Chapter 3: Process Description and Control

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Chapter 3 Process Description and Control

• CS 472 Operating Systems
• Indiana University Purdue University Fort Wayne

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Typical processes

• Batch job
• Perhaps a background application
• Interactive process
• Operating system function
• Monitor performance
• I/O manager
• Spawned subprocesses
• Children of a parent process

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Recall . . .

• A process consists of
• Activity
• Data
• Execution context
• The execution context includes all information
  the operating system needs to manage the process

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The execution context includes . . .

• Identifier
• Processor state (hardware PCB)
• Registers
• Program counter
• Stack pointer
• PSW
• Execution state (running, ready, blocked)
• Priority and privilege
• Memory pointers
• I/O status information
• Accounting information

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Process Control Block (PCB)

• Data structure containing the execution context
• Contains sufficient information to allow the
  process to be interrupted and later resumed
• Created and managed by the operating system
• Allows support for multiple processes

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Recall . . .

• Each process has an execution state
• Running
• Executing on the processor
• Ready
• Ready to run
• Waiting in a queue for time on the processor
• Blocked
• Waiting for an event to occur
• Cannot run until the event happens
• See next slide for a chronological scenario

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Process switching
I/O request
timer interrupts
Process A
Process B
Process C
Dispatcher
time
Interleaved execution of processes on a single
processor
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Seven-state process model

• We have seen that a process may be
• Running
• Ready
• Blocked
• A typical state transition model includes four
  additional states
• New
• Exit
• Ready/Suspended
• Blocked/Suspended

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Seven-state process model
Note all states but New have an implicit
transition to Exit
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Additonal states

• New
• The process control block of the new process has
  been created
• The process has not yet been admitted to the pool
  of executable processes
• Perhaps sufficient resources do not yet exist
• Exit
• The process has been released from the pool of
  executable processes
• Process information is available for performance
  analysis, accounting, or other clean up

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Additonal states

• Ready / Suspended
• A ready process is completely swapped out to
  secondary memory
• The process is inactive while in this state
• Blocked / Suspended
• A blocked process is completely swapped out to
  secondary memory waiting for an event
• Event completion moves the process to the
  Ready/Suspended state

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Reasons to suspend a process

• To free memory for the remaining processes to use
• This might be done to reduce the virtual memory
  page fault rate
• A utility process may not be needed for a while
• Perhaps only executed periodically
• User request or parent process request
• Lack of use
• Deadlock among processes
• Etc.

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Reasons to terminate a process
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Reasons to terminate a process
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Implementation of Running, Ready, and Blocked
states
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Implementation

• In a large operating system, there may be
  hundreds of processes in the blocked state
• It is more efficient to have one blocked queue
  for each event
• Queue holds all processes associated with that
  event
• Avoids the need to search for processes to
  release whenever an event occurs
• Occurrence of the event releases all processes in
  the associated queue
• If processes have priorities, there would be one
  ready queue for each priority level

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Implementation

• OS control system structures are needed to hold
  information about the current status of each
  process and resource
• Tables are constructed for each entity the
  operating system manages

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Memory tables keep track of . . .

• Main memory allocated to the OS and the various
  processes
• Secondary memory allocated to each process
• Any allocated shared memory
• This includes protection attributes for access to
  shared memory regions
• Information needed to manage virtual memory
• In paged virtual memory, this includes the page
  tables of the various processes

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I/O tables

• Availability of each I/O device
• Is the device assigned to a process?
• If so, which process is the owner?
• Status of I/O operations
• What is the location in main memory being used as
  the source or destination of the I/O transfer?
• Which process is associated with the I/O
  operation?

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File tables

• File tables provide information about . . .
• Existence of files
• Location on secondary memory
• Current status
• Closed or open
• If open, by which process?
• Attributes
• Sometimes this information is maintained by a
  file management system

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Process table

• Maintains the process image data structure for
  each process
• The process image includes . . .
• User data
• User activity (program)
• System stack (more user data)
• Local variables
• Parameters
• Process control block (PCB)

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Operating system control tables
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Process control block

• The process control block was discussed earlier
  in simplified form
• In more detail, the elements of the PCB are . . .
• Process identifiers
• Processor state information (hardware PCB)
• User-visible registers
• Control and status registers
• Stack pointers
• Process control information
• Scheduling and state information
• Related processes
• Interprocess communication information
• Process privileges
• Memory management information
• Resource ownership and utilization

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Process identifiers

• Identifiers that may be stored with the process
  control block include
• Internal identifier of this process
• Identifier of the process that created this
  process (parent process)
• User identifier

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Processor state info (hardware PCB)

• User-visible registers
• Control and status registers
• Program counter (PC)
• Process status word (PSW) containing . . .
• Condition codes
• Result of the most recent arithmetic or logical
  operation
• For example sign, zero, carry, equal, overflow
• Status information such as . . .
• Trap enable bits for various traps (overflow,
  underflow, trace)
• Interrupt priority level
• Execution mode (user, kernel)
• Interrupt enable / disable bit
• Stack pointer (SP)
• Possibly one SP for each execution mode

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Process control information

• Scheduling and state information
• Process execution state
• Running, ready, blocked, ready/suspended, etc.
• Priority relative to other processes
• Scheduling-related information
• The details are specific to each scheduling
  algorithm used
• For example, the length of time that the process
  has been waiting
• Events
• Identification of the events that must occur
  before the process can be resumed

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Process control information

• Related processes
• A process may be linked to other processes in a
  queue, ring, etc.
• A process may exhibit a parent-child
  (creator-created) relationship with another
  process
• The process control block may contain pointers to
  other processes that support these relationships
• Interprocess communication (IPC) information
• Various flags, signals, and messages may be
  associated with communication between two
  separate processes
• Some or all of this information may be maintained
  in the process control block

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Process control information

• Process privileges
• Memory that may be accessed
• The types of instructions that may be executed
• Allowed system utilities and services
• Memory management information
• References to segment and/or page tables that
  describe the virtual memory assigned to this
  process
• Resource ownership and utilization
• Open files
• Devices currently controlled
• History of utilization of the processor or other
  resources
• This may be needed by the scheduler

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Execution modes

• Systems typically have two execution modes
• User mode
• Less-privileged mode
• User programs typically execute in this mode
• Kernel mode
• More-privileged mode
• Kernel of the operating system executes in this
  mode
• Also called system mode or control mode

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Execution modes

• A user-mode process may run in kernel mode when
  necessary
• The way this is possible is summarized below
• The user-mode process executes a change-mode
  instruction
• This a special supervisor call instruction
• The change-mode instruction has an integer
  operand indicating a specific OS supervisor
  routine to be activated
• The supervisor routine runs in kernel mode
• For example, it may control a device
• When done, the supervisor routine executes a
  special return instruction that changes the
  processor mode back to user mode
• In this way, a user-mode process may temporarily
  gain necessary privilege in a controlled manner

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Mode switch

• An interrupt or supervisor call typically results
  in a mode switch to the interrupt handler or
  supervisor routine
• Also called a context switch
• A mode switch is typically hardware implemented
• Efficient to perform
• Easy to restore the old mode (context)
• The running process does not undergo a change of
  state from Running to Ready as a result of a mode
  switch

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Mode switch

• What is involved
• The hardware context (hardware PCB) of the
  interrupted process is saved
• The hardware PCB is loaded with a new context
  associated with the appropriate interrupt handler
  or supervisor call

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Typical causes for a mode switch

• Clock interrupt
• The process has executed for the maximum
  allowable time slice
• I/O interrupt
• Memory fault (or page fault)
• There is a reference to a page in virtual memory
  that has not been loaded into main memory
• Trap
• Error or exception occurred
• This may cause process to be moved to the Exit
  state
• Supervisor call
• For example, to open a file

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Process switch

• In contrast to a mode switch, during a process
  switch the running process does undergo a change
  of state
• Examples
• Running to Ready
• Running to Blocked
• A process switch is performed by operating system
  software

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Process switch

• What is involved
• A mode switch is performed to an interrupt
  handler or supervisor routine
• The handler/routine updates software PCB of
  running process
• Running process is moved to appropriate queue for
  its new state
• Ready, Blocked, Ready/Suspended, etc.
• A new process is selected to run next
• The software PCB for the new process is updated
• The new process is removed from its queue
  (typically Ready)
• Memory-management data structures are updated if
  needed
• A mode switch is performed from interrupt handler
  or supervisor routine to the new process

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Relationship of the OS to processes

• Is the OS kernel a process?
• Options
• Non-process kernel
• The OS kernel operates outside of every process
• Operating system code is executed as a separate
  entity that operates in kernel mode
• Execution within user processes
• Except for process switching activity, the OS
  kernel executes in the context of the current
  user process
• The OS code is in an shared area of virtual
  memory accessible to all processes
• Supervisor call overhead is just two mode
  switches
• Not two process switches
• Process-based operating system
• Except for process switching activity, the OS is
  a collection of processes
• Cleaner architecture in a multiprocessor or
  multicomputer system

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UNIX SVR4 process management

• Most of the operating system executes within the
  environment of a user process

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UNIX process states
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UNIX process states

• The states Preempted and Ready to run in
  memory are really the same (same queue)
• Reason for dotted line
• Preemption can only occur when a process is about
  to move from kernel mode to user mode
• Perhaps a higher priority process has become
  ready
• While a process is running in kernel mode, it may
  not be preempted
• This makes UNIX in this form unsuitable for
  real-time processing

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UNIX process states