by the O.S. to provide a Service like
  the creation of a Print File.
• Spawned /Created by any existing process for
  purpose of modularity or to exploit Time
  Sharing.
• e.g. a) User process calling an application
  by a command.
• b) User process executing a
  Process Creation
• Call (an SVC) like fork.
• In cases 3. 4. we have a Parent Process
  spawning a Child Process.

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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) Set up the processor state information
• a) Initialize Instruction Pointer Code
  Segment Register
• to the Code Entry Point.
• b) Initialize Stack pointers to the
  relevant stack boundaries.
• c) All other registers Flags are
  initialised to 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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Loading a Process into Main Memory

• NEW ? READY OR READY/SUSPEND ? READY
• This involves loading a Process / Its relevant
  portion from Files on Disk OR alternately from
  Disk Swap Space to Main Memory by the O.S.
  Modules involved are File Manager, Memory
  Manager , Long Medium Term Scheduler as well as
  the Loader . In a Multi programmed / Multi
  Tasked environment several user processes gets
  loaded and some portions of all of them reside in
  the main memory. Needs Block Level DMA based /
  Interrupt Driven data transfer.
• After coming into memory, all these
  Processes lie in Readiness waiting to be executed
  by the CPU. These Processes are maintained in
  one/ several Priority Queues each of which is
  termed as a READY Queue. Each READY Queue is
  actually a Queue of Pointers that resides in main
  memory. Each of these pointers are pointing to
  the relevant / concerned PCBs in main memory.
• If no Memory is currently available then the new
  Processes are put in the Swap Space
• NEW ? READY / SUSPEND

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Process Scheduling Dispatching

• READY ? RUNNING
• Scheduling Selecting the Process from the set
  of Ready Queues to be executed next . Performed
  by the Short Term Scheduler based on some
  predefined scheduling policy.
• Dispatching the Scheduled Process Allocating
  the CPU to the scheduled a process i.e. loading
  the Code Segment Register as well as the
  Instruction Pointer / Program Counter Register of
  the CPU with appropriate addresses. Performed by
  the Dispatcher/ Short Term Scheduler of the O.S.
  After being Dispatched , the Process starts
  running/ is being executed by the CPU.

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Process Pre- Emption Scenario

• Running ? Ready Caused by one of the
  following reasons
• a) Allotted Time Slice over signaled by
  Internally generated Timer Interrupt from the
  Programmed Interval Timer.
• b) Entry of a High Priority Process ( depends
  on the Scheduling Policy )
• Running ? Ready / Suspended. Same reasons as
  before in addition Non Availability of Main
  Memory due to Higher Memory demand of the next
  Scheduled Process.

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Reasons for Process being Blocked Running ?
Blocked/ Wait

• Requested for Peripheral Service using
  Supervisor Call SVC / Software Interrupt
  waiting for it to complete. This may include
  User Response.
• Requested for Peripheral Service using
  Supervisor Call SVC / Software Interrupt
  waiting for its availability in the respective
  WAIT Queue.
• Require some sort of message from some other
  process Waiting for the arrival of that
  Message.
• Waiting for the spawned child process(s) to
  complete.

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Running ? Blocked/ WaitAssociated Events

• a) Scheduling Dispatching of a New Process
  to prevent CPU idle time. This requires Context
  Switching.
• b) Mode Switching for the Process going into
  the Blocked / Waiting State.

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Reasons for process suspension / Swapped out of
Memory

• 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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Swapped Out Processes Associated State
Transitions - 1

• Blocked ? Blocked/ Suspend gt used to make
  available more memory to the Ready processes
  and/ or to the Running Process. Possible only for
  those processes which are currently either
  waiting for completion of Peripheral Operation OR
  waiting for the peripheral to be available OR
  waiting for some message. The concerned Process
  portions are normally swapped out to Disk Swap
  Space.
• Blocked/Suspend ? Ready Suspend gt Waiting
  is over ( Peripheral Operation Complete / Message
  has arrived) but no main memory is currently
  available. Process remains in Disk swap space.

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Swapped Out Processes Associated State
Transitions - 2

• Ready ? Ready/ Suspend adopted if that is the
  only way to make available more memory to a high
  priority / currently running process. The
  concerned Process portions are normally swapped
  out to Disk Swap Space.
• Blocked /Suspend ? Blocked used to bring a high
  priority waiting process currently either doing
  peripheral operation OR waiting for the
  peripheral availability OR waiting for some
  Message back into memory.

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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 Scheduler for picking out the
  desired Process the pre-specified Ready Queue to
  be Dispatched next based on a
  predefined short term scheduling policy.
• 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 for any Unfinished
  Process.
• 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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Mode Switching Scenario User ? Kernel

• Caused by one of the following
• 1) User process asks for an O.S. Service
  via a Supervisor Call /
• Software Interrupt. Running ?
  Blocked
• 2) A trap / Exception occurs in an
  instruction while executing in
• the user mode. Running ? Terminated
  .
• Abnormal Termination.
• 3) Normal Termination of the Running
  Process.
• In ALL these cases it is always followed by a
  Context Switch to invoke the Scheduler
  followed by the Dispatcher to bring in some new
  process into execution.
• 3) A device interrupt occurs signalling
  either end of Peripheral Operation or fresh
  request for Data Transfer from/to Peripheral
  Buffer.
• Only Mode Change takes place .

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Mode Switching Activity

• 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 Kernel ? User

• This normally happens either
• 1) By executing some privileged
• instruction executed by the Kernel.
• 2) By some RETURN instruction after
  completion of Peripheral service issued by the
  associated Interrupt Handler (Part of Kernel).

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The Kernel the User Relationships
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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